БЪЛГАРСКА АКАДЕМИЯ НА НАУКИТЕ ИНСТИТУТ ПО ЕЛЕКТРОНИКА “АКАДЕМИК ЕМИЛ ДЖАКОВ”

BULGARIAN ACADEMY OF SCIENCES INSTITUTE OF ELECTRONICS “ACADEMICIAN EMIL DJAKOV”

EMIL DJAKOV INSTITUTE OF ELECTRONICS

ANNUAL REPORT 2013

Editors: Ch. Ghelev and N. Guerassimov

3 Annual Report IE 2013

Contents Page

About the Academician Emil Djakov Institute of Electronics 4

Laboratories: 9 ● Plasma Physics and Engineering 11 ● Physical Problems of Ion Technologies 19 ● Physical Problems of Electron Beam Technologies 22 ● Superconductivity and Cryoelectronics 30 ● Micro- and Nano-photonics 35 ● Biophotonics 41 ● Laser Systems 55 ● Nonlinear and Fiber Optics 60 ● Laser Radars 64 ● Microwave Physics and Technologies 72 ● Microwave Magnetics 75 ● Physical Technologies 80

Selected Projects: 85 ● Modeling and Simulation of Gyrotrons for ITER 87 ● 3D Femtosecond Laser Microprocessing of Biomaterials for 95 Application in Medicine ● Light-induced Atomic Desorption (LIAD) for All-optical Control 99 of Light ● Velocity Distribution of Alkali Atoms in Micrometric Thin Cells 103 ● Room-temperature Multiferroics Based on Y-type Hexaferrites 108

Scientific Events: 115 ● Eighteenth International Summer School on Vacuum, Electron and 117 Ion Technologies (VEIT 2013)

Awards 119

4 About the Acad. E. Djakov Institute of Electronics

ABOUT THE ACADEMICIAN EMIL DJAKOV INSTITUTE OF ELECTRONICS

In its quality of a part of the Bulgarian techniques, research and industrial Academy of Sciences, the Institute’s equipment for micro- and nano- mission is to acquire, accumulate and structuring, thin films deposition and disseminate scientific knowledge and study, modification of surfaces, vacuum technologies in its research field, thus melting and welding of metals by intense contributing to Bulgarian people’s electron beams. The physical basis is intellectual and material enrichment and to being formed of creating nanostructures, widening humankind’s scientific horizons. nanomaterials and nanoelements by The Institute of Electronics at the using electron and ion beams. Bulgarian Academy of Sciences was Furthermore, fundamental properties are established in 1963 as a non-profit state being investigated of gasses and gas organization conducting research, plasma, plasma arcs and plasma torches education and dissemination of scientific in view of developing diagnostic knowledge in the fields of Physical techniques and applications in thin films Electronics, Photonics and Quantum deposition and plasma chemistry. Electronics and Radio Sciences. Soon, the The research in photonics and quantum Institute of Electronics evolved as a electronics comprises theoretical and leading scientific institution in these areas experimental studies on the interaction of of applied physics and engineering within short and ultrashort lasers pulses with the Bulgarian Academy of Sciences and in matter; development of novel Bulgaria. nanostructuring technologies; laser thin- Through the years, the Institute’s films deposition and treatment; light- research field and structure have induced absorption and transmission in developed dynamically in response to the alkaline vapors; development of complex changes taking place in the main trends laser systems for analysis and modification in applied physics and technologies: of semiconducting and superconducting materials science and technologies, materials; theoretical and experimental physics of nano-sized objects and investigation of non-linear optical nanotechnologies, nanoelectronics, phenomena; biomedical photonics. photonics, opto-electronics, quantum The research efforts in radiophysics are optics, environmental physics and directed to clarifying the processes of monitoring, biomedical photonics and interaction of optical and microwave biomedical applications. electromagnetic radiation with the The Institute’s main areas of scientific atmosphere and the Earth’s surface; development have been preserved, while developing experimental systems for laser the specific research today takes into remote sensing and monitoring of the account the contemporary problems and atmosphere; microwave remote requirements, the world-wide tendencies radiometric measurement of soil moisture and Bulgaria’s specificity and traditions. content; developing algorithms and The research efforts in physical techniques for signals and information electronics are concentrated on studying processing; constructing microwave units and solving the problems of generating and systems for radar and communication and controlling electron and ion beams applications; studying non-linear processes and their interaction with materials. This in optical communication media. New includes theoretical modeling, modern ferrite devices with micrometric 5 Annual Report IE 2013

dimensions were developed with Scientists from the Institute are possibility for a higher degree of delivering 35 academic courses in ten integration. Active research on gyro- universities in Bulgaria and have been magnetic materials is underway, in view of invited to lecture at universities in the reaching higher frequency ranges, European Union, Japan, etc. At present, especially mm-waves for wireless eight doctoral students are preparing their communications and protection from theses in the Institute. powerful microwave radiation. The Academician Emil Djakov In all three fields of research, the Institute of Electronics was where the first Institute’s scientists have achieved Bulgarian laser, lidar, plasma torch, internationally-recognized priority by ultrahigh vacuum pump, micro-channel discovering new physical phenomena and electron-optical converter, parametric relations, among which are ion channeling microwave amplifier, Josephson junctions in the semi-channels of a crystal surface and SQUID, portable microwave moisture and hyperchanneling at grazing angles; meter, magnetometer, installations for ion-stimulated sorption; plasma electrodes electron lithography, electron beam for gas lasers; particularities in the melting, refining, and welding were built, interaction of electron and ion beams with followed by the development of several materials; electromagnetically induced advanced e-beam technologies, novel absorption in Hanle configuration. types of optical gas sensors, pioneering Scientists from the Institute are achievements in nanostructuring and actively involved as experts in the work of nanoparticle formation, laser and plasma a number of governmental and high technologies. international organizations, such as the The Academician Emil Djakov National Scientific Fund, scientific boards Institute of Electronics aims to sustain and at other institutes within the Bulgarian advance previous pioneering work by Academy of Sciences, academic boards of promoting the theory, basic science and universities, editorial boards of Bulgarian technology of photonics, optoelectronics, and international scientific journals, expert environmental monitoring, laser bio- boards of the European Commission, medical research and applications. This program committees of national and involves searching for new materials, new international scientific events. techniques, new devices and new applications.

STRATEGIC PLAN AND PRIORITIES FOR 2014-2016

The development is envisaged of The Institute of Electronics’ strategic research subjects and short-and long-term plan for scientific research is based on the plans, including the prospects for results and achievements obtained by the strengthening the interdisciplinary most competitive researchers and cooperations within the Academy, at laboratories. It coincides nowadays with national level and internationally (in several emerging fields. The research Europe and worldwide). activity of the Institute has had the The physical and engineering sciences tendency of becoming more complex and are key driving forces for research and interdisciplinary. Bearing in mind the innovation, providing fundamental insight priorities of the National Strategy for and creating new applications. Scientific Development 2020 and of 6 About the Acad. E. Djakov Institute of Electronics

Horizon 2020 for the next three years the Topic 3. Physics and diagnostics of the Institute’s research priorities will be: plasma in controlled thermonuclear fusion (CTF) reactors. Theory, Topic 1. Electronic, ionic and optical modeling, experiments. (Within the techniques for development of new framework of the European Fusion materials and methods for their Development Agreement (EFDA) and characterization. Nanomaterials and EURATOM). nanotechnologies. 3.1. Development and application of 1.1. Electronic techniques; novel, highly-efficient techniques for 1.2. Ionic techniques; experimental determination of the 1.3. New magnetic materials and parameters of magnetically-confined superconductivity; plasma in CTF reactors of the 1.4. Optical techniques. tokamak (incl. ITER and DEMO) and stellarator types; Topic 2. Photonics for quality of life 3.2. Theoretical and numerical studies improvement: photonic techniques for of high-power gyrotrons for electron analysis of media and structures. cyclotron resonance heating (ECRH) 2.1. Biophotonics; and current drive (ECCD) in CTF 2.2. Remote sounding of the reactors and of high-frequency atmosphere: remote studies of aerosol gyrotrons for novel technological processes, cloud formations and applications and for fundamental gaseous pollution in the atmosphere scientific research. Development of over Republic of Bulgaria and across computer software and carrying out Europe as a part of the EARLINET numerical experiments for computer- European Lidar Network; aided design (CAD), analysis and 2.3. Quantum-optical techniques for optimization of new gyrotrons with monitoring the Earth’s magnetic field improved parameters. and the electromagnetic pollution; 2.4. New techniques for analysis of Topic 4. Social physics and application food contamination based on surface- of physical methods in economics. enhanced Raman spectroscopy.

INSTITUTE’S LEADERSHIP

DIRECTOR Assoc. Prof. Sanka Gateva, Ph.D. SCIENTIFIC SECRETARY Tel: +359 2 875 0077 Assoc. Prof. Ekaterina Borisova, Ph.D. e-mail: [email protected] Tel: +359 2 979 5894 e-mail: [email protected] DEPUTY DIRECTORS ● Assoc. Prof. Tanja Dreischuh, Ph.D. ADMINISTRATIVE DIRECTOR Tel: +359 2 979 5867 Mrs. Dora Mladenova e-mail: [email protected] Tel: +359 2 875 2678 ● Assoc. Prof. N. Nedyalkov, Dr.Sc. e-mail: [email protected] Tel: +359 2 979 5924 e-mail: [email protected] 7 Annual Report IE 2013

SCIENTIFIC COUNCIL1

Chairman: Assoc. Prof. O. Yordanov, Ph.D., IE BAS, e-mail: [email protected], [email protected] Deputy Chairman: Assoc. Prof. T. Koutzarova, Ph.D., IE BAS, e-mail: [email protected] Secretary: Assoc. Prof. I. Sirkova, Ph.D., IE BAS, e-mail: [email protected]

Members:

Prof. V. Gerjikov, Dr.Sc, Assoc. Prof. T. Dreischuh, Ph.D., INRNE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Prof. T. Nurgaliev, Dr.Sc., Assoc. Prof. S. Gateva, Ph.D., IE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Prof. P. Petrov, Dr.Sc., Assoc. Prof. L. Gurdev, Ph.D., IE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Prof. K. Vutova, Dr.Sc., Assoc. Prof. L. Kovachev, Ph.D., IE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Assoc. Prof. N. Nedyalkov, Dr.Sc., Assoc. Prof. L. Michov, Ph.D., IE BAS, e-mail: [email protected] S-W U B, e-mail: [email protected] Assoc. Prof. E. Alipieva, Ph.D., Assoc. Prof. S. Sabchevski, Ph.D., IE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Assoc. Prof. E. Balabanova, Ph.D., Assoc. Prof. M. Taslakov, Ph.D., IE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Assoc. Prof. E. Borisova, Ph.D., Sen. Assist. M. Damyanova, Ph.D., IE BAS, e-mail: [email protected] IE BAS, e-mail: [email protected] Assoc. Prof. A. Ognemirova-Dikovska, Ph.D., IE BAS, e-mail: [email protected]

1 Abbreviations: IE BAS –Acad. E. Djakov Institute of Electronics of the Bulgarian Academy of Sciences INRNE BAS – Institute for Nuclear Research and Nuclear Energy of the Bulgarian Academy of Sciences S-W U B – South-West University, Blagoevgrad 8 About the Acad. E. Djakov Institute of Electronics

LABORATORIES:

PLASMA PHYSICS AND ENGINEERING HEAD: Assoc. Prof. S. Sabchevski, Ph.D.

PHYSICAL PROBLEMS OF ION TECHNOLOGIES HEAD: Prof. S. Tinchev, Dr.Sc.

PHYSICAL PROBLEMS OF ELECTRON BEAM TECHNOLOGIES HEAD: Prof. K. Vutova, Dr.Sc.

SUPERCONDUCTIVITY AND CRYOELECTRONICS HEAD: Prof. T. Nurgaliev, Dr.Sc.

MICRO- AND NANO-PHOTONICS HEAD: Prof. P. Atanasov, Dr.Sc., Corresponding Member of BAS

BIOPHOTONICS HEAD: Prof. L. Avramov, Dr.Sc.

LASER SYSTEMS HEAD: Assoc. Prof. S. Gateva, Ph.D.

NONLINEAR AND FIBER OPTICS HEAD: Assoc. Prof. L. Kovachev, Ph.D.

LASER RADARS HEAD: Prof. D. Stoyanov, Dr.Sc.

MICROWAVE PHYSICS AND TECHNOLOGIES HEAD: Assoc. Prof. O. Yordanov, Ph.D.

MICROWAVE MAGNETICS HEAD: Prof. I. Nedkov, Dr.Sc.

PHYSICAL TECHNOLOGIES HEAD: Prof. P. Petrov, Dr.Sc.

LABORATORIES

● Plasma Physics and Engineering ● Physical Problems of Ion Technologies ● Physical Problems of Electron Beam Technologies ● Superconductivity and Cryoelectronics ● Micro- and Nano-photonics ● Biophotonics ● Laser Systems ● Non-linear and Fiber Optics ● Laser Radars ● Microwave Physics and Technologies ● Microwave Magnetics ● Physical Technologies

11 Annual Report IE 2013

LABORATORY

PLASMA PHYSICS AND ENGINEERING

HEAD: Assoc. Prof. S. P. Sabchevski, Ph.D.

TOTAL STAFF: 9 RESEARCH SCIENTISTS: 6

Assoc. Prof. E. Balabanova, Ph.D.; M. Dimitrova, Ph.D.; M. Damyanova, Ph.D.; P. Ivanova, Ph.D.; E. Vasileva, M.Sc.; I. Bozhinova, M.Sc.; K. Raykov, M.Sc.; E. Hasan, B.Sc.

RESEARCH ACTIVITIES energy distribution function (EEDF) on the basis of the first derivative probe technique (FDPT). 1. Diagnostics of edge plasma in As a result of these experimental tokamaks studies, significant new information about the plasma parameters was obtained that In recent years, an advanced novel provides a better insight in the underlying technique for diagnostics of magnetically physical processes near the inner wall of confined plasma based on the classical the tokamak vessel. It was found, for Langmuir probe method was developed example, that using a poloidal arrangement and is being actively applied to systematic of the probes the calculated EEDF is a bi- experimental investigations carried out in Maxwellian in the inner (high-field side) various fusion devices, namely, in the new and in the outer (low-field side) divertor COMPASS tokamak at the Institute of regions, while it is a Maxwellian (having a Plasma Physics of the Academy of single temperature of 9 eV) in the middle Sciences of the Czech Republic (IPP- between the two strike points, where the ASCR) as well as in the ISTTOK tokamak D-shaped plasma is in contact with the and the TJ-II stellarator (CIEMAT) in divertor. New data were obtained also for Spain. the plasma parameters in the so-called In 2013, a series of experiments was SOL (scrape-off layer) region that is carried out on the COMPASS tokamak, located between the last closed surface of during which a great number of Langmuir the magnetic field (LCFS) and the wall of current-voltage characteristics was the tokamak using both of the available registered using a set of different probes reciprocating manipulators. The first of placed in the plasma edge region. Among them is a vertical one with a single them were the standard (embedded Langmuir probe, while the second one is initially in the plasma vessel) 39 graphite positioned horizontally and has 3 ball-pen probes and a novel 60-chanel probe system probes and 2 single Langmuir probes. (designed and installed by the Bulgarian Such an experimental set-up allows one to research team). The interpretation of the collect rich information about the plasma measured signals was performed by parameters in this zone, which is of great processing the electron part of current- importance for the operation of the voltage (I-V) characteristics, which are tokamaks. In particular, the results show used in order to retrieve the plasma that in the proximity of the wall the EEDF potential and to evaluate the electron is Maxwellian. At the same time, it was 12 Laboratory Plasma Physics and Engineering

found that going inside the plasma the ECH and current drive (ECH&CD) system distribution changes its shape and can be of ITER (International Thermonuclear approximated by a bi-Maxwellian profile Experimental Reactor) requires 24 MW that corresponds to two groups of particles, installed power (20 MW delivered to the respectively. The first one (with a higher tokamak in a CW mode for 60 min) at 170 concentration) consists of slow electrons GHz and additional 3 MW at 120 GHz with a temperature of about 5 eV, while (~3 s) for assisting the initial breakdown. the second group is represented by faster Modeling, simulation and computer aided electrons with a temperature of 10-22 eV. design (CAD) based on numerical Studies were carried out as well of the experiments are indispensable tools for the non-thermal electrons in the boundary development, optimization and study of region of the TJII stellarator using I-V such high-power gyrotrons. The research Langmuir probe characteristics. The basic on this topic is being pursued as a Task parameters (plasma potential and 2.1.2 of the scientific program of the temperature, electron density) and the Association EURATOM-INRNE by a EEDF were evaluated at different stages of Bulgarian team from IE-BAS and from the plasma heating, namely, during the ECR Faculty of Physics of Sofia University phase and the NBI phase. (FP-SU) in collaboration with the Institute The data thus obtained allow a better for Pulsed Power and Microwave physical insight in the physical processes Technology at KIT (IHM-KIT), Karlsruhe, that affect the operation of the COMPASS Germany and Centre de Recherches en tokamak and the TJII stellarator. The Physique des Plasmas, École techniques applied in the investigations Polytechnique Fédérale de Lausanne mentioned above can be used in studying (CRPP-EPFL), Switzerland. other similar reactors for fusion research. The work on this project includes: (i) The work described is being pursued as theoretical studies focused on the a Task 2.2.1 “Edge plasma diagnostics” of development of adequate, self-consistent the Association EURATOM-INRNE (PI and informative physical models Tsv. Popov) as well as in the framework of formulated in two (2D) and three (3D) a joint research project between IE-BAS space dimensions; (ii) maintenance, and IPP-ASCR (PI M. Dimitrova). upgrade, improvement, testing and benchmarking of the available computer 2. Modeling, simulation and computer codes and the underlying numerical aided design (CAD) of powerful libraries, compilers, integrated gyrotrons for fusion research development environments (IDE); (iii) development of novel software tools for High-power microwaves generated by simulation of the electron-optical system megawatt-class gyrotron tubes operating in (EOS) of the tube and the electro- the millimeter band (with typical dynamical system (resonant cavity) using frequencies ranging from 120 GHz to efficient numerical methods and 170 GHz) in CW and long-pulse regimes algorithms, advanced computing are used in various reactors for controlled environments and programming thermonuclear fusion (e.g. tokamaks and techniques, as well as (iv) conducting stellarators) for electron cyclotron numerical experiments for analysis, resonance heating (ECRH) and electron computer aided design (CAD) and cyclotron current drive (ECCD) of optimization of megawatt class gyrotrons. magnetically confined plasmas as well as The codes under development are for plasma diagnostics and plasma integrated in several problem-oriented stabilization. For example, the planned software packages (most notably 13 Annual Report IE 2013

GYROSIM, GYREOSS, CAVITY (IHM) maintains a collaborative website of the and ESRAY (IHM)) for CAD and project (http://gyreoss.wikidot.com), numerical investigation of different which provides more details and subsystems of the gyrotron tube. During illustrative examples. 2013, the computational modules of these packages were subject to modifications directed towards an improvement and 3. Development, investigation and optimization of their program application of high-frequency gyrotrons implementation and adaptation to the upgraded versions of the underlying As the most powerful sources of numerical libraries, compilers and IDEs at coherent radiation in the sub-terahertz and the workstations of the research team. A terahertz frequency range operating in CW series of numerical experiments have been mode, the gyrotrons have demonstrated a carried out that illustrate both the remarkable potential for bridging the so performance and the functionality of the called THz-gap (a.k.a. T-gap) of the latest versions of the packages. electromagnetic spectrum in an area In recent years, GYREOSS (which between microwaves and light (or, in other stands for Gyrotron Electron Optical words, between electronics and photonics), System Simulation) has evolved from a which until quite recently had been problem oriented package for numerical considered as a “no man’s land”. The study of the EOS in both 2D and 3D to a recent progress in the development of sub- test bed for experimenting with different THz and THz gyrotrons has opened an numerical methods and algorithms in order avenue for many novel applications in to develop more efficient computational various scientific and technological fields. modules for particle-in-cell (PIC) One of the leaders worldwide in the field simulations. The field solver of of development, investigation and GYREOSS is based on the finite element application of such gyrotrons is the method and is realized as a FreeFEM++ Research Center for Development of the script for the integrated environment Far-Infrared Region (FIR FU Center) at FreeFEMM++-cs. In the relativistic the University of Fukui, Japan. The particle pusher of GYREOSS, a second- Institute of Electronics has a longstanding order leap-frog Boris-Bunemann scheme (more than 15 years) and fruitful for tracing the motion of the particles is collaboration with FIR FU in the implemented. The latest module developed framework of an Agreement for Academic and integrated in 2013 provides allocation Exchange and a Memorandum of (scattering) of the space charge to the understanding for creation of an nodes of the computational mesh using an international consortium for “Promoting efficient charge-conserving algorithm. international collaboration for The current status of the simulation development and application of sub- tools for CAD and numerical investigation millimeter gyrotrons”. In March 2013, the of gyrotrons were presented in an invited consortium held an “International lecture at the International Symposium on Symposium on Development of Terahertz Development of Terahertz Gyrotrons and Gyrotrons and Applications” at FIR FU Applications (March, 2013, Fukui, Japan) Center. The IE-BAS participates in the and in a conference report at the 18th Int. consortium together with several other Summer School on Vacuum, Electron and leading institutions from Japan, Germany, Ion Technologies (October, 2013, Sozopol, Russia, UK and USA. Bulgaria). The research team working on The gyrotrons developed in FIR FU the development of GYREOSS package Center form the so−called Gyrotron FU 14 Laboratory Plasma Physics and Engineering

Series, which includes several groups of 4. Investigation of the intermolecular devices, namely FIR FU (pulsed tubes), potentials and the thermophysical FIR FU CW (oscillators operating in a properties of gases and binary mixtures continuous wave mode), FIR FU CW C at low pressures (compact tubes), and FU CW G (radiation sources with internal mode converters that The investigations on the form well-collimated Gaussian beams). intermolecular interactions in dilute gases Among the novel applications of these and gas mixtures have always been among gyrotrons are several advanced the central research topics of the spectroscopic techniques (most notably Laboratory. As a result of these studies, ESR spectroscopy, DNP enhanced NMR novel data for both the intermolecular spectroscopy), X-ray detected magnetic potentials and the thermophysical resonance, and studies on the energy levels properties of various gases and mixtures of positronium (Ps). A good example of a have been obtained and published. The pioneering application of a gyrotron in a physical model, which has been used for medical technology is the recently the calculations of the thermophysical developed concept of a dual−beam properties is based on the (n-6) Lennard- irradiation facility for an experimental Jones temperature dependent potential cancer therapy based on the simultaneous (LJTDP) developed by B. Stefanov and L. and/or sequential application of two Zarkova and elaborated further by L. beams, namely a beam of neutrons and a Zarkova and M. Damyanova. The LJTDP CW (continuous wave) or intermittent sub- takes into account the influence of the terahertz wave beam produced by a vibrational excitation of the molecules on gyrotron (forming a so called “quantum the potential parameters, namely the beam”). The conceptual design and the equilibrium distance and potential well feasibility study of the hybrid (dual−beam) depth. Using this type of potential, an system were carried out using the software extensive data base of recommended package GYROSIM. The first prototype of values for the potential well depth, the gyrotron for this facility has already equilibrium distance, second virial been manufactured, tested and prepared for embedding in the complex for BNCT coefficient, viscosity and diffusion in the (Boron Neutron Capture Therapy). The temperature range 200-1200 K have been EOS of this tube has a low voltage obtained for many pure gases and binary magnetron injection gun (MIG) and a mixtures. During many years, the cryo-free superconducting magnet with a investigations have been carried out in maximum field intensity of 8 T. The collaboration with Professor Uwe Hohm gyrotron operates at the second harmonic from the Technical University of of the cyclotron frequency and delivers an Braunschweig, Germany. Recently, Dr. E. output power of 20-30 W at the vacuum Balabanova has been involved in these window and about 15 W at the applicator studies as a member of the research team. (the end of the transmission system). The In 2013, the efforts of the team were central frequency at which the gyrotron aimed at studying the interaction potential operates is 395 GHz and corresponds to a and the thermophysical properties of wavelength of 0.76 mm, i.e. belongs to the gaseous NF3 . Nitrogen trifluoride is a gas sub-millimeter range. It is expected that primarily used in the semiconductor building such novel irradiation facility will industry. Its usage has increased during the open many new opportunities for recent years because it is still not included experimenting with different therapeutic in the list of gases controlled by the Kyoto techniques utilizing a quantum beam. protocol. Because of the high percentage 15 Annual Report IE 2013

of its conversion to fluorine, it is used as a current of 60 A, and a voltage of 20 V. substitute for other fluorinated species The process was carried out at a welding (perfluorocarbons and SF6) for plasma speed of 35 mm/min using shielding gas etching and cleaning of CVD chambers. mixtures of argon and carbon dioxide (Ar- The results obtained were the potential 82%, CO2-18%) at a flow rate of 20 l/min. parameters, the second virial coefficient, The microstructural characteristics of the the viscosity and self-diffusion coefficients joints were analyzed by optical in a wide temperature-range between microscopy. The comparison between 200 K and 1200 K. The systematized and joints obtained by welding with and tabulated values compensate the lack of without the use of diamond nano-size systematic experimental data in such a particles shows that the addition of wide temperature interval and can be used nanoparticles slightly improves the quality for both interpolation and extrapolation, as of the welded beads. It is believed that well as for calculation of other potential- further progress can be achieved by an dependent properties of pure gases and optimization of the welding parameters mixtures at low pressures. and, more radically, by changing the welding method. 5. Nanotechnology and nanomaterials Another activity that should be mentioned is the expert work in the field The Laboratory is involved in studies of nanotechnology. In 2013, the 13-th on novel and advanced nanomaterials and issue of the periodical „Nanoscience & their application to the nanotechnologies. Nanotechnology−Nanostructured materials Here we mention only the latest activities application and innovation transfer” (ISSN that have been pursued in 2013. 1313-8995) with leading editor Е. An advanced technique was Balabanova was published. In recent years, investigated for welding T-joints between this periodical was recognized as a high carbon steel parts by the gas-metal-arc quality journal which is included in the (MAG) method using diamond databases of the Chemical Abstract nanoparticles with an average size of Services. It has been supported in the 3 nm. The work was carried out in a framework of the Science and Business program, funded by both the Development collaboration between the Physical of Human Resources Operational Program Problems of the Electron-Beam and the National Science Fund. Since Technologies Laboratory of the Institute of 2013, the journal has had its own web-site Electronics, the Acad. A. Balevski (visit http://nsc-nt.ipc.bas.bg/). As a Institute of Metal Science, Equipment and member of the Nano-Technologies Technologies (BAS) and our Laboratory. Technical Committee of the Bulgarian It should be mentioned that the usage of Institute of Standardization, E. Balabanova nano powders for modification of various has authored expert assessments of more metals and alloys has attracted strong than 15 standards and translated and edited interest and is an object of intense studies two ISO standards. because the nano-composites produced possess many desired advantageous PUBLICATIONS properties (e.g. high micro-hardness, stability at high processing temperatures) 1. Popov Tsv K, Mitov M, Bankova A, that are lacking or insufficient in the basic Ivanova P, Dimitrova M, Rupnik S, materials. In our experiments, the welding Kovačič J, Gyergyek T, Čerček M beads were made using an arc with a and Dias F M 2013 Langmuir probe 16 Laboratory Plasma Physics and Engineering

method for precise evaluation of the Chapter 2: Polymers and Ceramics, negative ion density in ed H Yuwono Appl. Mechanics and electronegative gas discharge Materials 789 262-8 (Trans Tech magnetized plasma Contrib. Plasma Publications) SBN: 978-3-03785-757-1 Phys. 53/1 51-6 DOI: 7. Idehara T and Sabchevski S P 2013 10.1002/ctpp.201310009 Online Development of high frequency ISSN: 1521-3986 gyrotrons in FIR FU covering sub- 2. Bozhinova I, Kolev St, Dimitrova M, THz to THz range for applications to Popov Tsv and Pashov A 2013 high power THz spectroscopy Proc. Discharge tube with coaxial geometry Int. Symp. Millimeter and for efficient production of metal Submillimeter Waves: Phys. Engin. hydrides Rev. Sci. Instrum. 84/9 Microwaves (June 2013 Kharkov 093107 DOI: 10.1063/1.4820959, Ukraine) pp16-26 http://dx.doi.org/10.- ISSN: 0034-6748 1109/MSMW.2013.66 21990. Print 3. Sabchevski S P, Idehara T, Ishiyama ISBN: 978-1-4799-1066-3 S, Miyoshi N, Tatsukawa T 2013 A 8. Tashev P, Valkanov S, Balabanova E dual-beam irradiation facility for a and Petrov P 2013 Microstructure of novel hybrid cancer therapy gas metal arc welded carbon steel J. Infrared Millimeter and Terahertz with nanoparticles Nanosci. & Waves 34/1 71-87 DOI: Nanotechnol. 13 199-202 ISSN: 10.1007/s10762-012-9945-3 ISSN: 1313-8995 1866-6892 9. Raykov K T and Takov T B 2013 4. Aripin H, Mitsudo S, Sudiana I N, Modernization of a wire bonder for Prima E S, Sako K, Fujii Y, Saito T, semiconductor chips National Forum Idehara T, Sano S, Sunendar B, on Electron, Information and Hernawan H and Sabchevski S 2013 Communication Systems 1 46-50 Microstructural and thermal ISSN: 1314-8605. Available online at: properties of nanocrystalline silica http://ecad.tusofia.bg/epubl/search/files/fi xerogel powders converted les/1013_EIKS_2013_wire_bonding.pdf from Sago waste ash material Mater. Sci. Forum 737 110-8 DOI: CONFERENCES 10.4028/www.scientific.net /MSF.737.110. ISSN: 1662-9752 18th International Summer School on 5. Aripin H, Mitsudo S, Endang P, Vacuum, Electron and Ion Technologies, Sudiana I N, Kikuchi H, Sano S and 7 - 11 October 2013, Sozopol, Bulgaria Sabchevski S 2013 Structural • Dimitrova M, Popov Tsv K, Ivanova P, characterization of mullite-based Vasileva E, Hasan E, Horáček J, ceramic material from Al2O3 and Vondráček P, Dejarnac R, Stöckel J, silica xerogel converted from Sago Weinzettl V, Havlicek J, Janky F and waste ash Adv. Mater. Res. 789 262-8 Panek R DOI: 10.4028/ www.scientific.net Evaluation of the scrape-off-layer /AMR.789.262. ISSN: 1662-8985 plasma parameters by horizontal 6. Aripin H, Mitsudo S, Prima E S, reciprocating Langmuir probe in the Sudiana I N, Kikuchi H, Sano S and COMPASS tokamak. Sabchevski S 2013 Structural • Dimitrova M, Silva C, Fernandes H, characterization of mullite-based Duarte P, Popov Tsv K, Stockel J and ceramic material from Al2O3 and Panek R silica xerogel converted from Sago Determination of the electron energy waste ash In: Advances in Materials, distribution function in the ISTTOK Processing and Manufacturing tokamak. 17 Annual Report IE 2013

• Bozhinova I, Dimitrova M, Kolev S, carbon steel with TiN, TiCN, Y2O3 Popov Tsv and Pashov A nanoparticles. Discharge tube with coaxial geometry • Bankova A, Andreev S, Raykov K, for efficient production of metal Mitov M, Videkov V, Tzaneva B and hydrides. Dimitrova E • Mitov M, Videkov V, Bankova A, Growth of thin films of nanostructured Dimitrova M and Popov Tsv oxide on dielectric base. Automated Langmuir probe system for plasma diagnostics and EEDF International Symposium on Development determination using the second- of Terahertz Gyrotrons and Applications, derivative probe technique. 14 - 15 March, 2013, Fukui, Japan • Damyanova M, Sabchevski S, • Sabchevski S Zhelyazkov I, Vasileva E, Balabanova E, Current status and an outlook for Dankov P and Malinov P further development of the simulation Current status of the development of tools for computer-aided design (CAD) the problem-oriented GYREOSS of high-performance gyrotrons for software package. novel applications. • Damyanova M, Hohm U and Balabanova E International Symposium on Physics and Temperature-dependent potential of Engineering of Microwaves, Millimeter interaction between nitrogen trifluoride and Submillimeter Waves, MSMW'13 molecules and thermo-physical 23 - 28 June, 2013 Kharkov, Ukraine properties of gaseous NF3. • Idehara T and Sabchevski S P • Raykov K and Videkov V Development of high frequency Laser activation of wire bonding surfaces. gyrotrons in FIR FU covering sub-THz to THz range for applications to high 10th International Workshop on Electric power THz spectroscopy. Probes in Magnetized Plasmas (IWEP2013), 9 - 12 July 2013, Madrid, 13th International Conference on Quality Spain in Research (QiR) in Conjunction with • Dimitrova M, Dejarnac R, Popov Tsv K, the ICCS 2013, Yogyakarta, Indonesia, Ivanova P, Hasan E, Kovačič J, 25 - 28 June 2013 Stockel J, Havlicek J, Janky F and • Aripin H, Mitsudo S, Endang P, Panek R Sudiana I N, Kikuchi H, Sano S and Plasma parameters on COMPASS Sabchevski S divertor during ohmic D-shaped Structural characterization of mullite- plasmas. based ceramic material from Al2O3 and • Popov Tsv K, Dimitrova M, Ivanova P, silica xerogel converted from Sago Hasan E, Horacek J, Dejarnac R and waste ash. Stöckel J Second National Congress on Physical Langmuir probe evaluation of the Science, 25 - 29 September 2013, Sofia, plasma potential in tokamak edge Bulgaria plasma at non-Maxwellian EEDF. • Bozhinova I and Pashov A Laser spectroscopy of metal hydrides. th 15 International Workshop on Nanosceience & Nanotechnology, 21 - 23 ONGOING RESEARCH PROJECTS November 2013, Sofia, Bulgaria • Tashev P, Petrov P, Valkanov S, Funded by EC as 7FP projects through Stefanov G and Balabanova E Contract of Association with Association Electron beam surface alloying of low EURATOM-INRNE 18 Laboratory Plasma Physics and Engineering

1. Task 2.1.2: Development of numerical Project under the Bilateral Scientific codes to describe the behavior of high Cooperation between St. Kl. Ohridski power gyrotrons. University of Sofia and Faculty of 2. Task 2.2.1: Edge plasma diagnostics. Electrical Engineering, University of Ljubljana, Slovenia. Funded by the Bulgarian Academy of 6. Edge plasma diagnostics on the Sciences COMPASS tokamak, 1. Edge plasma diagnostics on the Inter-academic collaboration BAS – COMPASS tokamak 2011-2013. ASCR, Bulgaria - Czech Republic. 2. Investigations in the field of the controlled thermonuclear fusion − LABORATORY VISITS powerful gyrotrons for plasma heating and probe plasma diagnostics in 1. S. Sabchevski, Research Center for tokamaks” 2011-2013. Development of Far-Infrared Region (FIR FU), University of Fukui, Japan, COLLABORATIONS 11-22 Mar 2013. 2. S. Sabchevski, Advanced Medical 1. Analysis and optimization of electron Device Research Center, Korea guns for compact shower devices and Electrotechnology Research Institute submillimeter−wave gyrotrons, (KERI), Ansan, Korea, 10-16 Jun in the framework of the Agreement for 2013. academic exchange between IE-BAS 3. M. Dimitrova, Institute of Plasma and FIR FU Research Center in Fukui, Physics, Czech Academy of Sciences, Japan. Prague, Czech Republic, 4 Mar 2013 – 2. Promoting international collaboration 31 Aug 2013 (post-doc), 10-28 Nov for development and application of 2013. submillimeter−wave gyrotrons, 4. P. Ivanova, Institute of Plasma in the framework of the Memorandum Physics, Czech Academy of Sciences, of Understanding between IE-BAS Prague, Czech Republic, 10-16 Nov and FIR FU Research Center in Fukui, 2013. Japan for establishing an international 5. E. Hasan, Institute of Plasma Physics, consortium. Czech Academy of Sciences, Prague, 3. Development of numerical codes to Czech Republic, 24 Aug. - 8 Sept. describe the behavior of high power 2013. gyrotrons, Task 2.1.2 of the Association EURATOM GUESTS – INRNE in collaboration with KIT-IHM, 1. Radomir Panek, Institute of Plasma Karlsruhe (Germany) and EPFL-CRPP, Physics, Czech Academy of Sciences, Lausanne (Switzerland). Prague, Czech Republic. 4. Edge plasma diagnostics, 2. Jan Stockel, Institute of Plasma Task 2.2.1 of the Association Physics, Czech Academy of Sciences, EURATOM– INRNE in collaboration Prague, Czech Republic. with the Institute of Plasma Physics, 3. Renaud Dejarnac, Institute of Plasma Czech Academy of Sciences, Prague, Physics, Czech Academy of Sciences, Czech Republic. Prague, Czech Republic. 5. Langmuir probe diagnostics of electron energy distribution functions in fusion edge type magnetized plasmas,

19 Annual Report IE 2013

LABORATORY

PHYSICAL PROBLEMS OF ION TECHNOLOGIES

HEAD: Prof. S. Tinchev, Dr. Sc.

TOTAL STAFF: 3 RESEARCH SCIENTISTS: 2

Assoc. Prof. T. Milenov, Ph.D.; K. Mukov.

RESEARCH ACTIVITIES and G peaks dominated by the G and D Raman bands of the a-C:H. This 1. Low temperature crystallization of conclusion is in line with the experimental diamond-like carbon films to graphene Raman spectra observed. Electrical field effect measurements of the samples Plasma surface modification was used showed ambipolar dependence, typical for to fabricate graphene on top of insulating single-layer graphene. diamond-like carbon films. We showed that it was possible to achieve 2. Surface resistivity of hydrogenated crystallization of amorphous carbon to amorphous carbon films: Existence of graphene by a combination of pulsed intrinsic graphene on its surface argon plasma treatment and thermal annealing at 350 oC. The Raman spectra The surface resistivity of hydrogenated observed were typical for defective amorphous carbon films was measured by graphene: split D- and G-peaks and a the Van der Pauw method. It was found broad 2D-peak. The interpretation of the that the surface resistivity depends on the Raman spectra of such complicated system applied electrical field. The dependence is not easy, so that we calculated Raman measured showed a sharp ambipolar peak signals of graphene on an amorphous at +0.3 V of the gate voltage and hydrogenated carbon film deposited on a sheet resistance at the peak maximum of Si substrate. Our simulation results 7.5 kΏ/sq. This value is in the same order showed that multiple reflections and of magnitude as the sheet resistance of a interference effects lead to an defect-free graphene monolayer. enhancement of Raman signal of the Therefore, the conclusion was drawn that system. The characteristic for graphene G intrinsic graphene exist on the surface of and 2D bands reached maximal hydrogenated amorphous carbon films. enhancement for thicknesses of the amorphous hydrogenated carbon film of 3. Intrinsic graphene field effect about 75 nm and 230 nm. We estimated transistor on amorphous carbon films that the interference enhancement of the 2D graphene Raman signal was very weak A graphene field-effect transistor was in contrast to that of the G band signal fabricated based on intrinsic graphene on simulated for the underlying diamond-like the surface of as-deposited hydrogenated carbon films on a silicon substrate amorphous carbon films. Clear modulation only. Therefore, the experimentally of the drain current by the gate voltage measured Raman spectra of the whole with unipolar characteristic was observed. graphene/a-C:H/Si system will probably The transistor characteristics changed consist of interference enhanced but still when the surface was etched in oxygen weak 2D graphene peak and stronger D plasma. This can open new prospects for 20 Laboratory Physical Problems of Ion Technologies

graphene electronics as amorphous carbon tellurium, Ag5Te3 and (Te+Ag5Te3) by films can be grown easily with unlimited thermal evaporation. The phase dimensions and no transfer of graphene is homogeneity of the specimens in all necessary. experiments and their composition were studied by SEM/EDX. The broadband

4. Growth of La2CoMnO6 crystals reflectance spectra of β-Ag2Te crystals doped with Pb were measured. In the region from FIR to MIR, we observed a small dip at a photon Crystals of La2CoMnO6 doped with Pb energy of about 0.008 eV; this was were successfully grown by means of the attributed to the free-carrier absorption in high temperature solution growth the THz region. In the region from MIR to thechique and their magnetic and transport NIR, there was another dip at a photon properties were studied. The crystals energy of about 0.097 eV, which might be examined were found to have a due to carrier-plasmon coupling. We used predominantly ordered Co2+/Mn4+ the Kramer-Kroing relation to analyze the 3+ structure with randomly distributed Mn complex dielectric function of β-Ag2Te substituting Co2+. A relaxor-like through the reflectance spectra. By temperature dependence of the dielectric calculating the absorption coefficient, we constant with relaxation maximum was obtained an energy band gap of β-Ag2Te established in the temperature interval 180 for direct transitions of ~1.44 eV. The – 210 K. Based on the dc-conductivity complex refractive indices were also data, it was assumed that the charge determined from the measured broadband transport in the interval 180 – 350 K is reflectance spectra. Some initial results of governed by small-polaron hopping, ultrafast dynamic studies in β-Ag2Te whose onset coincides with the Curie crystals were presented in. temperature − see [2]. The Raman spectra of β-Ag2Te, tellurium and Ag5Te3 were measured at 5. Synthesis of carbon phases by the three excitation wavelengths: 633 nm, hydrothermal technique 515 nm and 488 nm. The Raman active modes of β-Ag2Te, tellurium and Ag5Te3 We studied the synthesis of carbon were situated at frequencies below phases by the hydrothermal technique 300 cm−1, while vibrations of other phases using inorganic acids and organic appeared at higher frequencies. compounds. It was established that the reactions products were amorphic carbon, 7. Ar+ dc-plasma for thinning of micro-crystalline graphite, nano- and multilayer grapheme micro- crystalline diamonds, diamond-like + carbon, fullerens C60 and C70 and carbolite. We used Ar dc-plasma for thinning of The intermediate products included in multilayer graphene, as well as for addition some aromatic and naphthenic additional check of the quality of single- compounds. We suggested also a possible layer polygraphene films. The films were mechanism of formation by the modified by Ar plasma in a simple pulsed hydrothermal method of carbon phases bias diode system operating at a pressure with sp2-hybridized bonds [2]. of 0.3 Torr. The unipolar voltage amplitude used was 400 V (pulse 6. Investigations of β-Ag2Te crystals frequency 66 kHz and pulse duration 10 µs) in order to affect the upper 1-nm We produced β-Ag2Te crystal ingots thick layer of the film. The modification containing few grains using the Bridgman time was 8 s, 15 s, 30 s and 60 s in the method. We also deposited thin films of different experiments. We established that 21 Annual Report IE 2013

in the case of thinning of multilayer • Tinchev S graphene for 30 s, the Raman features’ Ion-modified Josephson contacts in relative intensities (at 633 nm wavelength high-temperature superconductors and laser excitation) were enhanced as follows: SQUIDs (oral presentation). I2D/ID increased by a factor of about 3 • Milenov T I, Valcheva E P and (from ~0.7 to ~2), I2D/IG increased by a Tinchev S S factor of about 2.5 (from ~1 to ~ 2.5) and Influence of the low-energy Ar+ on the full-width at half maximum of the 2D graphene phases (oral presentation). feature decreased about twice (from 80 − 90 cm-1 to 50 − 55 cm-1). Annual Meeting of the Physical Society of Taiwan, 29 – 31 January, 2013 National PUBLICATIONS Dong Hwa University, Hualien, Republic of China 1. Tinchev S, Valcheva E and Petrova E, Low temperature crystallization of • Hong Ch-Ch, Luo Ch-W and Milenov diamond-like carbon films to graphene T I 2013 Appl. Surf. Sci. 280 512-7 Reflectance spectroscopy studies of DOI 10.1016/j.apsusc.2013.05.019. Ag2Te crystals. 2. Yankova L, Milenov T I, Rafailov P M, Avdeev G V, Veleva M N and First Annual Conference of COST Action Gospodinov M M 2013 Magnetic and MP1204 and International Conference on electric field characterization of Semiconductor Mid-IR Materials and La2CoMnO6 crystals doped with Pb Optics SMMO2013, 27 February – 2 Crystal Res. Technol. 48 439–45 March 2013, Warsaw, Poland DOI 10.1002/crat.201300081 3. Tinchev S 2013 Surface resistivity of • Milenov T I and Luo Ch-W hydrogenated amorphous carbon Recent results on synthesis and films: Existence of intrinsic graphene characterization of monoclinic Ag2Te on its surface arXiv preprint crystals. arXiv:1311.0605 4. Tinchev S 2013 Intrinsic graphene ONGOING RESEARCH PROJECTS field effect transistor on amorphous carbon films arXiv preprint European Research Projects arXiv:1311.5529 COST MP 1204 ACTION: CONFERENCES “TERA-MIR Radiation: Materials, Gene- ration, Detection and Applications“. Second National Congress on Physical Science, 25 - 29 September 2013, Sofia, Financed by the BAS Bulgaria (Oral presentation). “Deposition, modification and • Tinchev S investigation of thin films of materials for Low temperature plasma modification electronics by ion methods”. of amorphous carbon to grapheme (oral presentation).

22 Laboratory Physical Problems of Electron Beam Technologies

LABORATORY

PHYSICAL PROBLEMS OF ELECTRON BEAM TECHNOLOGIES

HEAD: Prof. K. Vutova, Dr.Sc.

TOTAL STAFF: 12 RESEARCH SCIENTISTS: 7

Prof. P. Petrov, Dr.Sc.; Assoc. Prof. V. Vassileva, Ph.D.; Assoc. Prof. E. Koleva, Ph.D.; Assoc. Prof. Y. Gueorgiev, Ph.D.; Assoc. Prof. M. Beshkova, Ph.D.; T. Bobev, MSc; M. Naplatanova, MSc; V. Donchev, BSc; R. Nikolov; A. Stoimenov. Ph.D. Student: D. Dimitrov.

RESEARCH ACTIVITIES parameters of the material, the dimensions of the cylindrical metal ingot, etc. These 1. New materials and technologies – data are imported via the developed development, investigation, computational graphical user interface (GUI). The modeling and optimization application software simulates the variation of the temperature fields in the 1.1. New materials regeneration by metal ingot during the heating and, at the electron beam melting and refining same time, systemizes the data on the (EBMR) of metals and alloys. EBMR − variation of important parameters such as: computational modeling and optimization. the heat streams through the different ingot boundaries, the geometry of the liquid pool Computational modeling yields a in the cast block, etc. At the end of the possibility for better understanding and numerical experiment performed, data investigating the heat transfer mechanisms about the variation of important main and can be used for optimizing the electron parameters characterizing the EBM beam melting process in view of obtaining process is systemized. The results obtained new materials with improved give a possibility of studying and characteristics that have many applications recommending suitable EBMR regimes for in the power industry, medicine, process optimization and production of electronics, instrument engineering, pure materials. transport, etc. Based on theoretical A time-dependent 3D axis-symmetrical (adequate physical models development of heat model for simulation of thermal software tool, numerical experiments) and transfer in metal ingots solidified in a experimental studies, we developed a tool water-cooled crucible during EBMR was for modeling and simulating of heat improved and applied to the investigation transfer processes and optimizing the of EBMR of different metals (tantalum, EBMR of metals. hafnium, etc.). In order to optimize the Based on a non-stationary heat model technological regimes, different criteria proposed by us and a modified numerical were proposed for improving the quality scheme of “Pismen-Rekford” type, (structure and composition) of the new application software was developed. The materials produced by EBMR. Analytical input data are the EBMR technological problems of the mathematical optimization parameters (the power and radius of the were formulated, discretized and solved electron beam, heating time, casting via heuristic methods. Different criteria for velocity, etc.), the thermo-physical minimization of L1, L2 norms of the radial 23 Annual Report IE 2013

derivative of the temperature field were During the year, the main activities of used to investigate appropriate the experimental investigation on EBMR technological regimes. Criteria were of metals and alloys using the ELIT-60 proposed for minimization of the curvature equipment were: (i) experiments of the liquid/solid contour, which is concerning verification of the directly connected with the quality of the mathematical heat models developed, structure of the metal produced. development of new modules and The tool developed for modeling and optimization criteria formulation, simulating heat transfer processes and (ii) experiments on extraction of refractory optimizing the process of EBMR of metals metals (tungsten and molybdenum) from can be summarized as follows: waste alloys containing these metals. As a 1. Theoretical mathematical modeling result of the EBMR conducted of tungsten of the heating processes during EBMR of containing alloys Fe/WC-50/50 and metals and alloys, including adequate W/Cu/Zn, the achieved final physical models and equations; concentrations of tungsten in the refined 2. Construction of a modified Pismen- materials were 96.5 % (for Fe/WC-50/50) Rekford numerical scheme of the model; and 99.4 % (for W/Cu/Zn). The work was 3. Development of software tool for performed in the framework of an model simulation via the numerical Agreement for development of scheme; technologies for EBMR of metals and 4. Verification of the model; alloys in Bulgaria and for training of 5. Development of analytical criteria for specialists for high-technological work optimizating the quality of the new between IE-BAS and the companies IPPK materials obtained by achieving flatness of EOOD, Sofia, and Targets OOD, Plovdiv. the crystallization front shape; Based on the studies of the EBMR 6. Discretization of the criteria refining kinetics, metal samples were synchronically with the numerical analyzed together with our Indian discretization of the model and computer colleagues from the Centre for Materials implementation; for Electronics Technology (C-MET), 7. Choice of heuristic optimization Hyderabad, India. The research is being techniques and synchronization of all pursued in the framework of a joint project computer programs developed; under the Bulgarian-Indian inter- 8. Tests using different combinations of governmental program of cooperation in criteria, control variables and optimization science and technology. Data was obtained techniques. and analyzed about the concentration of Experimental data and simulation metal impurities (ICP MS), oxygen and results (from numerical experiments at nitrogen in the initial (raw) materials and different combinations of criteria, in the new materials produced after EBMR technological control parameters and of hafnium and titanium under the optimization techniques) were obtained technological conditions examined. The and analyzed. The software tool developed values were estimated of the coefficients and the results obtained are important and of relative volatility of metal impurities in open a possibility for better understanding hafnium and of the free energy for possible and exploring the heat transfer chemical reactions during EBMR of Hf mechanisms, for controlling the quality of scrap. The factors controlling the refining the new material and for optimizing the processes were investigated and criteria process parameters, as well as for more concerning the possibility for refining efficient collaboration between the were selected. A technological scheme for laboratory team, scientific organizations EBMR of hafnium scrap was proposed for and companies. a concrete electron beam equipment.

24 Laboratory Physical Problems of Electron Beam Technologies

1.2. Electron Beam Welding (MAG technique). The welding beads were produced using a current of 60 A, a 1.2.1. A user interface software for voltage of 20 V, a welding speed of electron beam welding (EBW) was 35 mm/min and 20 l/ min shielding gas – a developed, which includes modules for mixture of argon and carbon dioxide (Ar- studying the process parameters influence 82%, CO2-18%). The microstructure and on the geometry and quality of the EB microhardness characteristics of the joints welds, together with modules for were examined. simulation and optimization. The user The effect was studied of using interface is based on four sets of different nanopowders, such as TiN, experimental data obtained in the State TiCN, Y2O3, in electron beam surface National Research Polytechnic University alloying of low carbon steel samples. It of Perm, the Russian Federation. The was found that the use of nanoparticles expriments were conducted under different leads to changes in the microstructure of technological conditions of EBW of high- the treated zones of the samples, as well as strength steel (38Cr2H2Mo) and stainless to an increase of the strength and the steel. The geometry of the welding joints Vickers microhardness both in the width produced in the presence or absence of and the height of the samples. beam deflection oscillations, as well as during welding by two electron beams, was 2. Nano-patterning of structures by examined depending on the EBW process electron beam lithography (EBL) and conditions. Quality criteria for the welding nanoelectronics joints were formulated and studied. The stainless steel weld profiles were simulated. Among all fabrication methods, the Several optimization approaches were resist-based electron beam lithography implemented (graphical optimization, seems to be a suitable and flexible desirability function, Pareto optimization) technique when nanostructures are to be in view of obtaining optimal solutions produced for applications such as depending on the concrete task for development of sensors. Achieving sub- producing welds of both stainless steel and 100 nm structures using electron beam high-strength steel (38Cr2H2Mo). The lithography (EBL) is a very sensitive development of the user interface software process determined by various factors, will continue with the development of new starting with the choice of resist material functional and simulation modules and ending with the development process. involving MS students and trainees from To correct the pattern distortions caused the University of Chemical Technology and by electron scattering in the resist/substrate Metallurgy, Sofia, the Technical University material, estimation of the lithographic of Sofia and the University of Mining and parameters is required. We chose PMMA Geology, Sofia. The software was A2 (Microchem) as a positive resist for developed in Russian with the purpose of nanostructure pattern transfer due to its its being used in the EBW installation in the high resolution and high contrast for EBL State National Research Polytechnic exposures, uniform resist coating, long University of Perm, the Russian shelf life and good adhesion to most Federation. substrates. The thicknesses of the PMMA A2 resist layer and of the thin platinum 1.2.2. A technique was proposed of layer on Al2O3 were 70 nm and 20 nm, welding T-joints of carbon steel using a respectively. The limitations of the paste containing diamond nanoparticles electron beam nanostructure patterning with an average diameter of 3 nm. Gas were investigated for a Gaussian e-beam at metal arc welding was used with argon gas an accelerating voltage of 30 keV in the 25 Annual Report IE 2013

case of thermal emission. Data for the digital electronics, sensing, and energy deposition function was obtained photovoltaics. The results fall within the by Monte Carlo simulation. The following main directions, namely: backscattered electrons influence on the (i) optimization of a CMOS-compatible accurate definition of the structures top-down fabrication process for Si patterned in the resist layer was evaluated nanowire sensing devices with various by a Monte Carlo methodology. The numbers, lengths, and widths of the proximity parameters (βf, βb, ηE) were nanowires. The devices were fabricated by simulated for a PMMA A2 e-beam resist electron beam lithography and reactive ion on a 20-nm thin platinum layer. The etching on silicon-on-insulator (SOI) lowest value achieved for the line/space wafers. Due to the successful optimization, ratio was 77 nm/173 nm. The results of the their electrical characteristics were electron beam lithography process significantly improved. (ii) germanium optimization were applied to the patterning nanowire resistors on germanium on of structures with a minimal line-width of insulator (GeOI) were fabricated with 85 nm to be used in gas sensors. This nanowire width from 20 nm to 1000 nm. research is a result of our collaboration They were used for the development of with colleagues from the Institute of damage-free doping methods avoiding the Informatics of the Slovak Academy of use of the standard method of ion Sciences in the framework of the joint implantation. (iii) activities on fabrication project “Robust lithography of submicron of test quantum computing devices on SOI and nano-dimensional structures” under and GeOI substrates using electron beam the Bulgarian-Slovak inter-governmental lithography and reactive ion etching. program of cooperation in science and technology. PUBLICATIONS Our fruitful and long term (20 years) collaboration with the Hiroshima Institute 1. Vutova K and Donchev V 2013 of Technology, Department of Electronics Electron beam melting and refining of and Computer Engineering (EC-HIT), metals: Computational modeling and Hiroshima, Japan, is also in this field of optimization Materials 6 4626-40 research and is in the framework of an DOI:10.3390/ma6104626 Agreement for Academic Cooperation and 2. Vutova K, Donchev V, Vassileva V Exchange between IE-BAS and HIT for and Mladenov G 2013 Thermal "Computer simulation of the processes of transfer process through treated metal electron, ion and X-ray irradiation of regenerated by electron beam melting electronic materials." A report for the joint and refining J. Metal Sci. Heat research covering over 20 years of Treatment 11 53-60 collaboration was prepared together with 3. Trushnikov D, Koleva E, Mladenov Prof. Dr. Takeshi Tanaka, the Japanese G and Belenkiy V 2013 Effect of Coordinator. beam deflection oscillations on the A review related dealing with device weld geometry J. Material Proc. application of graphene on SiC (HF- Technol. 213 1623-34 transistors, amplifiers and modulators) was 4. Buitrago E, Fagas G, Fernández- prepared by a member of our laboratory, Bolanos Badia M, Georgiev Y M, Assoc. Prof. Dr. Milena Beshkova. Berthomé M and Ionescu A M 2013 During 2013, a member of our Junctionless silicon nanowire laboratory (Assoc. Prof. Dr. Yordan transistors for the tunable operation of Georgiev) continued working in Ireland on a highly sensitive, low power sensor top-down nanofabrication of nanowire Sensors and Actuators B 183 1–10 structures and devices for applications in http://dx.doi.org/10.1016/j.snb.2013.03.028 26 Laboratory Physical Problems of Electron Beam Technologies

5. Yu R, Georgiev Y, Das S, Hobbs R Criteria for refining hafnium scrap G, Povey I M, Petkov N, Shayesteh using electron beam melting J. of the M, O'Connell D, Holmes J D and Technical University – Sofia, Plovdiv Duffy R 2013 Junctionless nanowire branch 19 281-86 transistor fabricated with high 13. Donchev V and Vutova K 2013 mobility Ge channel Physica Status Application software based on non- Solidi Rapid Res. Letters available online stationary heat model for simulation 14.10.2013 DOI: 10.1002/pssr.201300119 of the heat processes during electron 6. Vutova K, Donchev V, Vassileva V, beam melting of metals J. of the Amalnerkar D, Munirathnam N and Technical University – Sofia, Plovdiv Prakash T 2013 Application of non- branch 19 293-8 stationary thermal model for 14. Koleva E, Mladenov G, Trushnikov simulation and investigation of heat D and Belenkiy V 2013 Increasing and refining processes of Ti during energy efficiency in welding EBMR 2013 Proc. EPD Congress J. Electrotechnica and Electronica TMS (Wiley USA) 253-60 5-6 40-46 7. Trushnikov D, Koleva E, Mladenov 15. Koleva E and Mladenov G 2013 G M, Belenkiy V Ya and Salomatova Energy efficiency and renewable E S 2013 Weld formation control at energy generation in Bulgaria electron beam welding with focal spot J. Electrotechnica and Electronica scanning Middle-East J. Sci. Res. 6 5-6 13-27 1062-68 16. Koleva E, Mladenov G, Trushnikov 8. Vutova K, Donchev V, Vassileva V, D and Belenkiy V 2013 Quality Koleva E and Mladenov G 2013 analysis for efficient electron beam Investigation of electron beam drip welding process J. Electrotechnica melting by a time-dependent heat and Electronica 5-6 190-204 model Proc. Int. Conf. High-Power 17. Belenkiy V, Trushnikov D, Mladenov G Electron Beam Technol. EBEAM and Koleva E 2013 Electron beam 2012 (Reno Nevada USA) 35-41 welding − the highly effective energy 9. Koleva E, Vassileva V, Mollov D, saving way of joining metal Vutova K and Mladenov G 2013 construction materials J. Electro- Model-based approach for quality technica and Electronica 5-6 47-51 optimization of EBMR process of Ta 18. Vutova K, Donchev V, Vassileva V, Proc. Int. Conf. High-Power Electron Koleva E and Mladenov G 2013 Beam Technol. EBEAM 2012 (Reno Regeneration of materials by electron Nevada USA) 16-24 beam melting and refining of 10. Koleva E, Trushnikov D, Belenkii V refractory metals and alloys in and Mladenov G 2013 Quality vacuum Annual Report 2012 Institute optimization at electron beam of Electronics, BAS, Eds. Ch. Ghelev welding of steels Proc. Int. Conf. and N. Guerassimov 91-6 High-Power Electron Beam Technol. 19. Koleva E and Mladenov G 2013 EBEAM 2012 (Reno Nevada USA) Energy efficiency and renewable 48-68 energy generation in Bulgaria Proc. 11. Vutova K, Mladenov G, Vassileva V, Int. Joint Symp. Renewable Energy Koleva E, Georgiev Y, Beshkova M Sources and Healthy Buildings and Donchev V 2013 20. Belenkiy V, Trushnikov D, Mladenov Nanoelectronics, new materials and G and Koleva E 2013 Energy technologies J. BAS 6 55-69 (in influence optimization of electron Bulgarian) beam at electron beam welding of 12. Vassileva V and Vutova K 2013 thick metals Mechanical Engineering, 27 Annual Report IE 2013

Material Sci. (Perm National Georgiev Y M, Yu R, Lotty O, Research Polytechnic University) Holmes J D, Nightingale A M and 15/3 33-40 (in Russian) Ionescu A M 2013 3D silicon 21. Tachev P, Valkanov S, Petrov P and nanostructures for biosensing Balabanova E 2013 Microstructure of applications: functionalized 3D 7x20- gas metal arc welded carbon steel array, vertically stacked SiNW FET with nanoparticles Nanoscience & for streptavidin sensing Proc. 71st Nanotechnology 13 199-201 Annual Device Res. Conf. DRC 2013 22. Yu R, Georgiev Y, Lotty O, (Notre Dame, IN, USA) doi: McCarthy B, Petkov N, O'Connell D, 10.1109/DRC.2013.6633887 Nightingale A, Das S and Holmes J 2013 Si junctionless transistor for CONFERENCES sensing application: subthreshold International Joint Symposium on region sensor Proc. Ninth Workshop Renewable Energy Sources and Healthy Thematic Network on Silicon on Buildings, 21-24 April 2013, Belgrade, Insulator Technology, Devices and Serbia Circuits EUROSOI (Paris France) Koleva E and Mladenov G 23. Yu R, Georgiev Y, Povey I M, Das S, • Energy efficiency and renewable Petkov N, Shayesteh M, O'Connell D, energy generation in Bulgaria. Holmes J D and Duffy R 2013 Ge

nanowire junctionless transistor Proc. 142nd Annual Meeting & Exhibition TMS Ninth Workshop Thematic Network 2013, 3-7 March 2013, San Antonio, on Silicon on Insulator Technology, Texas, USA Devices and Circuits EUROSOI (Paris France) • Vutova K, Donchev V, Vassileva V, 24. Georgiev Y, Yu R, Povey I M, Das S, Amalnerkar D, Munirathnam N and McCarthy B, Petkov N, O'Connell D, Prakash T Lotty O, Shayesteh M, Nightingale A, Application of non-stationary thermal R. Duffy R and Holmes J D 2013 model for simulation and investigation Silicon and germanium junctionless of heat and refining processes of Ti nanowire transistors for sensing and during EBMR. digital electronics applications Proc. 7th Int. Workshop Functional Third International Conference Nanomaterials and Devices and 2nd Engineering, Technologies and Systems, Ukrainian-French Seminar Semi- TECHSYS 2013, 29-31 May 2013, Plovdiv, conductor-on-Insulator Materials, Bulgaria Devices and Circuits: Physics, Technology & Diagnostic (Kiev, • Donchev V and Vutova K Ukraine) (invited talk) Application software based on non- 25. Koshro-Pour N, Kayal M, Jia G, stationary heat model for simulation of Eisenhawer B, Falk F, Nightingale A, the heat processes during electron de Mello J C, Georgiev Y M, Petkov beam melting of metals. N, Holmes J D, Nolan M and Fagas G • Vassileva V and Vutova K 2013 A miniaturised autonomous Criteria for refining hafnium scrap sensor based on nanowire materials using electron beam melting. platform: the SiNAPS mote Proc. SPIE 8763 Smart Sensors, Actuators, and International Conference on Mathematical MEMS VI doi:10.1117/12.2017520; Modeling in Physical Sciences - IC- http://dx.doi.org/10.1117/12.2017520 MSQUARE, Sept. 1-5, 2013, Prague, The 26. Buitrago E, Fernandez-Bolaños M, Czech Republic 28 Laboratory Physical Problems of Electron Beam Technologies

• Donchev V and Vutova K COLLABORATIONS Optimization method based on mathematical heat model for electron EU, NATO and other international beam melting and refining of metals. organizations projects

Eighteenth International Summer School Semiconducting nanowire platform for on Vacuum, Electron and Ion autonomous sensors (SiNAPS), 7 FP of Technologies - VEIT 2013, 7-11 October, EU (grant agreement no. 257856), Dr. 2013, Sozopol, Bulgaria Y.Fagas Coordinator.

Donchev V, Vutova K and Vassileva V • Other scientific institutions and companies Experimental and numerical investiga- tion of the refinement of Hf by EBM. 1. Computer simulation of the processes ONGOING RESEARCH PROJECTS of electron, ion and X-ray irradiation of electronic materials, Hiroshima Financed by the National Science Fund Institute of Technology, Hiroshima, Japan. 1. Grant BIn-5 New materials 2. Electron beam nanolithography, regeneration through electron beam Institute of Electrical Engineering melting and refining of refractory (IEE), Chinese Academy of Sciences, metal wastes in vacuum, with the Beijing, China. Centre for Materials for Electronics 3. Investigation of physical and heat Technology (C-MET), Hyderabad, processes at interaction of focused India, under Bulgarian-Indian inter- energy flows with materials, National governmental program of cooperation Institute for Lasers, Plasma and in science and technology. Radiation Physics, Romanian 2. Grant DNTS/Slovakia01/10 Robust Academy of Sciences, Bucharest, lithography of submicron and nano- Romania. dimensional structures, with the 4. Investigation of nanosystems and new Institute of Informatics, Slovak materials by neutron scattering, JINR, Academy of Sciences, under Dubna, Russia. Bulgarian-Slovak inter-governmental 5. Novel nanowire structures for devices, program of cooperation in science and financed by the Science Foundation technology. Ireland (SFI), (grant agreement No 09-IN1-I2602). Financed by the European Social Fund, 6. Agreement between IE-BAS and IPPK Human Resources Development EOOD, Sofia for joint development of Operational Program technologies for electron beam melting and refining of metals and Contract BG051PO001-3.3.07-0002 alloys in Bulgaria and for training of “Students practices”. specialists for high-technological work. Financed by the BAS 7. Agreement between IE-BAS and Targets OOD, Plovdiv for joint Pure metal obtaining, investigation of development of technologies for processes at electron beam modification of electron beam melting and refining of materials, modeling of nano-lithography precious metals and alloys in Bulgaria processes and thin film deposition and and for training of specialists for high- study. technological work. 29 Annual Report IE 2013

LECTURE COURSES LABORATORY VISITS

1. Assoc. Prof. Dr. E. Koleva: Lecture 1. E. Koleva courses on statistics (in English) – 30 h State National Research Polytechnic and seminars – 20 h, Univ. Chem. University of Perm, Perm, Russia, Technol. and Metallurgy, Sofia, May 2013. Bulgaria. 2. K. Vutova and E. Koleva 2. Assoc. Prof. Dr. E. Koleva: Seminars Institute of Informatics, Slovak on methods for experimental Academy of Sciences, Bratislava, investigations – 75 h, Univ. Chem. Slovakia, Nov. 2013. Technol. and Metallurgy, Sofia, Bulgaria. GUESTS 3. Assoc. Prof. Dr. E. Koleva: Seminars on modeling and optimization of 1. Dr. Ivan Kostic, Institute of technological objects – 60 h, Univ. Informatics, Slovak Academy of Chem. Technol. and Metallurgy, Sciences, Bratislava, Slovakia, March Sofia, Bulgaria. 2013 and Oct. 2013. 4. Assoc. Prof. Dr. E. Koleva: Lecture 2. Dr. Liu Junbiao, Institute of Electrical courses on statistical methods – 20 h, Engineering (IEE), Chinese Academy Univ. Chem. Technol. and of Sciences, Beijing, China, July 2013. Metallurgy, Sofia, Bulgaria. 3. Dr. Kong Xiangdong, Institute of 5. Assoc. Prof. Dr. E. Koleva: Seminars Electrical Engineering (IEE), Chinese on industrial management − 69 h, Academy of Sciences, Beijing, China, Univ. Chem. Technol. and July 2013. Metallurgy, Sofia, Bulgaria. 4. Dr. Dinesh Amalnerkar, Executive 6. V. Donchev: Seminars on Director, Centre for Materials for biomathematics – 30 h, St. Kl. Electronics Technology (C-MET), Ohridski Univ. of Sofia. Pune, India, Sept. 2013 7. V. Donchev: Seminars on algebra 1 – 90 5. Nagegownivari Munirathnam, h, St. Kl. Ohridski Univ. of Sofia. Director (A), Centre for Materials for 8. V. Donchev: Seminars on algebra 2 – 45 Electronics Technology (C-MET), h, St. Kl. Ohridski Univ. of Sofia. Hyderabad, India, Sept. 2013 9. V. Donchev: Seminars on 6. Dr. Dmitriy, N. Trushnikov, State mathematical analysis – 45 h, St. Kl. National Research Polytechnic Univ. Ohridski Univ. of Sofia. of Perm, Perm, Russia, Sept. 2013. 10. V. Donchev: Seminars on 7. Dr. Tatiana V. Olshanskaia, State mathematics – 60 h, St. Kl. Ohridski National Research Polytechnic Univ. Univ. of Sofia. of Perm, Perm, Russia, Sept. 2013. 11. K. Vutova, P. Petrov, V. Vassileva, 8. Hartmut Schacke, Vistec Electron E. Koleva – training of more than 45 Beam GmbH, Jena, Germany, Nov. students from St. Kl. Ohridski Univ. 2013. of Sofia, HTMU-Sofia, TU-Sofia, T. 9. Dr. Liana Socaciu-Siebert, SPECS Kableshkov VTU, A. Kanchev RU Surface Nano Analysis GmbH, Berlin, Ivan Rilski MGU and 360 h lectures Germany, Dec. 2013. in the framework of the Project BG051PO001-3.3.07-0002 “Students practice”.

30 Laboratory Superconductivity and Cryoelectronics

LABORATORY

SUPERCONDUCTIVITY AND CRYOELECTRONICS

HEAD: Prof. T. Nurgaliev, Dr.Sc.

TOTAL STAFF: 7 RESEARCH SCIENTISTS: 6

Assoc. Prof. E. S. Mateev, Ph.D.; Assoc. Prof. N. Donkov, Ph.D.; B. S. Blagoev, Ph.D.; L. I. Neshkov; I. G. Gostev; P. Petrov

RESEARCH ACTIVITIES the LSMO component. In spite of the presence of the twin structure, the films During the year reported, the grown under optimal conditions were Laboratory team continued its research in characterized by a quite smooth surface. the fields of “Nanoscience, New Materials Magneto-optical imaging of the magnetic and Technologies” and “Nanoelectronics flux distribution in the YBCO film and New Materials”, which were among revealed that the twin structure can the priority fields of the National Scientific modulate the critical current strength, but Program for 2009-2013. Details of the does not interrupt the current flow. team tasks in this respect were given in the IE 2011 Annual Report. 2. Conductivity of intergrain channels in LSMO and YBCO/LSMO films 1. Twin boundary effect in HTS and FM deposited onto sapphire substrates manganite films sputtered on LAO substrates There is little information available in the literature concerning the characteristics The growth conditions and physical of layered HTS/manganite structures properties were investigated of high deposited on non-traditional substrates, temperature superconducting (HTS) such as Al2O3 (ALO). On the other hand, YBa2Cu3O7 (YBCO) and ferromagnetic studying such kind of artificial structures (FM) manganite La0.7Sr0.3MnO3 (LSMO) is of great importance for the fundamental thin films and YBCO/LSMO bi-layers physics and for possible device with thicknesses of several tens of nm, applications. prepared by magnetron sputtering on The LSMO layers and YBCO/LSMO LaAlO3 (LAO) substrates. It is known that bi-layers were grown on sapphire at room temperature a LAO substrate substrates. The layers were polycrystalline. contains a twin structure. This structure is The magnetic properties of the formed at a ferroelastic (cubic − YBCO/LSMO bi-layer indicated the rhombohedral) transition occurring at presence of a nonuniform magnetic state in ~ 813 K and serves as a template for the LSMO film at low temperatures, which forming the film microstructure. was similar to a cluster-spin-glass system The twin structures were observed by with a characteristic spin-glass transition optical microscopy in the free area of the temperature Tirr ~ 250 K. LaAlO3 substrate and in the areas The temperature dependences of the containing YBCO, LSMO single films and resistance of LSMO films grown on ALO a YBCO/LCMO bi-layer. The main spatial substrates were typical for polycrystalline period of the twin structure seemed to be manganite materials. Depositing a top somewhat different in the areas containing YBCO layer led to an increase of the 31 Annual Report IE 2013

resistance. The temperature dependence of and Structural Research, Polish Academy the resistance of these LSMO films and of Sciences, and the Tallinn University of YBCO/LSMO bilayers was interpreted in Technology, Estonia. the framework of a phenomenological two The sample’s hysteresis loop showed a conductance channel model. One channel soft magnetic behavior with a coercive represents a spin-polarized tunneling field Hc of 260 Oe at 4 K. The remanence between neighboring ferromagnetic grains magnetization Mrem was 8 emu/g at 4 K. At with good contact, and the other, a 300 K, the hysteresis and the coercive field thermally activated transport through an became insignificant and the remanence insulator (semiconductor) phase situated magnetization Mrem became less than mainly at the grain boundary or between 1 emu/g. grains. The very small values of the ratio The resistance’s temperature a1/a2 (where a1, a2 are the effective cross- dependence showed a metal-to-insulator section constants of these channels) in transition peak at TI-M = 201 K; it was YBCO/LSMO samples, determined in the shifted to 208 K when a 10 kOe DC framework of the above model, confirmed magnetic field was applied. No upturn of the strong suppression of the conductivity the resistance curve at 10 kOe was of the channels responsible for a spin- observed. The resistance was reduced as polarized tunneling in the LSMO layers of the field was increased and an observable this polycrystalline sample. The high hysteresis arose at low temperatures. resistivity of these samples and the step- When the sample was in a ferromagnetic like behavior of the R vs T dependences at insulating state at T = 300 K (i.e. 350>T > low temperatures were interpreted as due TI-M), only the intrinsic magnetoresistance to the effect of Coulomb charging energy (MR) effect took place with MR300K ≈ −3% EC to the intergrain conductance at a field H = 10 kOe {where mechanism. MR= 100x[R(H)-R(0]/R(0)}. When the temperature was lowered, the 3. Investigation of the magnetic and non-linearity and the hysteretic character magnetoresistive characteristics of of R-H dependence became more polycrystalline La0.6Pb0.4MnO3 pronounced, with the linear part practically disappearing at 4 K. The magneto- It is known that the partial substitution resistance increased with the temperature in a LaMnO3 compound of the trivalent decrease: for example, the values of the La3+ ions by divalent metal ions, as Sr, Ca, magnetoresistance obtained at H = 10 kOe Ba and Pb, can cause a mixed valence state were MR = −10.5%, −18.3%; −32.3% at of Mn and leads to the appearance of 200 K, 120 K and 4 K, respectively. magnetic properties. While the Sr and Ca doped lanthanides (La1-xSrxMnO3 and La1- 4. Magnetron sputtering of GdFeO3 xCaxMnO3) have been investigated films thoroughly, information lacks in the literature about the Pb doped manganites. The orthoferrites RFeO3 (where R is a We prepared a bulk La0.6Pb0.4MnO3 rare-earth element) demonstrate original sample and studied its magnetic and physical characteristics. They can behave resistive characteristics in a large as ferroelectrics, semiconductors, collinear temperature range in collaboration with the and non-collinear antiferromagnets. Institute of Solid State Physics, BAS, the Coexistence of magnetism and electrical Institute of Mineralogy and polarization (which is characteristic for Crystallography, BAS, the University of multiferroic materials) is possible in doped Chemical Technology and Metallurgy, orthoferrites. The orthoferrites also Sofia, the Institute of Low Temperature demonstrate a pronounced Faraday effect, 32 Laboratory Superconductivity and Cryoelectronics

while coherent light can stimulate the dispersion components were determined formation of spin polarized charge carriers by means of the Owens-Wendt-Rabel- in them. This is why the orthoferrites are Kaelble method. The total abrasive wear of considered as promising materials for composite ceramic coatings at abrasion application in spintronics devices, memory action condition was investigated. elements, sensors, etc. The cell adhesive potential on the The experiments on preparation of thin various substrate materials (with films of the gadolinium orthoferrite roughness parameters in the range 20- GdFeO3 started in collaboration with the 400 nm) on glass, Al2O2, ZrO2, Ta2O5 Shanghai Institute of Ceramics, China substrates was determined. Academy of Sciences. RF magnetron sputtering was used for deposition of a PUBLICATIONS series of amorphous and polycrystalline

GdFeO orthoferrites on dielectric 3 1. Blagoev B, Terzieva S, Nurgaliev T, substrates. The aim of the experiments was Shivachev B, Zaleski A, Mikli V, an optimization of the technological parameters of magnetron sputtering for Staneva A and Stoyanova-Ivanova A 2013 Magnetic and transport preparing GdFeO3 films. X-ray analysis was used to study the structural parameters characteristics of oxygenated poly- of the films. Information was obtained on crystalline La0.6Pb0.4MnO3, JMMM the dependence of the crystalline 329 34-8 DOI: 10.1016j jmmm.2012.10.014 characteristics of the magnetron sputtered 2. Donkov N, Zykova A, Safonov V, GdFeO3 films on the deposition Luk`yanchenko V and Kolesnikov D conditions. This information will be used 2013 The surface properties of Ta2O5 for the further optimization of the ceramic coatings and next deposition process. correlations with cell response in vitro and in vivo tests J. Problems of 5. Deposition of Al2O3, AlN and Atomic Sci. Technol. 1/83 195-7 AlN/Al2O3 coatings on Ti4Al6V and (ISSN: 16829344) stainless steel samples 3. Donkov N, Safonov V, Zykova A, Smolik J, Rogovska R, Goltsev A, The nano-structured composite coatings Dubrava T, Rassokha I and based on Al, Zr, Ti oxides exhibit unique Georgieva V 2012 Nano scale properties, such as high inductivity, high surface modification of plastic density, bio- and chemical inertness, which substrates for advanced tissue are very important for biomedical engineering applications, J. Phys.: applications. Depositions of Al2O3, AlN Conf. Series. 398 012031 and composite nitride/oxide (AlN/Al2O3) coatings on titanium-based alloys Ti DOI:10.1088/1742-6596/356/1/012046 4. Tinchev S, Nurgaliev T 2013 New (Ti4Al6V) and stainless steel samples (1H18N9) were performed (MS) in a thin film materials for electronics, J. vacuum pumping system at a base pressure Bulgarian Academy of Sciences 6 of about 10−3 Pa. The surface properties ISSN 0007-3989 (in Bulgarian) and the structure of Al2O3, AlN films were 5. Blagoev B, Nurgaliev T, Štrbik V, investigated by means of SEM, AFM, XPS Mateev E and Zaleski A 2013 and XRD. The advancing contact angle Magnetic and transport parameters of was measured by Wilhelm’s technique LSMO and YBCO/LSMO films (Kruss K12) at a temperature of 20 oC. The deposited on sapphire substrates surface free energy (SFE) and its polar and Bulg. J. Phys. 40 337-47 33 Annual Report IE 2013

6. Nedkov I, Slavov L, Blagoev B and HTS and magnetic perovskite films. Krezhov K 2013 Surface effects • Štrbík V, Blagoev B, Mateev E and in superparamagnetic magnetite Nurgaliev T particles Bulg. J. Phys. 40 177-89 Electrical transport in epitaxial and polycrystalline LSMO thing films. COLLABORATIONS • Donkov N, Zykova A, Safonov V, Smolik J, Rogowska R, 1. Investigation of the electrical current Luk`yanchenko V and Yakovin S flowing conditions in HTS/manganite Tribological performance of ceramic heterostructures and the electrical coatings deposited on metal surfaces current effects on the resistivity for micro-bearing and biomedical (2012-2014), application. Institute of Solid State Physics, • Donkov N, Zykova A, Safonov V, Russian Academy of Sciences, Kolesnikov D, Goncharov I, Yakovin Chernogolovka, Russia. S and Georgieva V 2. Development of a technology for Modification of the structure and deposition of nanostructured dielectric oxide coatings of new generation used composition of Ca10(PO4)6(OH)2 as biomaterials and biosensors ceramic coatings by changing the ЕР - 12а (2011-2013), deposition conditions in a mixture of National Scientific Center “Kharkov O2 and Ar gases. Institute of Physics and Technology”, • Georgieva V, Gadanova V, Kharkov, Ukraine. Grechnikov A, Donkov N, Sendova- 3. Investigation of the proximity effect Vassileva M, Stefanov P and Kirilov R and spin injection in epitaxial bi-layer Sol-gel TiO2 films as NO2 gas structures of ferromagnetic manganites sensors. and high temperature superconductors, (2012-2014), European Biotechnology Congress, 16-18 Institut of Electroengineering, Slovak May, 2013, Bratislava, Slovakia Academy of Sciences, Bratislava, • Angelova R, Slavov L, Iliev M, Slovakia. Blagoev B, Kovacheva D, Abrashev M, 4. Preparation and spintronic property of Nedkov I and Groudeva V perovskites оxide materials, Biogenic iron oxides produced by (2012-2014), neutrophilic iron-oxidizing bacteriа Shanghai Institute of Ceramics, China under laboratory conditions. Academy of Sciences, Shanghai,

China. Second National Congress on Physical CONFERENCES Sciences, 25-29 September 2013, Sofia, Bulgaria Eighteenth International Summer School • Angelova R, Nedkov I, Slavov L, on Vacuum, Electron and Ion Iliev M, Blagoev B, Buchkov K and Technologies (VEIT 2013), 7-11 October, Groudeva V Sozopol, Bulgaria Magnetic properties and structure of • Blagoev B, Gostev I, Nurgaliev T, biogenic subproducts of the Štrbík V, Bineva, I, Uspenskaya L, Sphaerotilus-Leptothrix bacteria Mateev E, Neshkov L, Dobročka E group metabolism. and Chromik Š • Terzieva S, Stoyanova-Ivanova A and Deposition and characterization of Blagoev B 34 Laboratory Superconductivity and Cryoelectronics

Phase formation and microstructure of of Physics and Technology, Ukraine BSCCO superconductors with Academy of Sciences, Kharkov, ferromagnetic additive. Ukraine. • Nedkov I, Slavov L, Blagoev B and 2. Dr. B. Blagoev, Krezhov K 7 days, November 2013, Institute of Vervey transition in superpara- low temperature and structural magnetic particles of magnetite. research of the Polish Academy of • Vlakhov E, Nenkov K, Marinov Y, Sciences, Wroclaw, Poland.

Neshkov L, Mateev E, Blagoev B, Nurgaliev T, Lakhov L and Toncheva K VISITS OF FOREIGN SCIENTISTS

Cobaltite NdBaCo2O5+x thin films deposion, characterization and their 1. Assoc. Prof. V. Strbik, application potential. 7 days, October 2013, Institute of Electrical Engineering, Slovak Academy International Conference Bioscience – of Sciences, Bratislava, Slovak Republic. Development and New Opportunities – 2. Prof. A. Zikova, Kliment days 10 days, October 2013, Kharkov • Angelova R, Slavov L, Blagoev B Institute of Physics and Technology, and Iliev M Kharkov,Ukraine. Neutrophilic iron-oxidizing bakteria 3. Prof. V. Safonov, Sphaerotilus-leptothrix materials for 10 days, October 2013, Kharkov application in nano and bio Institute of Physics and Technology, tecnologies. Kharkov, Ukraine.

LABORATORY VISITS

1. Assoc. Prof. N. Donkov, 7 days, March 2013, Kharkov Institute

35 Annual Report IE 2013

LABORATORY

MICRO- AND NANO-PHOTONICS

HEAD: Prof. Dr.Sc. P. A. Atanasov, Corresponding Member of BAS

TOTAL STAFF: 11 RESEARCH SCIENTISTS: 7

Assoc. Prof. N. N. Nedyalkov, Dr.Sc.; Assoc. Prof. A. Og. Dikovska, Ph.D.; Assoc. Prof. M. E. Koleva, Ph.D.; N. E. Stankova, Ph.D.; A. S. Nikolov, Ph.D.; E. L. Pavlov, MS; Т. R. Stoyanchov, MS. Ph.D. students: Ro. G. Nikov; Ru. G. Nikov.

RESEARCH ACTIVITES and fluencies in the range of 60 – 410 mJ/cm2. The optical properties of 1. Nanoscience and nanotechnologies these low-density close-packed NP structures were characterized by 1.1. Preparation of metal nanorods microscopic observation, measurement of substrates for SERS application the absorption and Raman spectra, and modeling of the near-field intensity Pulsed laser deposition was used, distributions. The absorption bands together with a glancing-angle deposition centered within 540 – 570 nm were technique, to produce metal substrates assigned to resonant absorption by surface suitable for surface enhanced Raman plasmons. A dependence was found of the scattering (SERS) applications. The band positions, half widths and intensities surface morphology, crystallinity, optical on the NPs shape (partial spheres), size properties, and enhancement factor of and size distribution, as well as on the these substrates were investigated. excitation energy. The structures are Rodamine-6G was used as a SERS active characterized by markedly reduced molecule pumped at 785 nm. The Au dephasing times of ∼3 fs.We thus showed nanorods with average lengths of 50 – that laser annealing of thin Au films is a 70 nm and diameters in the range of 10 – reliable and cost effective method for 20 nm exhibited a SERS enhancement controlled preparation of semi-regular NP factor of 105. The as-prepared metal arrays suitable for photonic applications. substrates were found to meet the criteria for a sensitive, uniform, and easily 1.3. Laser nanostructuring of Au/Ag and produced SERS substrate. Au/Ni films for application in SERS

1.2. Engineering and study of Au The results of our studies showed that nanoparticle arrays laser nanostructuring of Au/Ag and Au/Ni thin films leads to the formation of arrays Semi-regular arrays of Au nanoparticles of bimetallic nanoparticles. These (NP) were studied as obtained by UV laser structures were prepared using a two step irradiation of thin Au films on glass procedure whereby, initially, thin Au/Ag substrates. The NP structures were or Au/Ni films were deposited on quartz prepared from films of thicknesses up to substrates by classical pulsed laser 60 nm produced by discharge sputtering or deposition using targets consisting of two pulsed laser deposition, and annealed by sections of different metals. Thin films nanosecond laser pulses at 266 or 308 nm with different concentrations of the two 36 Laboratory Micro- and Nano-photonics

metals were obtained by changing the area enhancement was confirmed of the different sections on the target. The experimentally. as prepared films were then annealed by nanosecond laser pulses delivered by a 1.5. Tuning the optical properties of gold Nd:YAG laser system operating at nanostructures fabricated on flexible λ = 355 nm. It was found that the laser substrates annealing may lead to nanostructuring of the deposited films, since under certain A laser-based technique was used to conditions decomposition into monolayers form metal nanostructures on flexible of nanoparticles with a narrow size substrates. The analysis of the structures distribution was achieved. The EDX obtained was focused on their optical analyses performed indicated that the properties. The nanostructures were fabricated particles consist of a bimetallic fabricated by laser processing of thin gold system of the basic metals used. The films, the latter being deposited on PMMA transmission spectra of the structures and PVC substrates by a classical PLD prepared showed evidence of plasmon technology. The films produced were then excitation. The bimetal nanostructures annealed by nanosecond pulses delivered were covered by Rhodamine 6G and then by a Nd:YAG laser system operating at tested as active substrates for surface λ = 355 nm. Under specific conditions, the enhanced Raman spectroscopy (SERS). laser treatment led to the formation of a discrete nanostructure on the substrate 1.4. Near-field intensity enhancement and surface. The optical properties of samples localization in noble metal nanoparticle fabricated at different conditions were ensembles examined by optical spectroscopy. Bending the substrates at different angles Based on a numerical simulation using was used to modify the characteristics of the finite-difference time-domain the structure produced, which led to a approach, a theoretical analysis was change in its optical properties. The performed of the electromagnetic field plasmon band of the nanostructures properties in the vicinity of noble metal obtained could thus be efficiently tuned nanostructures. The systems under within a range of about 30 nm. The consideration were two- and three- experiments on using these structures in dimensional arrays of gold or silver SERS showed that they could be an nanoparticles. The near-field intensity efficient alternative of those already distribution and its enhancement were developed. calculated for structures with different characteristics – particle size, inter-particle 2. Production of noble-metal distance, and under different conditions nanoparticles by laser ablation in a related to the incident irradiation – liquid environment polarization and geometry of excitation. The goal of the analysis was to define Au nanoparticles (NPs) were created these structures’ optimal parameters from by nanosecond pulsed laser ablation of a the viewpoint of application in SERS. It solid target in double distilled water. The was shown that manipulating the geometry influence was examined of the laser of excitation of the nanoparticle system wavelength on the size, shape and optical could be used as a key parameter in properties of the resulting NPs. Three improving the efficiency of the SERS in different wavelengths: the fundamental classical configuration. The predicted (λ = 1064 nm), and the second effect of the nanoparticle system’s (λSHG = 532 nm) and third (λTHG = 355 nm) properties on the Raman signal harmonics of a Nd:YAG laser at the same 37 Annual Report IE 2013

fluence were utilized to produce various band was achieved, which is evidence of a colloids. The ablation at the wavelength of narrowing of the particles size distribution 532 nm was investigated in more detail to and of their shape approaching the ideal reveal the influence of self-absorption by spherical one. the already created NPs on their characteristics. The colloid produced was 3. Plasmonic properties of Ag/ZnO irradiated by λirrad = 532 nm (laser energy nanocomposites prepared by laser 40 mJ) for different times up to 25 min methods after the end of ablation. The initial structure of the welded NPs forming wires Composite nanostructures of double was thus modified. Transmission electron layer, multilayer and mosaic Ag/ZnO microscopy and optical transmission containing nano-sized particles were measurements were used to evaluate the produced via laser deposition and shape and the size distribution of the NPs. annealing procedures. The samples were Contamination-free Ag nanoparticles examined, with the results indicating that were produced by nanosecond pulsed laser laser annealing of Ag or Ag/ZnO layers is ablation (10 Hz repetition rate, 15 ns pulse a method suitable for tuning the optical duration) of a solid target immersed in properties of the nanostructures. Their double distilled water. The size morphology and optical absorption were distribution observed of the NPs was wider investigated as a function of the annealing compared to those obtained by other regime and preparation procedure, leading preparation methods, mainly chemical. To to different silver arrangement. All modify it and make the NPs more suitable annealed samples showed surface plasmon for biological application, the suspensions resonance (SPR) properties. A red-shifted fabricated were irradiated by an unfocused SPR was observed as the number of layers laser beam The experiments on Ag NPs was increased, which we associated with a preparation were performed by using a reduction in the interparticle distance. A Nd:YAG laser system operating at the broadening was observed of the mosaic fundamental wavelength (λ = 1064 nm). It samples’ absorption minimum towards the lies outside the NPs surface plasmon longer wavelengths as the thickness was absorption band and was so chosen to reduced, due to the broader NPs size avoid self-absorption during ablation by distribution. A ZnO excitonic feature seen the already formed NPs. Excluding a in the spectra of double layer and possible absorption in the preparation multilayer structures disappeared in the stage provides a better control over the spectrum of a mosaic sample. X-ray irradiation influence. The subsequent photoelectron spectroscopy was used to irradiation of the suspension obtained was study the composition and chemical state performed by the third (λTHG = 355 nm) of the double layer Ag/ZnO harmonic of the laser system, which was nanocomposites on the surface and in chosen due to its being positioned within depth. The binding energy of Ag3d5/2 was the corresponding surface plasmon shifted to the lower values compared to the absorption band. The changes in the NPs pure metallic Ag due to the interaction characteristics caused by various laser between Ag and ZnO. fluences and durations of irradiation were traced by transmission electron 4. Application of Au nanoparticles in microscopy (TEM) and confirmed by biophotonics optical transmission measurements in the near UV and the visible ranges. A Plasmonic near-field tweezers in water considerable narrowing (by about a factor with gold nanosphere pairs on various of three) of the surface plasmon extinction substrates were proposed and studied. An 38 Laboratory Micro- and Nano-photonics

enhanced near field localized in the The data demonstrated for the first time nanometric gap space pumped by 800 nm the transplacental size-dependent femtosecond laser pulses was formed with clastogenic and epigenetic effects of the intention of trapping and killing AuNPs in mouse fetus, thus highlighting viruses. The maximal optical trapping new aspects concerning the putative force obtained exceeded 20 pN at an genotoxicity of AuNPs during a vulnerable incident optical peak intensity of period of life. 2 1 mW/µm . We also proposed a new system consisting of a gold nanosphere PUBLICATIONS and a gold nanoridge. In this system, the enhanced near field stems mainly from the 1. Grochowska K, Śliwiński G, image charge effect, exhibiting optical Iwulska A, Sawczak M, Nedyalkov trapping in an asymmetric space. The N, Atanasov P, Obara G and Obara calculated trapping force was equivalent to M 2013 Engineering Au that of the system of gold nanosphere nanoparticle arrays on SiO2 glass by pairs. The trapped viruses may easily be pulsed UV laser irradiation made inactive using an unfocused 800-nm Plasmonics 8/1 105-13 fs laser pulses. 2. Nikov Ru, Nedyalkov N, Atanasov The wide application of P A, Terakawa M, Shimizu H and nanotechnology in medicine, biology, and Obara M 2013 Tuning the optical pharmacology is leading to a dramatic properties of gold nanostructures increase of the risk of direct contact of fabricated on flexible substrates nanoproducts, including gold nanoparticles Appl. Surf. Sci. 264 779-82 (AuNPs), with the human organism. Our 3. Koleva M E, Dikovska A Og, study aimed at evaluating in vivo the Nedyalkov N N, Atanasov P A and genotoxicity of AuNPs with average size Atanasova G B 2013 Ag/ZnO of 40 nm and 100 nm. A single nanocomposites prepared by laser intraperitoneal treatment of adult male and methods Proc SPIE 8770 female Swiss mice (strain H) with AuNPs, 4. Nikov R G, Nikolov A S, Nedyalkov at a dose of 3.3 mg/kg body weight, had no N N, Atanasov P A, Alexandrov M effect on the frequency of micronucleated T and Karashanova D B 2013 polychromatic erythrocytes (MN PCEs) in Processing condition influence on bone marrow. Conversely, the the characteristics of gold transplacental treatment with AuNP-100 nanoparticles produced by pulsed nm, but not with AuNP-40 nm, applied laser ablation in liquids Appl. Surf. intraperitoneally at a dose of 3.3 mg/kg to Sci. 274 105-9 pregnant mice on days 10, 12, 14, and 17 5. Dikovska A Og, Avdeev G V, of gestation, resulted in a significant Nedyalkov N N, Koleva M E and increase in the frequency of MN PCEs in Atanasov P A 2013 Preparation of both liver and peripheral blood of mouse metal nanorods substrates for SERS fetuses. In addition, the same treatment application Proc. SPIE 8770 877006 with AuNP-100 nm, but not with AuNP-40 6. Nedyalkov N N, Nikov Ru G and nm, produced significant changes in the Atanasov P A 2013 Near field microRNA expression. In particular, out of intensity enhancement and 1281 mouse microRNAs analyzed, 28 localization in noble metal were dys-regulated more than two-fold and nanoparticle ensembles Proc SPIE to a statistically significant extent in fetus 8770 877005 lung, and 5 were up-regulated in fetal 7. Nikov Ru G, Nedyalkov N N, liver. Let-7a and miR-183 were Atanasov P A, Grochowska K, significantly up-regulated in both organs. Iwulska A and Sliwinski G 2013 39 Annual Report IE 2013

Laser nanostructuring of Au/Ag and University, Yokohama, Japan. Au/Ni films for application in SERS 2. Nanoparticles generation by Proc SPIE 8770 87700D ultrashort laser ablation of metals 8. Nikolov A S, Nikov R G, Dimitrov I and dielectrics: theory and G, Nedyalkov N N, Atanasov P A, experiments, Coherentia-INFM, Alexandrov M T and Karashanova D Istituto Nazionale per la Fisica della B 2013 Modification of the silver Materia; 80126 Napoli, Italy. nanoparticles size-distribution by 3. Fabrication and characterization of means of laser light irradiation of noble metal nanoparticles arrays, their water suspensions Appl. Surf. Institute of Fluid-Flow Machines, Sci. 280 55-9 Polish Academy of Sciences, 9. Hirano K, Shimizu H, Enami T, Gdansk, Poland. Terakawa M, Obara M, Nedyalkov 4. Pulsed laser deposition of thin oxide N N and Atanasov P A 2013 films, Institute of Lasers, Plasma and Plasmonic nanometric optical Radiation Physics, Romanian tweezers in an asymmetric space of Academy of Sciences, Bucharest, gold nanostructured substrates J. Romania. Nanotechnol. in Diagnosis and 5. Laser nanostructuring of different Treatment 1 2-10 materials and nanoparticles 10. Balansky R, Longobardi M, generation, Institute of Physics, Ganchev G, Iltcheva M, Nedyalkov Czech Academy of Sciences, Prague, N, Atanasov P, Toshkova R, De Czech Republic. Flora S and Izzotti A 2013 Transplacental clastogenic and GUESTS epigenetic effects of gold nanoparticles in mice Mutation 1. Ph.D. student, Debora Palioti, Research – Molecular Mechanisms Coherentia-INFM, Istituto Nazionale of Mutagenesis 751-752 42-8 per la Fisica della Materia; Napoli, Italy, Complesso Univeristario di ONGOING RESEARCH PROJECTS Monte S. Angelo, 3 days, 31.07.2013 and 15 days, 12.10.2013. Financed by the Bulgarian Academy of 2. Dr. Stefano Letieri, Coherentia- Sciences INFM, Istituto Nazionale per la Fisica della Materia; Napoli, Italy, Pulsed laser deposition and processing of Complesso Univeristario di Monte S. thin films. Angelo, 3 days, 31.07.2013.

Financed by the National Science Fund 3. Prof. Ion Mihailescu, National Institute for Lasers, Plasma and Nanostructures and nanostructured Radiation Physics, Magurele, surfaces of noble metals and their Romania, 5 days, 28.08.2013 and 5 application in high resolution analyses of days, 11.12.2013. materials, CNRS, LP3, Marseilles, France, 4. Dr. Felix-Nicolae Sima, National “Rila” Program (2013-2014). Institute for Lasers, Plasma and Radiation Physics, Magurele, COLLABORATIONS Romania, 5 days, 28.08.2013 and 5 days, 11.12.2013. 1. Femtosecond laser processing and 5. Prof. Dr. Carmen-Georgeta Ristoscu, novel methods for nanofabrication, National Institute for Lasers, Plasma Department of Electronics and and Radiation Physics, Magurele, Electrical Engineering, Keio Romania, 5 days, 28.08.2013 and 5 40 Laboratory Micro- and Nano-photonics

days, 11.12.2013. Coherentia-INFM, Istituto Nazionale per la Fisica della Materia; Napoli, LABORATORY VISITS Italy, 9 days, 04.11.2013. M. E. Koleva: P. A. Atanasov: National Institute for Materials Coherentia-INFM, Istituto Nazionale Science, Tsukuba, Japan, The per la Fisica della Materia; Napoli, Matsumae International Foundation Italy, 9 days, 04.11.2013. PostDoc Grant, 3 months, N. N. Nedyalkov: 01.04.2013.

41 Annual Report IE 2013

LABORATORY

BIOPHOTONICS

HEAD: Prof. Latchezar Avramov, Dr.Sc.

TOTAL STAFF: 11 RESEARCH SCIENTISTS: 7

Assoc. Prof. E. G. Borisova, Ph.D.; Assoc. Prof. D. G. Slavov, Ph.D.; A. T. Daskalova- Shivarova, Ph.D.; I. A. Bliznakova, Ph.D.; I. I. Balchev, Ph.D.; A. I. Gisbrecht; D. S. Petkov; Al. Zh. Zhelyazkova; L. P. Angelova; Ts. I. Genova.

RESEARCH ACTIVITIES tissue destruction and often has recurrence after initial treatment. SCC, the second 1. Biophotonics: Laser medical and most widely spread cutaneous tumor biomedical research and development of (about 20% of all new cases), is associated new diagnostic and therapeutic methods with a risk of metastasis and is usually and equipment more aggressive and difficult to treat in comparison with BCC lesions. 1.1. Endogenous fluorescence skin cancer The excellent diagnostic possibilities diagnostics for clinical applications offered by the autofluorescence spectroscopy technique notwithstanding, Fluorescence spectroscopy is a very no reliable and universal systems for sensitive tool, whose applicability to skin fluorescence detection of skin cancer have cancer detection depends on many factors appeared on the medical market. related to the lesions investigated – their The problems related to the pigmentation, morphology, localization development of such diagnostic and stage of growth. When one discusses autofluorescence systems for skin cancer skin cancer optical diagnostics and detection have to do with the great variety treatment, the most common of benign and malignant forms of skin differentiation is based on the tumor type pathologies. For example, basal cell and pigmentation. Malignant melanoma carcinoma (BCC) lesions have more than (MM) lesions are the most severe kind of 15 sub-types, squamous cell carcinoma skin tumors; they originate from (SCC) lesions have about 10 different sub- melanocytes, with the highest risk for the types, and all of them have a variety of patients being due to the fast growth and benign and dysplastic forms, which differ risk of metastases. Non-melanoma skin by morphology, appearance and metabolic cancer (NMSC) lesions are a type of statement, including by their conditions that are a promising goal for the autofluorescence properties, on the fluorescence spectroscopy for diagnosis of different stages of the lesion growth. skin tumors. This common term (NMSC) The fact that one could use LIAFS not usually covers basal cell carcinoma (BCC) only for initial diagnosis, but for and squamous cell carcinoma (SCC) evaluation of the lesion stage is very tumors. These lesions are among the most promising for the clinical needs. However, common cancers within the white-skinned one will need to compare the specific population. BCC covers about 70% of all situation and the specific spectrum new skin cancer cases; it could detected with a great variety of other occasionally grow aggressively causing possibilities, such as lesion kind, stage of 42 Laboratory Biophotonics

growth, and even patient skin general attached to the spectrometric system conditions, such as influence of medicines, (HORIBA JobinYvon Ltd., France). age, cutaneous phototype, etc., because all Excitation-emission matrices (EEM) were of them influence the spectra detected in detected for the need of endogenous vivo. fluorophores origins evaluation. Excitation During the last several years, our group was applied in the region from 260 nm to directed a part of its efforts to the 500 nm, with a 5 nm step between the development of a spectral database excitation bands (2 nm); the emission was containing the fluorescence properties of detected in the 260 nm − 800 nm region. In different benign and malignant skin the EEMs obtained, we could address all pathologies for the needs of skin cancer major skin fluorophores existing in the non-invasive diagnostics. samples based on their coordinates Two types of skin samples were used in (excitation max; emission max). In such a our investigations – for in vitro and in vivo way we could obtain a complete picture of measurements. The measurements of both the autofluorescence properties of the kinds of samples were approved by the normal skin and tumors investigated and ethical committees of the two hospitals could use the spectral matrices obtained where the studies were carried out – The for the subsequent comparison and National Oncological Medical Center, diagnostics based on the autofluorescence Sofia and Queen Giovanna-ISUL signals from the different cutaneous University Hospital. pathologies. This spectrofluorimeter configuration 1.1.1. In vitro fluorescence studies. could be used for in vivo measurements as Surgically removed malignant cutaneous well, but, due to the long period of pathologies were investigated up to two measurements (more than 15 minutes per hours after their excision. These samples EEM), this option is not appropriate for contained a part of the tumor lesion and a clinical applications. part of the non-tumorous skin − the safety area, which is kept around tumor lesions 1.1.2. In vivo fluorescence studies. The in during surgical excision. The normal vivo investigations were a part of a clinical tissue, the border area and the tumor were trial for introducing a spectral diagnostic investigated by steady-state fluorescence system for skin cancer detection in the spectroscopy. All samples were initially daily practice of The National Oncological clinically diagnosed and proved by Medical Center, Sofia and the histological examination. A part of the dermatological department of Queen excised sample was used for post- Giovanna-ISUL University Hospital. operation histological re-examination and Autofluorescence spectroscopy was confirmation of the initial diagnosis and applied to several different classes of surgical removal control. All our results benign, dysplastic and malignant were compared with the histological data cutaneous lesions. Initially, they were obtained. Freshly excised tumors were classified visually and dermatoscopically placed immediately after removal from the using ABCD scoring criteria. The second body in a liquid, keeping the vital step was detection of the lesion’s and the properties of the samples up to 24 hours surrounding normal skin’s auto- and transported from the hospital to the fluorescence spectra using excitation at laboratory in a standard Dewar vessel. 365 nm, 385 nm, and 405 nm by means of The autofluorescence measurements in narrow-band light-emitting diodes a steady-state mode were carried out using (Roithner LaserTechnik GmbH, Austria). a FluoroLog3 spectrofluorimetric system The histological examination was used with a F3000 fiber-optic stand and a probe as a “gold standard”. Spectra and 43 Annual Report IE 2013

dermatoscopic evaluations have so far The spectra forming the EEM data and been obtained from more than 500 the identification of the endogenous patients. fluorophores observed using excitation The autofluorescence signals were wavelengths from 270 nm to 500 nm are recorded and stored using a USB4000 presented in figure 1. The evaluation of fiber-optic microspectrometer with their origin was based on our previous standard SpectraSuite software (Ocean experimental experience working with Optics Inc., Dunedin, FL, USA). The pure samples of a given compound – normal tissue spectra detected were used collagen type I, porphyrins, keratin. as a basis for comparison with the The origin of all other compounds’ spectra pathologies observed. Several spectra were taken from each suspicious area and averaged to reduce the influence of the lesions’ inhomogeneity. Up to seven spectra were recorded and averaged from every lesion depending on its size and peculiarities, together with up to five spectra from a given normal skin area. The averaged spectra from the healthy skin area were used as indicators of the spectral changes in the pathological areas and for assessing the effects related Figure 1. Normal skin autofluorescence to inter- and intra-patients spectral spectra at different excitation wavelengths variations. (from 270 nm to 500 nm) and identification of Ordinarily, the whole procedure of the major endogenous fluorescence measuring the fluorescence spectra of compounds observed, using knowledge about tumorous and normal skin at all excitation their excitation and emission maxima. wavelengths used took about three minutes including measuring and storing the were evaluated using the knowledge about spectra in ASCII format on the computer the excitation and emission maxima cited that controlled the microspectrometer and in the literature for the different native visualized the spectra detected. endogenous fluorescent compounds Three major areas could be existing in the human cutaneous tissues. distinguished – one related to the We were thus able to reveal the autofluorescence signal of amino-acids in presence of amino-acids – tyrosine and the UV spectral region, another resulting tryptophan in the UV spectral region, from the inner fluorescence of co-enzimes, collagen – at the 400-nm emission proteins and their cross-links, which has an maximum, co-enzimes FAD and NADH at important role in the formation of the 400 nm and 450 nm, respectively, normal skin autofluorescence signal in the structural proteins – elastin and elastin and visible spectral range, and a very weak collagen cross-links at 500 nm, green third area of endogenous porphyrins fluorescence of keratin and flavins (530 detected in the red spectral region (>600 nm − 570 nm), and a low-intensity signal nm). The latter signal is very weak in the in the red spectral region (>600 nm), normal tissues and could only be detected which can be associated with endogenous if a very sensitive spectrofluorimeter was porphyrins. used; this is why it was not seen in the Significant differences between normal spectra detected in ordinary in vivo clinical and abnormal skin fluorescence spectra investigations. were observed when basal cell carcinoma 44 Laboratory Biophotonics

lesions excitation-emission matrices were range. Firstly, this is the destruction of the obtained (figure 2). extra-cellular matrix in the tumor tissue, which leads to an overall decrease of the structural protein fluorescence due to the lower proteins concentration in the volume from where the fluorescence signal was obtained. This is related to the signal obtained from collagen, elastin and their protein cross-links. The second very important reason is related to the decrease of NADH in excised tissue samples, which also leads to a decrease in the signal

intensity. However, a similar signal Figure 2. BCC tumor autofluorescence spectra decrease related to NADH auto- at different excitation wavelengths (from fluorescence is observed in tumors in vivo 270 nm to 500 nm). as well; several authors have discussed this phenomenon in the framework of NADH The most pronounced change seen was /NAD+ and flavins redox ratio, as the in the fluorescence intensity – it was much reduced form NAD+ does not fluoresce. lower than that of normal skin, which is in In table 1 we present the summarized a very good agreement with the reports of fluorescence diagnostics statistical values other investigators working in this area of for a group of 536 patients. The most research. However, more interesting and, interesting results, in our opinion, are the hopefully, more diagnostically important, malignant BCC (137 lesions), SCC (29 are the changes in the spectral shape of the lesions), and MM (41 lesions), as well as autofluorescence signals detected. The their benign duplicates (194 lesions) and amino-acids fluorescence intensity is much dysplastic precancerous conditions, such higher with respect to the maxima related as keratoacanthoma (KA) (11 lesions), to structural proteins and co-enzymes. which is a pre-cursor of SCC, or dysplastic There exist two major reasons for such a nevus (124 lesions) – pre-cursor of MM. decrease of the signal in the blue spectral

Table 1. Statistical data for diagnostics of benign, dysplastic, non-melanoma and malignant melanoma cutaneous tumors.

Experimental SE (%) SP (%) PPV (%) NPV (%) DA (%) methods used NMSC vs. benign 100 99,2 99,4 100 99,3 MM vs. benign 100 99,2 97,6 100 98,4 MM vs. NMSC 99,1 90,5 98,4 95,1 93,6

For melanin-pigmented benign and correspond to the lesion type and stage of dysplastic nevi and malignant melanoma development. For this kind of pathologies, lesions, the autofluorescence is suboptimal the diffuse reflectance spectroscopy is a from the point of view of specificity. very useful and highly sensitive technique There is a strong decrease of the with good diagnostic accuracy. Therefore, fluorescence intensity in the whole spectral in our clinical investigations we have region, related to the lesions’ usually combined autofluorescence and pigmentation, but no spectral shape diffuse reflectance measurements of skin changes are observed that could lesions in order to increase the diagnostic 45 Annual Report IE 2013

value of the optical biopsy approach and to optimistic. Despite the many technological develop a sensitive tool for identifying a advances that have taken place, based on variety of low and highly pigmented skin improved detectors and image analysis of neoplasia. better discrimination, the conventional The results obtained for skin tumor white-light GIT endoscopy is still fluorescence detection were used in the suboptimal and usually detects lesions preparation of two doctoral theses, which already show symptoms related to defended in 2013. The first one, for PhD in tumor growth, such as obstruction, Physics and defended in March 2013 (Irina bleeding and pain. Misdiagnoses, having Bliznakova), presented different to do with difficulties in differentiating the approaches and discrimination inflammatory from the initial stage of spectroscopic algorithms for the needs of adenocarcinoma, have also negative effect skin fluorescence diagnosis. The second on the diagnostic accuracy. Experienced one, for the PhD degree, was defended in gastroenterologists only, with a long October 2013 by Elmira Pavlova, a MD in practice in endoscopy observations, could Queen Giovanna-ISUL University be expected to discern the slight initial Hospital and a participant in the team changes to dysplastic and neoplastic stages conducting the research discussed under of esophageal, stomach or colon mucosa. project #DMU-03-46/2011 financed by the The visualization and discrimination Bulgarian National Science Fund with problems arising in open surgical coordinator Assoc. Prof. Dr. Ekaterina interventions during tumor excision Borisova, who was also Dr. Pavlova’s procedures for lower GIT are also a scientific supervisor during her doctoral significant clinical problem and require the research. development of new tools and diagnostic and monitoring modalities to improve the 1.2. Excitation-emission matrices for selectivity and accuracy of these investigation of internal fluorescent procedures. The existing clinical properties of gastrointestinal tumors limitations are one significant technical challenge which initiated the development Optical spectroscopic techniques have of new diagnostic modalities based on lately been widely applied as supporting different spectral and optical techniques. tools in the clinical practice of diagnosing One of the most sensitive optical different types of tumors in various detection approaches is the light-induced anatomic areas or tissues of interest, such fluorescence spectroscopy (LIFS) of as skin, prostate bland, breast, bladder, gastrointestinal mucosa for neoplasia brain, etc. The improved detection of detection. This technique is the most gastrointestinal tract (GIT) tumors is an widely studied one among the important goal in the development of spectroscopic techniques in general, modern, easy-to-use, objective and because of its rapid and highly sensitive clinically applicable spectroscopic response to early biochemical and equipment. morphological changes in the tissues. This According to the annual statistics of advanced method goes beyond the cancer incidence, GIT tumors have a major standard endoscopic techniques and is place among the newly developed cancers: characterized by better image resolution colon cancer is on third place, stomach and contrast, higher sensitivity, and tissue cancer is on fifth place, and esophageal penetration; it could even provide cancer is in the “top ten” as well. Usually, biochemical, structural and molecular the GIT tumors are detected in the information about the mucosal lesions advanced III and IV stage, when the investigated. prospects for the patients are not very 46 Laboratory Biophotonics

In 2013, we focused our efforts on is a part of the safety area excised during experiments on autofluorescence colon the tumor removal. Autofluorescence cancer diagnosis and differentiation vs. signals were detected using different corresponding normal mucosa. A excitation wavelengths in the feasibility analysis was carried out of the 280 − 440 nm region in view of fluorescence spectroscopy technique distinguishing between tumorous and chosen as a diagnostic tool for GIT tumors healthy tissues, thus forming an detection in endoscopic and open-surgery excitation–emission matrix of data. The observations. All samples investigated in fluorescence signal was measured between vitro were obtained by a surgical excision 300 nm and 800 nm. The last step of the of malignant tumors of esophagus, study was a comparison between the stomach and colon in such a way as to histological analysis and the spectral data contain tumor mass and surrounding obtained to verify the spectral results and normal mucosa areas. Endoscopic clinical identify the most significant diagnostic observation and biopsy sampling for the features observed. subsequent histological analysis were Figure 3a and 3b present the excitation- carried out in the clinic. The “gold emission matrix (EEM) and steady-state standard” histological analysis proved the autofluorescence data from normal colon tumor type and stage of growth in all cases mucosa, while fig 4a and 4b show the investigated by the spectroscopic EEM and autofluorescence spectra of techniques considered. After the excision, colon carcinoma for different excitation the samples were placed in a preserving solution and transported to the laboratory for fluorescent measurements, which were carried out up to two hours after tissue removal. Samples without an exogenous fluorophore applied for the needs of autofluorescence measurements were obtained after removal of proved tumors during excision of the lesions in open surgical procedures. The samples prepared for the fluorescence analysis were of

1,5 cm diameter and were obtained from a) the border areas of the tumors excised. Thus, the samples contained a tumor area, a normal mucosa area and a transient border zone, which allowed us to obtain a more precise and complete picture of possible spectroscopic differences in these three areas of diagnostic interest. Using a FluoroLog 3 spectrofluorimeter (HORIBA Jobin Yvon, France), we measured excitation-emission matrices of normal and tumorous GIT mucosa samples obtained by excision during procedures for b) removal of GIT neoplasia lesions. Point- by-point measurements were taken from Figure 3. (a) EEM and (b) steady-state the excised tumor lesions and outwards autofluorescence spectra of excised normal from the normal surrounding mucosa that colon mucosa. 47 Annual Report IE 2013

wavelengths. Several major auto- mucosal layer, which screens and reduces fluorescence sources in the tissues the 480 − 500 nm fluorescence from the investigated could be identified. The submucosa layer. amino acids tryptophan and tyrosine were Another change observed was the excited by deep UV – 280 – 300 nm and general decrease of the NADH emitted in the 320 − 360 nm range. The fluorescence vs. that of normal mucosa, structural proteins − elastin and collagen, because in tumor cells NADH undergoes a were excited within 320 – 360 nm and transition to its oxidized non-fluorescent emitted at 400 nm and in the 460 − 500 nm form NAD+ (see figure 4a and 4b). range, while their protein cross-links were excited within 360 − 400 nm and emitted with a maximum intensity within 480 − 500 nm. A fast decrease of the NADH and flavins was observed in the excised tissue samples, due to the co- enzymes’ degradation in such tissues. Autofluorescence could only be detected in the freshly excised samples by means of excitation in 340 − 380 nm, emission being detected in 440 − 480 nm for NADH; flavins were excited within 340 − 400 nm and emitted in 500 − 530 nm. a) The autofluorescence intensity from the tumorous area was much lower than that from the normal mucosa. About twice as low signal was typically observed from the carcinoma area as compared to the signal from the normal mucosa when studying the same tissue sample containing normal mucosa and a carcinoma area surgically removed from a given patient. Similar results of a fluorescence signal reduction have been reported by other research groups and have been proposed to be used b) as an indication of a tumor lesion presence Figure 4. (a) EEM and (b) steady-state when autofluorescent topography of GIT is autofluorescence spectra of excised colon carried out. However, changes were carcinoma. observed not only in the intensity, but in the spectral shapes as well, which can be The National Center of Biomedical assigned to a decrease of the signal Photonics of the Institute of Electronics- detected from a unit volume of the tissue BAS collaborates with Queen Giovanna- from collagen fibers and collagen cross- ISUL University Hospital with the aim of links. In the case of a tumor lesion, the developing clinically applicable intercellular matrix is relatively loosened fluorescence diagnostics techniques. The due to the increased tumor cells size and studies presented above were a part of a the general reduction of the collagen and clinical trial intended for introducing a elastin concentration in a unit volume. The spectroscopic diagnostic endoscopy fluorescence signal of structural proteins is system and a system for fluorescence also reduced due to the thickening of the image-guided surgery of GIT pathologies 48 Laboratory Biophotonics

in the clinical practice of the University for cells to differentiate. Hospital. Fluorescence spectroscopy and We conducted structural studies of self- topography could open new dimensions in organized micropores generated in thin the diagnostics and real-time resection, not gelatin, collagen, and collagen-elastin only of the entire tumor mass, but also of films after single and multishot irradiation the lymph nodes where micro-metastases by laser pulses with duration ranging from are detected in the case of open-surgery 30 fs to 100 fs at 800 nm. We studied fluorescent monitoring. This will make the systematically the effect of the laser entire procedure more individualized, parameters, namely, laser energy, number patient-friendly and efficient. The main of pulses and pulse duration on the objective of our efforts was to acquire a development of the micropores. We are significant database of the main spectral currently exploring further the possibility characteristics of GIT neoplasia and to of structuring these biomaterials by develop algorithms for benign/malignant varying the time delay between the tissue differentiation using auto- separate pulses. fluorescence and exogenous fluorescence Such studies allow one to derive tumor detection. Our role was to find the detailed information on the mechanisms of optimal modes and regimes of work using energy coupling into the material and the fluorescent spectroscopy techniques for redistribution channels. GIT tumors detection that will be useful Initial investigations were performed on and helpful to the clinicians. The results the surface modification of thin gelatin obtained can be considered when decision films for different time delays (0, 200 fs, are made on the fluorescence mode of 1 ps) and a fixed number of pulses diagnosis. The studies presented were (N = 10). Using a femtosecond time- performed in the framework of project resolved technique, we measured the DMU-03-46/2011 supported by the evolution of the laser-induced surface Bulgarian National Science Foundation. modification. This technique allows one to make use of the dynamic processes to 1.3. Ultra-short time-resolved micro- control the thermal effects in order to structuring of biomaterials and achieve the desired modification of the applications to the regenerative medicine material. The results point to the possibility of tailoring the surface The ability to produce idealized cellular modification by adaptive temporal constructs is essential for understanding separation. The outcome is of great interest and controlling intercellular processes and, in view of the controlled preparation of ultimately, for producing engineered tissue thin films with surface structures of replacements. The fabrication of artificial enhanced porosity. The results revealed scaffolds with the appropriate biological, the evolution of the formation of surface mechanical and structural properties is an foam for N = 10 and for time delays from important consideration in designing 0 to 1 ps. We assume that, in the tissues for biomedical applications. interaction region, the host material Natural and synthetic polymers are used properties are in a transient state, i.e., a as substitute biomaterials in tissue liquid or vapor phase at high temperatures. engineering applications. It is known that, The last step in this process is returning in comparison with the unmodified from this phase of the material to a solid scaffold, the laser-modified surface of a phase. We observed the porous matrix thin film of biomaterial is characterized by formation evolution in dependence of a higher degree of biocompatibility and different time delays between consecutive roughness, enhanced cell attachment and pulses. For N = 10, it was clearly seen that proliferation and provides an adequate host increasing the delay from 0 – 1 ps led to 49 Annual Report IE 2013

different way of nanopores modification, Carbon monoxide (CO) is a highly namely, creation of pores with a larger size toxic odorless and colorless gas. Treating was observed for a 1-ps time delay; CO poisoning is based on restoring the gas moreover, for the same delay, wave-like balance in the blood by providing a highly- structures were formed on the material’s oxygen-enriched environment. The CO surface. As a further proof-of-principle for molecules dissociated from Hb can bind to the possibility of temporal optimization, the same or different Hb, but this process we investigated the material’s behavior at is slow because the rate of binding is longer time delays, τ = 10 ps as of the exponential and depends on the CO pumping pulse energy deposition, when concentration. Hyperbaric oxygen therapy the microscopically visible material is considered as one of the relatively modifications have reached their final efficient methods of treatment. However, state. Most materials have an electron- the problems associated with CO phonon coupling time of between a poisoning are yet to be solved; therefore, picosecond and a nanosecond, with the novel, more efficient methods of treatment typical heat diffusion times falling need to be developed. between a nanosecond and a microsecond. Our research in 2013 included The thermal relaxation time for biological experimental in vivo measurements of the materials is in the order of 1 µs. Since rates of concentration of HbCO in blood. thermal diffusion is too slow to dissipate The results showed a decrease in the HbO2 the laser energy during this plasma concentration in blood under the influence lifetime, the thermal energy is confined to of transcutaneous laser light irradiation, as the zone of the plasma and no thermal the laser-induced photo-dissociation of interaction occurs in the adjacent regions. HbO2 led to the photolysis of nearly 50 % The successive pulse energy deposition of HbCO molecules. and heat conversion should take place The spectral effectiveness of the within a short time interval with respect to photodissociation approximately correlates the heat diffusion times. We postulated with the absorption spectrum of HbCO and that the presence of enhanced absorption the transmission spectrum of skin tissue. by the host material following a laser- It was shown that the significant induced breakdown will lead to a higher difference in the quantum yields of energy deposition by the probe pulse. The photodissociation of hemoglobin time-resolved results allowed us to complexes makes possible a selective conclude that by optimizing the temporal decomposition of HbCO in the blood interval between the consecutive pulses, as stream with a minimum effect on the HbO well as the other laser parameters (energy, 2 component. It should be noted that the beam homogeneity, fluence), one could small reduction in the concentration of choose an appropriate regime to eliminate HbO due to laser-induced excessive photo-thermal and photo- 2 mechanical effects and obtain in a photodissociation could be easily controlled way the desired surface compensated using a hyperventilation by structure and porosity. pure oxygen during laser irradiation. This work was performed as a part of The results obtained allowed us to the implementation of project DMU propose a new method for increasing the 03/15/2011. effectiveness of carbon monoxide treatment. Phototherapy of CO poisoning 1.4. Development of a novel method of in combination with oxygen laser-induced carboxyhemoglobin photo- hyperventilation and hyperbaric oxygen dissociation for reduction of carbon therapy may become a powerful method in monoxide intoxication the photomedicine. 50 Laboratory Biophotonics

PUBLICATIONS Lueftenegger S and Husinsky W 2013 Controlling the porosity of 1. Mantareva V, Angelov I, Wöhrle D, collagen, gelatin and elastin Borisova E and Kussovski V 2013 biomaterials by ultrashort laser Metallophthalocyanines for anti- pulses Appl. Surf. Sci. 292 367-77 microbial photodynamic therapy: an DOI:10.1016/j.apsusc.2013.11.145 overview of our experience J. 8. Chandra N, Daskalova A, Bliznakova I, Porphyrins Phthalocyanines 17 399– Lueftenegger S, Zhelyazkova A, 416 Enikoe S Ganz T and Husinsky W 2. Borisova E, Angelova L and Pavlova 2013 Defined nano-structuring with E 2013 Endogenous and exogenous ultrashort pulses in gelatine fluorescence skin cancer diagnostics biopolymer films for tissue- for clinical applications IEEE engineering MATEC Web of Conf. J. Select Topics Quant. Electr. 20/2 EDP Sci. 8 DOI: DOI: 10.1109/JSTQE.2013.2280503 10.1051/matecconf/20130803009 3. Borisova E, Plamenova L, 9. Daskalova A, Selimis A, Manousaki A, Keremedchiev M, Vladimirov B and Gray D, Ranella A and Fotakis C Avramov L 2013 Endogenous and 2013 Surface modification of exogenous fluorescence of collagen-based biomaterial induced gastrointestinal tumors: initial by pulse width variable femtosecond clinical observations Proc. SPIE. laser pulses Proc. SPIE 8770 DOI: 8770 87701C DOI: 10.1117/12.2012699 10.1117/12.2016389 10. Asimov M M Asimov R M and 4. Angelova L, Borisova E, Gisbrecht A I 2013 The impact of Zhelyazkova Al, Keremedchiev M, myoglobin on the efficiency of the Vladimirov B and Avramov L 2013 therapeutic effect of laser radiation Fluorescence spectroscopy of Acta Physica Polonica A 124/1 151- gastrointestinal tumors – in vitro 4 ISSN: 0587-4246 (printed) ISSN: studies and in vivo clinical 1898-794X (electronic) applications Proc. SPIE 9032 9032-5 11. Asimov M M Asimov R M and DOI: 10.1117/12.2044449 ISSN: Gisbrecht A I 2013 The impact of 1605-7422 myoglobin on the efficiency of the 5. Zhelyazkova A, Borisova E, therapeutic effect of laser radiation Angelova L, Pavlova E and Proc. SPIE 8770l 87701B Keremedchiev M 2013 Excitation- DOI: 10.1117/12.2014220 ISBN: emission matrices measurements of 9780819495686 human cutaneous lesions – tool for 12. Mamilov S A, Yesman S S Asimov fluorescence origins evaluation M M and Gisbrecht A I 2013 Venous Proc. SPIE 9032 90320A-1 saturation and blood flow behavior DOI: 10.1117/12.2044450 ISSN: during laser-induced photodisso- 1605-187422 ciation of oxyhemoglobin 6. Mamilov S A, Esman S S, Proc. SPIE 8770 877018 Veligodski D V, Asimov M M, DOI: 10.1117/12.2013418 ISBN: Borisova E G and Gisbrecht A I 9780819495686 2013 The impact of laser radiation on 13. Gurdev L, Dreischuh T, Vankov O, the photo-dissociation of Bliznakova I, Avramov L and carboxyhemoglobin in blood Proc. Stoyanov D 2013 Turbid media SPIE 9031 9031-30 optical properties derived from the 7. Daskalova A, Nathala C, Bliznakova I, characteristics of propagating laser Stoyanova E, Zhelyazkova A, Ganz T, radiation beams Appl. Phys. B 51 Annual Report IE 2013

DOI: 10.1007/s00340-013-5624-x Appl. Phys. (23-26 October 2013 14. Vandendriessche S, Brullot W, Kiev Ukraine) pp 91-2 Slavov D Valev V K and Verbiest T 21. Vanbel M, Vandendriessche S, van 2013 Magneto-optical harmonic der Veen M A, Slavov D, Heister P, susceptometry of superparamagnetic Paesen R, Valev V K, Ameloot M materials Appl. Phys. Lett. 102/16 and Verbiest T 2013 Second- 161903-5 DOI: 10.1063/1.4801837 harmonic generation from complex 15. Cartaleva S, Krasteva A, Moi L, chiral samples Proc. SPIE 8770 Sargsyan A, Sarkisyan D, Slavov D, 87701F DOI:10.1117/12.2014745 Todorov P and Vaseva K 2013 Laser http://dx.doi.org/10.1117/12.2014745 spectroscopy of sub-micrometre- and 22. Krasteva A, Slavov D, Todorov G micrometre-thick caesium-vapour and Cartaleva S 2013 Velocity layers Quantum Electr. 43 875 selective optical pumping resonance DOI:10.1070/QE2013v043n09 sign reversal Proc. SPIE 8770 ABEH014772 87700N DOI:10.1117/12.2013506 16. Slavov D, Sargsyan A, Sarkisyan D, http://dx.doi.org/10.1117/12.2013506 Mirzoyan R, Krasteva A, Wilson- 23. Slavov D, Krasteva A and Cartaleva S Gordon A D, Cartaleva S 2013 Sub- 2013 Bi-chromatic spectroscopy natural N-type resonance in cesium in micrometric optical cells atomic vapor: splitting in magnetic Proc. SPIE 8770 87700L fields arXiv:1309.4420 DOI:10.1117/12.2013432 17. Borisova E, Pavlova E, http://dx.doi.org/10.1117/12.2013432 Keremedchiev M, Angelova L, 24. Cartaleva S, Krasteva A, Sargsyan A, Zhelyazkova A and Genova Ts 2013 Sarkisyan D, Slavov D et al 2013 Optical biopsy of cutaneous tumors – Sub-natural width resonances in Cs from laboratory experiments to vapor confined in micrometric clinical applications Proc. 4th Int. thickness optical cells Symp. Topical Problems of Proc. SPIE 8770 87700M Biophotonics pp 291-3 ISBN: 978-5- DOI:10.1117/12.2013496 8048-0093-3 http://dx.doi.org/10.1117/12.2013496 18. Borisova E, Angelova L, Pavlova E, Keremedchiev M, Troyanova P and CONFERENCES Vladimirov B 2013 Fluorescence diagnosis of cutaneous and mucosal Prof. Dr.Sc. Latchezar Avramov tumors – clinical experience Proc. BioPIC (25-27 March 2013 Progress reports delivered to nd Ireland) pp 60-1 • 2 Korea-Bulgaria Science & 19. Mamilov S A, Asimov M M and Technology Forum “Science Bridge Gisbrecht A I 2013 Biomedical to Innovation”, Korea Institute of effect of laser-induced Science and Technology (KIST), photodissociation of oxymyoglobin June 2-4, 2013, Seoul, Korea, oral in the muscle tissue Proc. 12th Int. presentation; Conf. Appl. Biophysics (28-29 March • 9th Insulin Potentiated Teraphy (IPT) 2013 Kiev Ukraine) p 8 ISBN: 978- Global Conference, 24-27 October 966-388-467-7 2013, Madrid, Spain – oral 20. Mamilov S A, Esman S S, Gisbrecht presentation. A I and Velygotsky D V 2013 Photolysis efficiency dependence of Member of the Organizing Committee of HbCO in vivo from the wavelength • 23rd Int. Workshop LASER PHYSICS- Proc. 9th Int. Conf. Electronics and 2014, Sofia, Bulgaria, 14-18 July 2014. 52 Laboratory Biophotonics

Assoc. Prof. Dr. Ekaterina Borisova Sci., 25-29 September 2013, Sofia, Bulgaria, oral presentation; Member of the Program Committee of • 18th Int. Summer School on Vacuum, • Int. Conf. Photonics, Optics and Laser Electron and Ion Technologies, 07-11 Technologies - PHOTOPICS'2013, October, 2013, Sozopol, Bulgaria, February 2013, Barcelona, Spain; poster presentation. • Int. Conf. LAT2013-Laser Applications and Technologies, June 2013, Aleksandra Zhelyazkova Moscow, Russia; • 21th Int. Conf. Advanced Laser Member of the Organizing Committee of Technologies ALT'2013, September • 23rd International Workshop LASER 2013, Budva, Montenegro; PHYSICS-2014, Sofia, Bulgaria, • Second Int. Conf. Biomedical 14-18 July 2014. Engineering and Biotechnology (ICBEB 2013), October 2013, China; Progress report delivered at • Int. Conf. Photonics, Optics and Laser • First Int. Conf. Biophotonics - Riga Technologies - PHOTOPICS'2014, 2013, 25 − 31 August 2013, Riga, January 2014, Portugal; th Latvia, poster presentation. • 21 Int. Conf. Advanced Laser Technologies ALT'2014, October Liliya Angelova 2014, Casis, France. Member of the Organizing Committee of Chair of rd • 23 Int. Workshop LASER PHYSICS- Organizing Committee of 23rd • 2014, Sofia, Bulgaria, 14-18 July 2014. International Workshop LASER

PHYSICS-2014, Sofia, Bulgaria, 14-18 Progress report delivered at July 2014. • First Int. Conf. Biophotonics - Riga

2013, 25 - 31 August 2013, Riga, Lectures and progress reports delivered at Latvia, oral presentation. • Second Int. Biophotonics and Imaging

Conf. (BioPIC'2013), 24-28 March Aleksander Gisbrecht 2013, Dublin, Ireland – 1 oral report;

Int. Conf. Topical Problems of • Member of the Organizing Committee of Biophotonics - TPB'2013, 20-29 July th • 12 Int. Conf. Appl. Biophysics, Kiev, 2013, Nijnii Novgorod, Russia, 1 Ukraine, 28-29 March, 2013. invited lecture; Tsanislava Genova • Int. Simp. Optics and Biophotonics -

Saratov Fall Meeting- SFM'2013, 22- Member of the Organizing Committee of 29 September 2013, Saratov, Russia – 23rd International Workshop LASER 1 invited lecture, 1 oral report and 1 • PHYSICS-2014, Sofia, Bulgaria, 14-18 poster. July 2014.

Dr. Albena Daskalova ONGOING RESEARCH PROJECTS Progress reports delivered at • Int. Conf. Progress in Ultrafast Laser Contract DMU-03-46/2011 Development Modifications of Materials, 14-19 and introduction of optical biopsy system April 2013, Cargese, France, oral for early diagnostic of malignant tumors, presentation; 2011-2013, Assoc. Prof. Dr. Ekaterina • Second National Congress on Physical Borisova – coordinator. 53 Annual Report IE 2013

COST project TD1104 - EP4Bio2Med- Department, University of Aveiro, European network for development of Portugal; electroporation-based technologies and • Institute of Organic and treatments, 2011-2015, Assoc. Prof. Dr. Macromolecular Chemistry, Ekaterina Borisova – team member. University of Bremen, Germany; • Biophotonics Laboratory & Head Contract № DMU 03/15, 3D Femtosecond Atmospheric Sciences Division, laser microprocessing of biomaterials for Centre for Earth Science Studies, application in medicine, 2011-2013. India; • Biophysics and PDT group, Contract DNTS/Austria 01/1, Ultra-short Institute for Cancer Research, microprocessing of collagen thin films for Norwegian Radium Hospital, tissue engineering applications, 2013- Norway; 2015. • A.M. Prokhorov General Physics Institute, Russian Academy of Contract DNTS/Ukraine 01/0006/14.11.2012, Sciences, Moscow; Study of the influence of laser radiation on • School of Science and peripherical blood vessel for phototherapy Technology, University of Sussex, improvement. UK; • Physics Department, National COLLABORATIONS Technical University of Athens, Greece; 1. In the framework of the National • Department of Constructive and Center on Biomedical Photonics Technological Engineering, Project: National Institute of Research and • Center for Optical Diagnostics Development for Optoelectronics and Therapy, Erasmus Medical – INOE - 2000, Romania; Center, Rotterdam, the 2. Institute for Electronic Structure and Netherlands; Laser, FORTH, Heraklion, Greece. • School of Pharmacy & Bio- 3. Coherent Population Trapping Effect molecular Sciences, University of in Potassium” – CNR, Pisa, Italy. Brighton, UK; 4. Atomic vapors in metamaterials – • University of Ioannina, Greece; University of Leuven, Belgium. • Institute of General Physics, 5. High resolution spectroscopy in TUV, Austria; quantum optics and metrology” - • Optics and Biomedical Physics Institute of Physics, Belgrade. Department, Research-Educa- 6. P09073A: HCI-induced ToF-SIMS tional Institute of Optics & studies of hard dental tissues, France. Biophotonics, Saratov State 7. ITSLEIF-exchange programme of the University, Russia; EU Study of mechanisms of ultra- • Department of Electrical short laser ablation and desorption of Engineering and Electronics, biomolecules, 2010. University of Liverpool, 8. Development of new methods for laser Liverpool, UK; diagnostics and therapy of • Frederick University, Department dermatological and oncological of Mechanical Engineering, diseases, Institute of Physics, National Cyprus; Academy of Sciences of Belarus, • University of Siena, Italy; Minsk, Belarus. • Organic and Natural Products 9. Photodissociation of oxygen for a Chemistry Group, Chemistry phototherapy improvement, Institute 54 Laboratory Biophotonics

of applied physics and biophysics, the framework of BG051PO001-3.3.06 Natonal Ukrainian Academy of - 0048/04.10.2012 project on Human Sciences, Kiev, Ukraine Resources Development Project of the Institute of Experimental Morphology, LECTURE COURSES Pathology and Anthropology with Museum, Bulgarian Academy of Assoc. Prof. Dr. E. Borisova Sciences. • Photochemical interactions in biomedicine – lectures (15 h) and A. I. Gisbrecht practice (30 h), MS degree course, for • Institute of Applied Problems of Medical Physics program of the Physics and Biophysics, National Faculty of Physics, University of Ukrainian Academy of Sciences. Sofia. • Institute of Physics, National Academy • Optical methods and technologies used of Sciences of Belarus, Minsk, in medicine – practice (24 h), BS Belarus. degree course, for Physics and Engineering Technologies department of Plovdiv University. • Lasers in medicine – lectures (8 h) and practice (2 h), PhD students course, in

55 Annual Report IE 2013

LABORATORY

LASER SYSTEMS

HEAD: Assoc. Prof. S. Gateva, Ph.D.

TOTAL STAFF: 13 RESEARCH SCIENTISTS: 10

Assoc. Prof. S. Cartaleva, Ph.D.; Assoc. Prof. E. Alipieva, Ph.D.; Ch. Andreeva, Ph.D.; E. Taskova, Ph.D.; P. Todorov, Ph.D.; N. Nikolov; O. Vankov; K. Vaseva; N. Petrov; V. Sarova. Ph.D. students: A. Krasteva; S. Tsvetkov.

RESEARCH ACTIVITIES wavelength references can be developed. A new resonance in the fluorescence of a The growing interest in coherent- closed transition was studied population-trapping (CPT) resonances, demonstrating its high sensitivity to elastic prepared and detected under different atom-atom and atom-dielectric surface conditions, is stimulated by their numerous collisions. The theoretical modeling applications in high-resolution laser performed is in agreement with the spectroscopy, quantum information experimental observations. The application storage and processing, metrology (atomic of the nanometric cell provides a new clocks), magnetometry, lasing without opportunity for significant enhancement of inversion, laser cooling, ultraslow group the resolution of the laser spectroscopy of velocity propagation of light, etc. Building thermal cells, without application of miniature and high-sensitivity sensors complex atomic beam or laser cooling ensuring reliable operation requires good systems. A new collaboration with Toptica knowledge of the CPT resonance shape company was started, related to the results and the processes influencing it. mentioned. In 2013, in collaboration with The implementation continued of a Armenian, Italian and French colleagues, project financed by the EC (7th FWP), systematic theoretical and experimental entitled “Coherent optics sensors for investigations were performed in order to medical applications” (COSMA). The clarify the possibility of using alkali vapor project aims to develop optical atoms confined in nano-metric and micro- magnetometers specifically designed for metric cells, as well as in spin preserving medical applications. Transfer of polymeric coatings, for miniaturization of knowledge is carried out to train photonics sensors based on coherent researchers in the design of novel effects, without significant reduction of instrumentation for medical applications their sensitivity. It was shown that when and start comprehensive and coordinated the thickness of the alkali vapor cell research activities aimed at making approaches the wavelength of the biomagnetism detection and analysis a irradiating light, effects of basic interest very important medical technique. The occur (like Dicke narrowing in the optical project involves ten research groups from region, velocity-selective optical pumping Italy, the United Kingdom, Bulgaria, resonances with sub-Doppler width, Israel, Armenia, Russia, India, Poland and depolarization of atomic states by atom- the USA. surface interactions). Based on the sub- Related to COSMA project, studies of Doppler-width resonance, precise CPT resonances prepared in alkali micro- 56 Laboratory Laser Systems

layers were performed, demonstrating the N-resonance process. The N-type unexpectedly narrow high-contrast resonance in a magnetic field was shown features. The physical processes behind to split into seven or eight components, the strong resonance narrowing are still depending on the magnetic field and laser not well clarified. In cooperation with radiation directions. The approach Indian colleagues, theoretical analysis and introduced makes it possible to study in modeling of the CPT resonance formation detail the behavior of the ground-state in miniaturized alkali metal vapor cells levels of alkali atoms in a wide range of were conducted. To simulate the spectra, the magnetic field strength. The work was the five-level optical Bloch equations were carried out in collaboration with used. The Doppler convolution includes all colleagues from Armenia and Israel. possible orientation of atoms with respect In collaboration with Laboratoire de to the laser beam direction. Atoms moving Physique des Lasers, Université Paris 13, nearly parallel to the windows and an experimental method was proposed of perpendicular to the collinear pump and observing and counting “slow atoms” probe beams have a much lower Doppler (atoms moving at a grazing incidence shift and, hence, produce a considerable towards or away from the surface, i.e. narrowing of the Doppler background in atoms with a small normal velocity) the fluorescence spectra. The coherence relying on a mechanical discrimination by decay rate is also low for such atoms as spatially separated pump and probe beams they do not reach the walls. The narrow in a thin cell. The contribution of the EIT resonance in a micrometric cell may "slow" atoms is essential for the also result from a weaker radiation development of spectroscopic methods for trapping effect as compared to probing a dilute atomic vapor in the conventional cells. vicinity of a surface, thus enabling a sub- As a result of a joint experiment with Doppler resolution under irradiation at Italian and Indian colleagues, the first normal incidence. observation was reported of atomic spin The studies on the CPT resonance randomization of Rb atoms released by shapes performed at the Institute of LIAD. Rb atoms contained in paraffin or Physics in Warsaw with laser cooled PDMS coated glass cells were irradiated atoms in a magneto-optical trap (MOT) by a free-running diode laser light tuned concern small volumes without Doppler on the Rb D2 resonance line. The broadening. In the past year, modeling was transmission spectrum of the Rb vapor was conducted of the pump-probe spectra. The thus modified and showed a strong investigation showed that a state which is enhancement of the hyperfine optical not directly involved in the probe pumping as the light intensity was absorption can still considerably affect the increased and the laser frequency scanning absorption spectra shape (due to rate was decreased. The D2 line spectra multiphoton transitions) and its influence were compared for two cases: with and has to be carefully considered, even if the without illumination of the cell walls. A coupling-field induced Rabi frequency simple theoretical model based on rate values do not exceed the natural linewidth. equations was developed in order to These studies are important because analyze the experimental results. resonances of this type are of interest for, The sub-natural-width N-type e.g., developing quantum memory resonance on the D2 line of Cs atoms was protocols. studied for the first time in the presence of In a joint work with Russian colleagues, a buffer gas and the radiation of two the luminescence spectra of CdSe/ZnS continuous narrow-band diode lasers. An nanocrystals (called also quantum dots, or eight-mm-long cell was used to investigate QDs) of various sizes and concentrations 57 Annual Report IE 2013

were investigated. The interest in the 87700N ISSN: 1996-756X fluorescence of the nanocrystals is 6. Todorov P, Petrov N, Maurin I, connected with the problem of Saltiel S and Bloch D 2013 Detection implementing new protective of slow atoms confined in a cesium luminescence markers and labels. It was vapor cell by spatially separated found that large QDs extinguish very pump and probe laser beams Proc. efficiently the luminescence of QDs of SPIE 8770 87700P-1 ISSN: 1996- smaller sizes. Decreasing the pore size of 756X the matrix where quantum dots are 7. Gateva S, Taslakov M, Sarova V, introduced leads to a narrowing of the Marriotti E and Cartaleva S 2013 luminescence spectra. These results show Light-induced atomic desorption for that the informational capacity of the luminescence labels can be increased using miniaturization of magneto-optical multi-sized compositions of quantum dots sensors Proc. SPIE 8770 87700O on a proper matrix. ISSN: 1996-756X 8. Żaba A, Paul-Kwiek E, Kowalski K, PUBLICATIONS Szonert J, Woźniak D, Gateva S, Cao Long V and Głódź M 2013 The role 1. Cartaleva S, Krasteva A, Moi L, of a dipole-coupled but not dipole- Sargsyan A, Sarkisyan D, Slavov D, probed state in probe absorption with Todorov P and Vaseva K 2013 Laser multilevel coupling Eur. Phys. J. spectroscopy of sub-micrometre- and Special Topics 222 2197-2206 Print micrometre-thick caesium-vapour ISSN: 1951-6355, Online ISSN: layers Quantum Electronics 43/9 1951-6401 875–84 Online ISSN: 1468-4799 9. Żaba A, Cao Long V, Głódź M, Paul- Print ISSN: 1063-7818 Kwiek E, Kowalski K, Szonert J, 2. Cartaleva S, Krasteva A, Sargsyan A, Woźniak D and Gateva S 2013 Sarkisyan D, Slavov D and Electromagnetically induced trans- Vartanyan T 2013 Sub-natural width parency and Autler-Townes effect in resonances in Cs vapor confined in a generalized λ-system: A five-level micrometric thickness optical cell model Ukrainian Journal of Physical Proc. SPIE 8770 87700M ISSN: Optics 14/3 135-45 Print ISSN 1609- 1996-756X 1833, Online ISSN 1816-2002 3. Sargsyan A, Mirzoyan R, Cartaleva S 10. Żaba A, Paul-Kwiek E, Kowalski K, and Sarkisyan D 2013 Simultaneous Szonert J, Woźniak D, Gateva S, Cao observation of N- and EIT- Long V and Głódź M 2013 Pump- resonances in a 40-micron thin cell probe spectra modeled with inclusion filled with Rb and buffer gas 2013 of a dipole-coupled but not dipole- Proc. SPIE 8770 87700K ISSN: probed F' state, for the case of 85Rb 1996-756X 5S1/2(F)↔5P3/2(F') transitions Proc. 4. Slavov D, Krasteva A and Cartaleva S SPIE 8770 87700Q ISSN: 1996- 2013 Bi-chromatic spectroscopy in 756X micrometric optical cells Proc. SPIE 11. Alipieva E, Zlatov A S, Polischuk V A, 8770 87700L ISSN: 1996-756X Briukhovetskiy A P and Grigoriev D E 5. Krasteva A, Slavov D, Todorov G 2013 Influence of quantum dots size and Cartaleva S 2013 Velocity dispersion on the fluorescence selective optical pumping resonance spectrum Proc. SPIE 8770 87700T sign reversal Proc. SPIE 8770 ISSN: 1996-756X 58 Laboratory Laser Systems

12. Gurdev L, Dreischuh T, Vankov O, Patent No.EP2165647A1 Bliznakova I, Avramov L and Applicant - Siemens AG, Germany Stoyanov D 2013 Turbid media optical properties derived from the CONFERENCES characteristics of propagating laser th radiation beams Appl. Phys. B: 18 Int. Summer School on Vacuum, Lasers and Optics Print ISSN 0946- Electron and Ion Technologies, VEIT 2013 2171, Online ISSN 1432-0649. 7-11 October 2013, Sozopol, Bulgaria: • Taslakov M, Tsvetkov S and Gateva PATENTS SUPPORTED S Light-induced atomic desorption at 1. Biancalana V, Dancheva Y, Mariotti E, different illumination Moi L, Cartaleva S and Andreeva Ch • Tsvetkov S, Gateva S, Taslakov M, Mariotti E and Cartaleva S, Method and device for measurement Light-induced atomic desorption in of magnetic induction, BG Patent Reg. different coated PDMS cells No 107350/03122008 • Dancheva Y, Marinelli C, Mariotti E, 2. Biancalana V, Dancheva Y, Mariotti E, Marmugi L, Zampelli M, Gosh P, Moi L, Cartaleva S and Andreeva Ch Gateva S, Krusteva A and Cartaleva Method and device for measurement S, of magnetic induction, Eur. Patent No Spin randomization of Rb atoms EP1570282 Priority 03122002/2009 released by LIAD 3. Stoyanov D, Gurdev G, Dreischuh T, Vankov O and Protochristov Ch 6th Workshop on Quantum Chaos and Radar on single spontaneously emitted Localisation Phenomena, 24-26 May gamma-photons, Reg. № 65770 В1/2009 Warsaw, Poland: 4. Stoyanov D, Gurdev G, Dreischuh T, • Żaba A, Paul-Kwiek E, Kowalski K, Vankov O, Avramov L and Borissova E Szonert J, Gateva S, Cao Long V and Optical multichannel transceiving Głódź M, system, Reg № 65769 В1/2009 Level coupled but not probed in multilevel L-type system EUROPEAN PATENTS PENDING ONGOING RESEARCH PROJECTS 1. Stoyanov D, Dreischuh T, Gurdev L, Financed by the National Science Fund Vankov O, Avramov L, Borisova E

and Bliznakova I, Method for 1. Grant No: DO-02-108, determining optical and spatial Coherent spectroscopy of alkali nano- characteristics of an inclusion in a layers for miniaturization of photonics turbid medium using multiple- sensors. scattering optical tomography, 2. Grant No: BIn-2-07, Patent No.EP2153772A1, All-optical diode-laser-based magneto Applicant - Siemens AG, Germany meter: miniaturization and parameters 2. Stoyanov D, Dreischuh T, Gurdev L, optimization, Bulgarian-Indian inter- governmental program of cooperation Vankov O, Avramov L, Borisova E in science and technology (2008- and Bliznakova I, Apparatus for 2011). determining optical and spatial 3. Grant No: DMU-02-17, characteristics of an inclusion in a Velocity distribution of alkali atoms in turbid medium using multiple- micrometric thin cell. scattering optical tomography, 4. Grant No: DO-02-107/2009, 59 Annual Report IE 2013

Improving the resolution of Thomson 7. College of Human and Health scattering lidars by deconvolution- Sciences, Swansea University, UK. based algorithms. 8. Department of Physics, University of California at Berkeley, USA. Financed by the European Commission 9. Department of Physics and Astronomy, University College of London, UK. 1. Coherent optics sensors for medical 10. Department of Chemistry, Bar-Ilan applications (COSMA) 7FP 2012- University, Israel. 2016. 11. Department of Physics, Jagiellonian 2. Improving the resolution of Thomson University, Poland. scattering LIDARs by application of 12. Institute of Physics, Polish Academy novel deconvolution-based algorithms, of Sciences, Warsaw, Poland. Contract of Association between the European Atomic Energy Community LABORATORY VISITS (EURATOM) and INRNE under the 7th Framework Program of the Krasteva A European Atomic Energy Community • University of Siena, Siena, Italy, 90 (Euratom), No.FU07-CT-2007-00059. days. • Institute for Physical Research, NAS Financed by the BAS of Armenia, Ashtarak-2, 45 days.

1. Coherent processes under conditions Cartaleva S of strong optical excitation of • University of Siena, Siena, Italy, 30 multilevel atomic structures BAS-PAS days. (Poland). • Institute for Physical Research, NAS 2. Coherent Ramsey resonances in of Armenia, Ashtarak-2, 45 days. coated cells for application in optical magnetometry BAS-CNR (Italy). Gateva S 3. Coherent spectroscopy of alkali nano- • National Institute of Optics (INO- layers for miniaturization of photonics CNR), Pisa, Italy, 21 days. sensors. 4. Nonlinear magnetooptical effects on GUESTS Zeeman sublevels of alkali atoms in

antirelaxation coated cells. Gozzini S, National Institute of Optics (INO-CNR), COLLABORATIONS Pisa, Italy, 8 days.

1. University of Siena, Siena, Italy, Framework agreement of academic Marmugi L, cooperation, Establishment of PhD National Institute of Optics (INO-CNR), School on Laser Spectroscopy – Pisa, Italy, 15 days. Common Doctorate. 2. National Institute of Optics (INO- Gosh P, CNR), Pisa, Italy. University of Calcutta, Kolkata, India, 42 3. University of Calcutta, Kolkata, days. India. 4. Jadavpur University, Kolkata, India. 5. Institute for Physical Research, NAS Bloch D, of Armenia, Ashtarak-2, Armenia. Université Paris 13, France, 12 days. 6. Institute of Automation and Electrometry, RAS, Siberian Branch Novosibirsk, Russia.

60 Laboratory Nonlinear and Fiber Optics

LABORATORY

NONLINEAR AND FIBER OPTICS

HEAD: Assoc. Prof. Lyubomir Kovachev, Ph.D.

TOTAL STAFF: 5 RESEARCH SCIENTISTS: 3

Assoc. Prof. L. M. Ivanov, Ph. D; Assis. Prof. K. L. Kovachev, Ph. D; A. Dakova; V. Slavchev.

RESEARCH ACTIVITIES appear, presenting cross-modulation and degenerate four-photon parametric mixing. 1. Nonlinear polarization and The self-action process broadens the pulse polarization rotation of broad-band spectrum − starting with a narrow-band laser pulses pulse, the stable filament becomes broad- band far away from the source. Later As demonstrated in several papers, high works [3, 4] have shown, however, that the intensity femtosecond pulses generate in evolution of broad-band pulses, such as gases stable filaments with a broad-band filaments, cannot be described correctly by spectrum. Various models explaining this a nonlinear polarization of the kind (1). It phenomenon have been presented (e.g., is more correct to use the generalized see the paper of Couairon and Mysyrowicz and the references therein [1]). The nonlinear operator: standard filamentation model is based on r r r r nl (2) plasma generation and multi-photon P = n2 (E ⋅ E)E , processes and includes also a nonlinear polarization of the kind: which includes additional processes associated with third-harmonic generation r r r r 1 r r r nl  *  (1) (THG). The more precise analysis P = n2 (E ⋅ E )E + (E ⋅ E)E .  2  performed by us demonstrated that the polarization of kind (2) is not applicable to The polarization (1) was proposed by a scalar model, because the corresponding Maker and Terhune in 1965 [2]. If the Manley-Rowe (MR) conservation laws are electrical field contains one linear or not satisfied. This is why we substituted circular component, the polarization (1) into the nonlinear operators (1) and (2) a describes only a self-action effect, while in two-component electrical vector field at the case of a two-component electrical r one carrying frequency of the form: field E = (Ex , Ey )0, additional terms

r (A exp(iω t)+ .cc ) r (Ay exp(iω0t)+ .cc ) r (3) E = x 0 x + y , 2 2 where the amplitude functions and ω0 is the Ax = Ax tzyx ),,,( and Ay = Ay tzyx ),,,( are carrying frequency of the laser source. 61 Annual Report IE 2013

The nonrestricted nonlinear polarization (2) generates the following components:

1  ()A 2 + A 2 A exp ()2iω t +  r  x y x 0  nl 3 3 Px = n 2   exp ()iω 0 t + cc ,. 8  2 2 2  1 * 2   Ax + A y  Ax + Ax A y  3  3  (4) 1 .  ()A 2 + A 2 A exp ()2iω t +  r  x y y 0  nl 3 3 Py = n 2   exp ()iω 0 t + cc .. 8  2 2 2  1 * 2   Ay + Ax  Ay + A y Ax  3  3 

The nonlinear theories based on the nonlinear polarization of type Maker and Terhun type polarization (1) r r r r Pnl = n (E ⋅ E)E, some restrictions on the satisfy the Manley-Rowe relations. This 2 means that, during the process of energy components of the electrical field are exchange, the total energy is conserved for imposed. We demonstrated this on the arbitrary localized smooth complex fields. basis of a two-component vector field (3). Additional conservation quantities are also If we rewrite the generalized nonlinear possible. To satisfy the MR relations of the polarization (2) using circularly polarized truncated equations with a generalized components [4]

(5) A+ = (Ax + iAy ) ,2 A− = (Ax − iAy ) 2 , we obtain: The truncated equations can be written as

∂ A 2 i + = n (A 2 A ), P = n (A A ) exp[ik (z − v t)], 2 + − + 2 + − 0 ph (6) ∂ t (7) 2 ∂A− 2 P− = n2 (A− A+ ) exp[ik0 (z − v pht)]; i = n2 ()A− A+ . ∂t

P+ and P− correspond to left-hand circular The equations for the square modulus and right-hand circular polarization. of the components are:

∂ A 2 + 2 * * i = n2 A+ ()A+ A− − A+ A− = ,0 ∂t (8)

∂ A 2 i − = n A 2 ()A A − A* A* = ,0 ∂t 2 − + − + − following from the fact that the operator of kind (2), the possible initial components A+ and A− are complex- conditions and solutions should be conjugated fields (5). Thus we proved that, complex conjugated fields. The to satisfy the MR conditions for the conservation laws give us additional nonlinear system (7) (or other conservative information on the behavior of the vector nonlinear equations) with nonlinear amplitude function. 62 Laboratory Nonlinear and Fiber Optics

Only componentsr of the vector pulse spectrum. The broad-band spectrum A = (A , A )0, ∆ ≈ amplitude field x y r which ( kz k0 ) is one of the basic present rotation of the vector A in the characteristics of the stable filament. The plane (x;y), satisfy the MR conditions (7). evolution of the filament so obtained can This is why in our numerical experiments, as no longer be described in the framework of well in our analytical investigations, we will the nonlinear paraxial optics because the use complex conjugated components only. latter works correctly for narrow-band laser pulses only. The dynamics of broad- 2. Long range filament as vector band pulses can be presented properly solitary wave within different models of non-paraxial optics, such as UPPE models [3] or non- The stable filament propagation in paraxial envelope equations [4]. Another gases is realized in the sub-pico and standard restriction in the filamentation femtosecond ranges, while in the nano- theory is the use of one-component scalarr and picosecond regions there appears the approximation of the electrical field E . well known self-focusing. The dynamics This approximation, however, proved to be of the narrow-band laser pulses can be in contradiction with recent experimental accurately described in the framework of results, where rotation of the polarization the paraxial optics. The filamentation vector was observed [5]. Thus, in our experiments demonstrate a typical pulse investigations we employed the non- spectrum evolution. The initial laser pulse paraxial vector model up to the second

(t0 > 30 − 50 fs) possesses a relatively order of dispersion, whereby the nonlinear narrow-band spectrum ( ∆kz << k0 ), where effects are described by the nonlinear ∆kz is the spectral pulse width and k0 is polarization components (6). The set of the carrying wave number. During non-paraxial equations of the amplitude the filamentation process, the initial functions A+ and A− of the two-component self-focusing broadens significantly the electrical field has the form:

 ∂A 1 ∂A  β +1 ∂2 A − 2ik  + − +  = ∆A − + +γA2 A , 0  ∂z v ∂t  + v2 ∂t 2 + −  gr  gr (9)  ∂A 1 ∂A  β +1 ∂2 A 2ik  − − −  = ∆A − − +γA2 A , 0  ∂z v ∂t  − v2 ∂t 2 − +  gr  gr

3 connected with the fact that, following the where 2 2 is the nonlinear γ = n 02 kA 0 8 MR conditions, A+ and A− are obtained coefficient, v is the group velocity, gr from the complex-conjugated Ax and Ay 2 ′′ ′′ β = 0vk gr k0 and k0 is the group velocity fields. In air β << 1, so that when β = 0 dispersion. The sign (−) in the front of the the system of equation (9) admits an exact left-hand side of the first equation in (9) is vector vortex soliton solution of the kind:

2 A = exp[]− i∆k()z − vt + ~ 2 1+ r (10)

2 A = exp[]∆ ()zki − vt , − 1+ ~r 2 63 Annual Report IE 2013

2   k (v − v ) ~2 2 2 2 2 k ωnl 0 ph gr where ∆k = k , r = x + y + ()z − ik − v t − i nl  and k = = . 0 nl gr  v  nl v v  gr  gr gr

REFERENCES 5. Kovachev L M, Ivanov L M and Kovachev K L 2013 Broad-band laser [1] Couairon A and Mysyrowicz A 2007 pulses. Linear and nonlinear regime Femtosecond filamentation in Proc. Second Nat. Congress Phys. Sci. transparent media Phys. Rep. 441 47- (Sept 2011 Sofia Bulgaria) pp 224- 189 225 [2] Maker P D and Terhune R W 1965 6. Dakova A and Dakova D 2013 Study of optical effects due to an Propagation of broad-band pulses in induced polarization third order in the nonlinear dispersive media Proc. electric field strength Phys. Rev. 137 Second Nat. Congress Phys. Sci. (Sept A801 2011 Sofia Bulgaria) p 226 [3] Kolesik M and Moloney J V 2008 7. Slavchev and Kovachev L 2013 Perturbative and non-perturbative Nonlinear regime of propagation of aspects of optical filamentation in optical pulses phase-modulated by a bulk dielectric media Optics Express double-convex lens Proc. Second Nat. 16 2971 Congress Phys. Sci. (Sept 2011 Sofia [4] Kovachev L M 2009 New mechanism Bulgaria) p 230 for THz oscillation of the nonlinear 8. Kovachev L 2013 Shock wave in refractive index in air: particle-like Euler-Heisenberg-Köckel nonlinear solutions J. Modern Optics 56 1797- vacuum Proc. Second Nat. Congress 1803 Phys. Sci. (Sept 2011 Sofia Bulgaria) [5] Sheinfux A H, Schleifer E, Papeer J, pp 453-4 Gibich G, Ilan B and Zigler A 2012 Measuring the stability of PROJECTS polarization orientation in high intensity laser filaments in air Appl. Financed by the Bulgarian Academy of Phys. Lett. 101 201105 Sciences

PUBLICATIONS 1. Propagation of ultrashort optical pulses in media with non-stationary 1. Kovachev L M, Georgieva D A and optical and magnetic response (2011- Serkin V N 2013 Parametric solitons 2013). in isotropic media AIP Conf. Proc. 1561 289 LECTURE COURSES 2. Dakova A M and Dakova D I 2013 Nonlinear regime of propagation of Fiber Optic Communication Systems; femtosecond optical pulses in a single- Optics; Electricity and Magnetism; mode fiber Proc. SPIE 8770 87701J South-Western University, Blagoevgrad, 3. Kovachev L M and Georgieva D A Bulgaria. 2013 The long range filament stability: balance between non- paraxial diffraction and third-order nonlinearity Proc. SPIE 8770 87701G 4. Ivanov L M and Kovachev L M 2013 Improved four photon mixing methods for optical fibre’s parameters control Proc. 5-th Int. Sci. Conf. FMNS 2013 3 pp 57-64 64 Laboratory Laser Radars

LABORATORY

LASER RADARS

HEAD: Prof. D. Stoyanov, Dr.Sc.

TOTAL STAFF: 13 RESEARCH SCIENTISTS: 12

Assoc. Prof. L. Gurdev, Ph.D.; Assoc. Prof. T. Dreischuh, Ph.D.; Assoc. Prof. V. Mitev, Ph.D.; Assoc. Prof. V. Pencheva, Ph.D.; Assoc. Prof. A. Deleva, Ph.D.; Z. Peshev, Ph.D.; S. Penchev, Ph.D.; N. Kolev, Ph.D.; Ts. Evgenieva, Ph.D.; I. Grigorov; E. Тoncheva.

RESEARCH ACTIVITIES vertical profiling of the atmospheric backscattering coefficient. The working 1. Lidar remote sensing of the program included the activities listed atmosphere within the European below: Aerosol Lidar Network (EARLINET) 1. Regular climatologic lidar measurements, performed three times a week (at noon on Lidar systems are being used widely for Mondays and around the sunset on remote studies of atmospheric parameters. Mondays and Thursdays), under favorable The importance of the lidar information meteorological conditions. provided by a single lidar station is 2. Lidar detection and characterization of enhanced considerably if working within a Saharan dust atmospheric loadings (in lidar network. Two elastic backscatter terms of height, structure, and evolution of lidar systems of the Laser Radar dust layers) in cases of dust transport Laboratory (an aerosol lidar with a CuBr- events. vapor laser and a Raman-aerosol lidar with 3. Regular lidar measurements in the a Nd:YAG-laser), as a part of the framework of the “CALIPSO QPQ” EARLINET network, have been program. These are scheduled implementing tasks under the ACTRIS measurements synchronized with project (Aerosols, Clouds, and Trace overpasses of the CALIPSO gasses Research Infra Structure Network) meteorological satellite over the Balkan of the EC’s FP7 since 2011. Vertical Peninsula. The lidar data obtained, profiles of the atmospheric aerosol, together with those of other ground-based resulting from simultaneous measurements lidars, are used for calibrating the performed by the EARLINET network CALIPSO lidar downward measurements. lidar stations, provide a 3D picture of the 4. Processing of lidar measurements data. aerosol distribution in the atmosphere over A part of the results obtained is presented Europe. as height-time diagrams of the aerosol Sofia Lidar Station participates in two stratification (Quick Looks) at the web- of the ACTRIS project working programs: page of IE-BAS (http://www.ie- • WP2 – Remote sensing of aerosol bas.dir.bg/Departments/LidarData/Quicklo vertical distribution. oks.htm). The retrieved vertical profiles of • WP22 – Investigation of aerosol- the atmospheric backscattering coefficient cloud interactions. are uploaded to the common EARLINET The main goal of the remote sensing database. observations conducted during 2013 by 5. Conducting recurrent verifications of using the CuBr-laser-based lidar was the lidar receiving telescope adjustment by 65 Annual Report IE 2013

a standard procedure adopted in whole week of the days of lidar EARLINET called “Quadrant Test”. observations. 6. Mounting new mechanisms for lidar Several events of Sahara dust incursions telescope adjustments. above Sofia were registered by the Raman- Using the CuBr-laser based lidar, in aerosol lidar equipped by a Nd:YAG-laser 2013, measurements of total duration of emitting at 1064 nm. The experimental 260 hours were implemented resulting in lidar data were processed and results were 520 aerosol backscattering profiles, each obtained in terms of vertical backscattering corresponding to a 0.5-h measurement. coefficient profiles and color maps of the More than 200 of these files were atmospheric mass stratification evolution. uploaded to the EARLINET database. The The location, altitude and thickness of the files were distributed as follows: layers observed were determined. The climatology – 171, Saharan dust – 49, results from the Saharan dust monitoring cirrus clouds – 46, forest fires – 5. The were reported to the media and published. color Quick Looks uploaded to the IE- The results of the 3-day systematic two- BAS web-page were 31. Eight Quadrant wavelength lidar measurements on the tests were accomplished. forest wild-fire raging in Vitosha The aerosol load distribution in the Mountain conducted in the period atmosphere above Sofia originating from 09 − 11.10.2012 were partially processed. the forest fires around the village of A number of range-time color-maps Lokorsko (August 22, 2013) and tracing the distribution and spreading of Kremikovtsi monastery (August 24, 2013) fire aerosols over Vitosha Mountain and was studied using the two lidars operating Sofia were constructed at the two lidar at the wavelengths of 510.6 nm and wavelengths. The processing of the fire- 1064 nm. The difference in the lidars’ smoke lidar measurements will be wavelengths, as well as their specific completed. An approach was developed location with respect to the fires, was for determining reference values of the important in view of obtaining aerosol backscattering coefficient (BSC) comprehensive information on the aerosol within the zone in close vicinity to the fire stratification in the atmosphere. HYSPLIT source. The methodology was tested (HYbrid Single-Particle Lagrangian successfully by using modeling and real Integrated Trajectory) backward lidar data. Currently being at a stage of trajectories and DREAM (Dust REgional optimization, it will be applied in Atmospheric Model) forecasts for the days retrieving range profiles of BSC and BSC- of measurements were employed to draw related Ångström exponent (BAE) for the conclusions about the aerosol’s origin. The measurements performed, as well as in lidar data showed the presence of dense other cases of quasi-horizontal lidar aerosol layers not only in the lower soundings. The BSC- and BAE profiling troposphere to a height of 3 − 4 km, but will allow a qualitative characterization of also in the 8 − 11 km range of altitudes. the fire-smoke aerosols size distribution Upon the analysis of the lidar results and based on a frequency distribution analysis HYSPLIT- and DREAM-forecasts, we of BAE occurrences. The approach reached the conclusion that the increased developed is promising for applications in concentration of aerosols detected was the cases of lidar profiling of zones containing result of both regional transport of smoke coarse aerosols, such as fogs, clouds, from the fires around Sofia and of trans- cirrus clouds, snow, etc., in the absence of border transport of fire aerosols and other a molecular reference. particles. The latter coincided with Three complex experimental campaigns wildfires in Spain, Portugal, and in North were carried out in June 2010, June 2011 America taking place throughout the and June 2012 at three sites in the city of 66 Laboratory Laser Radars

Sofia (Institute of Electronics, ELM-evolution curves were built, using Astronomical Observatory in the Borisova Thomson Scattering LIDAR data, of the Gradina Park and National Institute of electron temperature, density and pressure Geophysics, Geodesy and Geography). of fusion plasma in the thermonuclear The data were obtaind using a CHM15k TOKAMAK reactors. The evolution ceilometer, two Microtops II sun curves were compared and shown to be in photometers and an automatic accordance with those obtained from meteorological station. The ceilometer radiometric data from the plasma pedestal backscatter data analyses rеvealed that the area. Experimental estimates were height of the mixing layer varied from obtained of the spatial changes of the 1500 m to 2500 (3000) m above ground velocity of plasma H-mode retrieval in level. The height of the residual layer different spectral channels, depending on ranged from 800 m to 2000 m. The stable the evolution of the electron temperature boundary layer extended to 200-400 m in the plasma pedestal area. during the campaigns. The aerosol optical depth at wavelength λ = 500 nm ranged 3. Photothermal characterization of from 0.38 to 0.66 in the first campaign, magnetoelectric nanofilms from 0.24 to 0.55 in the second one, and from 0.11 to 0.23 in the third one. The The results were analyzed on corresponding ranges for the water vapor photothermal characterization of content were from 1.26 cm to 2.6 cm. magnetoelectric nanofilms conducted Different types of behavior of the aerosol under Project DO 224/2008 “New optical depth and the water vapor content magnetic and magnetoelectric materials were observed. The total ozone content for next-generation electronic devices” varied from 240 DU to 370 DU during the financed by the National Science Fund. campaigns. The ground-based The parameters of the study included observations using the Microtops II photomodulated surface reflectance and ozonemeter were compared with satellite displacement of various nanostructures. A observation over Sofia using ozone subsequent application was developed for monitoring instruments. assessment of the film thickness making use of the optical absorption coefficient. 2. Lidar diagnostics of thermonuclear The method refers to nanofilms deposited plasma on transparent substrates, where the laser power is absorbed selectively in the film. Efficient observation and control of the The analytical approach is based on an fusion process is only possible on the basis ultrasensitive laser double-heterodyne of adequate plasma diagnostics involving probe featuring a resolution of thickness the determination with high accuracy and measurements of up to fractions of a resolution of the plasma electron nanometer with an improved translational temperature Te and density ne. An resolution compared to ellipsometry. important task of the plasma diagnostics is A model was developed where part of the observation of the H-mode restoration the laser power is transferred as a thermal after each edge localized mode (ELM) wave from the heated film to a volume at a event. The analysis of the sensitivity of the certain depth below the surface of the plasma parameters to ELM and their transparent substrate. The dependence of evolution during the restoration process the absorbed laser power on the can be effectively performed using a magnetoelectric film thickness is based on Thomson scattering LIDAR approach. The Beer’s law. The films are considerably results we obtained in 2013 using such an thinner than the thermal diffusion length in approach were the following: the relevant bulk material, so that their 67 Annual Report IE 2013

photothermal displacement is determined turbid medium. Based on the small-angle solely by the expansion of the substrate. approximation, the detected spatial power The assessment of the thickness is distribution of the forward-propagating conducted assuming a sample film of light, at relatively small or large optical homogeneous chemical properties and depths along the beam axis, is obtained small thickness variation at a constant asymptotically in an analytical form absorption coefficient over the laser scan. allowing one to derive relatively simple The minimal detectable gradient is expressions of the extinction, reduced- inversely proportional to the signal-to- scattering and absorption coefficients and noise ratio (SNR) by a fixed factor the anisotropy factor of the medium determined by the absorption coefficient through the characteristics of the and the mean thickness of the given propagating light beam. In the sample film. The resolution evaluated for experiments, using Intralipid dilutions of an LSMO film of 100 nm thickness and a different concentrations, the cross- SNR of 55 dB was set at 0,2 nm. The sectional radial distribution of the detected results were presented as curves for light power was determined at different various ferromagnetic and multiferoic depths along the beam axis. The films. The calibration procedure avoids corresponding on-axis detected light errors of parameters that are difficult to power profiles were measured assess, such as the angular aperture and the independently as well. The experimental focal size under different experimental results are consistent with the analytical conditions. The calibrated photo- expressions obtained that allow one to displacement is based on AlBK7 and AgBK7 estimate the optical coefficients and the mirrors for all referenced magnetoelectric anisotropy factor of the media investigated nanofilms. A mirror of higher reflection on the basis of the measured beam raises the measurable range and, characteristics. The values obtained are respectively, lowers the resolution of the close to those predicted by other film thickness measurements. The laser researchers. The investigations performed heterodyne probe resolution is higher for are of essential importance for the magnitudes of photodisplacement outside development of methods of measuring the the area of saturation. The relative optical characteristics of turbid media, thickness variation is assessable by the such as tissues and tissue-like phantoms. higher resolution that is independent of the They are also important in the process of accuracy of the optical constants. With the establishing the relations governing the majority of tasks focussed on deposition of radiative transfer in optically-investigated homogeneous nanofilms, these results are biological objects. of importance for the nanotechnologies in various areas of optics and electronics. 5. Lidar hardware & software 4. Laser probing of turbid media for optical tomography applications The detection system start-time delay with respect to the triggering pulse was A straightforward analytical approach determined, in view of increasing the CuBr was developed to the determination of the lidar ranging precision. This was carried optical properties of liquid turbid media out by using a calibrated optical delay line having forward-peaked scattering consisting of a 317-m-long single-mode indicatrices. The approach is based on optical fiber. As a result of the experiment, investigating the in-depth behavior of the a time correction of 0.2 µs was introduced, radius and the axial intensity of a laser thus improving the time-range calibration radiation beam propagating through the of the lidar. 68 Laboratory Laser Radars

A patent application was submitted for post-processing ensuring sufficient time new optical locator for remote for determining the wind turbines’ optimal determination of the velocity of operation. statistically homogeneous environments and objects. A correlation measurement of the spatial shift of the realizations defines PUBLICATIONS the position of the correlation function maximum. The correlation function 1. Gurdev L, Dreischuh T, Vankov О, between spatial realizations assumes a Bliznakova I, Avramov L and maximum value for certain spatial Stoyanov D 2013 Turbid media motions. They are used for defining two optical properties derived from the components of the velocity vector, but characteristics of propagating laser only after introducing the distance to the radiation beams Appl. Phys. B: object, which is carried out by means of an Lasers and Optics Online DOI: additional measuring device, such as a 10.1007/s00340-013-5624-x laser range finder. By means of a 2. Kolev N, Evgenieva Ts, Miloshev N, stereocamera, the optical locator makes it Muhtarov P, Petkov D, Donev E, possible to determine simultaneously the Ivanov D, Danchovski V and Kolev I velocity vector of heterogeneous 2013 Aerosol optical depth, water environments and objects and the distance vapor content and total ozone to them, avoiding the necessity of an measurements over Sofia (Bulgaria) additional measuring equipment. The from three campaigns 2010-2012 correlation measurement of the spatial Comptes Rendus de l'Academie shift of realizations for velocity Bulgare des Sciences 66/11 1603-12 measurement is distributed between the 3. Penchev S, Pencheva V, Nedkov I two channels of a stereo receiver. The and Krezhov K 2013 Laser double- distance to the object is measured heterodyne integrated microscopy of simultaneously by the same image stereo magnetoelectric nanofilms Vacuum receiver. /1 7-11 A new housing of the wavelength- 96 4. Romanelli F and JET EFDA separating lidar module was designed and Contributors, incl. Dreischuh T and incorporated in the lidar system. The Stoyanov D 2013 Overview of the module was optically adjusted and optimized for correct lidar measurements. JET results with the ITER-like wall The problem related to the presence of Nuclear Fusion 53 104002 electromagnetic noise in the lidar photo- 5. Deleva D and Grigorov I 2013 Lidar receivers was thus solved successfully. observations of high-altitude aerosol The theoretical prospects were assessed layers (cirrus clouds) Proc. SPIE concerning the control of wind turbines, 8770 877011 utilizing the experimental studies 6. Dreischuh T, Gurdev L and conducted of signal detection by a Doppler Stoyanov D 2013 Efficiency of lidar. The study aims at increasing the determining electron temperature and energy efficiency of wind parks – a task concentration in thermonuclear directly related to the environmental plasmas by Thomson scattering lidar protection. The architecture of the system Proc. SPIE 8770 877012 developed increases the potential of the 7. Grigorov I and Kolarov G 2013 selected method of control on the basis of Rayleigh-fit approach applied to neural networks. The innovative approach improving the removal of allows one to preserve all prior background noise from lidar data information for real-time processing and Proc. SPIE 8770 877010 69 Annual Report IE 2013

8. Kolev N, Evgenieva Ts, Miloshev N, ISSN: 1311-8668 (in Bulgarian) Muhtarov P, Petkov D, Donev E, 16. Stoyanov D, Dreischuh T, Grigorov I, Ivanov D and Kolev I 2013 Peshev Z, Deleva A, Kolarov G, Ceilometer, sun photometer and Gurdev L, Kolev N, Evgenieva Ts, ozonometer measurements of the Mitev V, Toncheva E and Pencheva V aerosol optical depth, angstrom 2013 LIDAR sensing of aerosol coefficients, water vapor and total processes over Sofia region in the ozone content over Sofia (Bulgaria) frame of the European Aerosol Proc. SPIE 8894 88940W Research Lidar Network 9. Peshev Z, Dreischuh T, Toncheva E (EARLINET) Bulgarian J. Phys. 40 and Stoyanov D 2013 Lidar 186-92 observations and characterization of 17. Romanelli F and Laxåback M on biomass burning aerosols over Sofia: behalf of the JET EFDA Long-range transport of forest Contributors, incl. Dreischuh T and wildfire smoke Proc. SPIE 8770 Stoyanov D 2013 Overview of JET 87700Z results Proc. 24th IAEA Fusion 10. Stoyanov D, Grigorov I, Deleva A, Energy Conf. (8-13 October 2012 Kolev N, Peshev Z, Kolarov G, San Diego, USA) IAEA-CN-197 Donev E and Ivanov D 2013 Remote Paper OV/1-3 (International Atomic monitoring of aerosol layers over Energy Agency, Vienna, Austria) Sofia during Sahara dust transport 18. Stoyanov D, Dreischuh T, Gurdev L, episode (April, 2012) Proc. SPIE Grigorov I, Peshev Z, Deleva A, 8770 87700Y Kolarov G, Kolev N, Pencheva V, 11. Deleva A 2012 Lidar investigations Mitev V, Evgenieva Ts and of the troposphere performed during Toncheva E 2013 Lidar remote summer 2011 measurement sensing of the atmosphere, campaign Bulgarian Geophysical J. thermonuclear plasma and optically 38 79-92 opaque dense media J. Bulgarian 12. Deleva A 2012 Lidar monitoring of Academy of Sci. CXXVI/6 83-96 (in clouds and aerosol layers Bulgarian Bulgarian) Geophysical J. 38 21-33 19. Dreischuh T and Daskalova A (eds) 13. Deleva A 2013 Lidar montoring of 2013 Laser Physics and Applications the aerosol load above Sofia J. Proc. SPIE 8770 (SPIE Bellingham Ecological Engin. and Environment WA USA) Protection 2 11-20 ISSN: 1311-8668 20. Dreischuh T 2013 XVII International (in Bulgarian) School on Quantum Electronics 14. Penchev S, Pencheva V and Ivanov L “Laser Physics and Applications”, 2013 Portable lidar systems for auto- Nessebar, 24–28 September 2012, tuning of the controlling electronics The World of Physics 2 227-30 (in of wind turbines J. Ecological Engin. Bulgarian) and Environment Protection 3-4 47- 55 ISSN: 1311-8668 (in Bulgarian) 15. Pencheva V, Penchev S and PATENTS SUPPORTED Grigorova I 2013 Boundary layer pollution with fine dust particles: 1. Stoyanov D, Gurdev L, Dreischuh T, ecological assessment of their Vankov O and Protochristov C influence on human health and Radar on single spontaneously emitted climate J. Ecological Engin. and gamma-photons, Environment Protection 3-4 25-33 Patent No 65770 B (2009) 70 Laboratory Laser Radars

2. Stoyanov D, Gurdev L, Dreischuh T, CONFERENCES Vankov O, Avramov L and Borisova E, Optical multichannel transceiving Second National Congress on Physical system, Sciences, 25─29 September 2013, Sofia, Patent No 65769 B (2009) Bulgaria • Grigorov I, Deleva A, Kolev N, EUROPEAN PATENTS PENDING Stoyanov D, Kolarov G, Donev E and Ivanov D LIDAR observation of the distribution 1. Stoyanov D, Dreischuh T, Gurdev L, of aerosol loading in the atmosphere Vankov O, Avramov L, Borisova E above Sofia due to the forest fire in and Bliznakova I, Vitosha mountain (July, 2012). Method for determining optical and • Stoyanov D, Grigorov I, Deleva A, spatial characteristics of an inclusion Peshev Zand Kolarov G in a turbid medium using multiple- Remote monitoring of Sahara dust aerosol layers in the atmosphere above scattering optical tomography, Sofia in May. Patent No.EP2153772A1, • Stoyanov D, Dreischuh T, Grigorov I, Applicant - Siemens AG, Germany. Peshev Z, Deleva A, Kolarov G, 2. Stoyanov D, Dreischuh T, Gurdev L, Gurdev L, Kolev N, Evgenieva Ts, Vankov O, Avramov L, Borisova E Mitev V, Toncheva E, Pencheva V and Bliznakova I, and Penchev S LIDAR sensing of aerosol processes Apparatus for determining optical and over Sofia region in the framework of spatial characteristics of an inclusion the European Aerosol Research Lidar in a turbid medium using multiple- Network (EARLINET). scattering optical tomography, Patent No.EP2165647A1 Lidar Technologies, Techniques, and Applicant - Siemens AG, Germany. Measurements for Atmospheric Remote Sensing IX, 23─26 September 2013, Dresden, Germany PATENTS PENDING • Kolev N, Evgenieva Ts, Miloshev N, Muhtarov P, Petkov D, Donev E, 1. Mitev V, Ivanov D and Kolev I Method and lidar system for Ceilometer, sun photometer and registration of turbulences, Reg. No ozonometer measurements of the aerosol optical depth, angstrom coefficients, 110786/04.11.2010. water vapor and total ozone content over 2. Mitev V, Sofia (Bulgaria). Method and lidar for measurement of aerosol turbulence in the atmosphere. Third National Conference Ecological Reg. No 110925/ 29.09.2011. Engineering and Environment Protection 3. Mitev V, (EEEP’2013) 13-14 June 2013 Adjustable imaging receiver. Reg. No • Pencheva V, Penchev S and Grigorova I Boundary layer pollution with fine dust 111198/10.08.2012. particles: ecological assessment of their 4. Mitev V, influence on human helth and climate. Lidar for remote determination of • Penchev S, Pencheva V and Mihov L vector transfer velocity of Portable lidar systems for auto-tuning homogeneous media and objects. Reg. of the controlling electronics of wind No. 111424/12.03.2013. turbines. 71 Annual Report IE 2013

ONGOING RESEARCH PROJECTS European projects

Financed by the National Science Fund 1. Improving the resolution of Thomson scattering LIDARs by application of 1. DNS - 7FP 01/17, Task 2.2.2, novel deconvolution-based algorithms, Improving the resolution of Thomson Task 2.2.2, Contract of Association scattering LIDARs by application of between the European Atomic Energy novel deconvolution-based algorithms. Community (EURATOM) and INRNE, FP7, Project No. FU07-CT- Financed by the Steering Council of the 2007-00059. Bulgarian Academy of Sciences 2. Aerosols, Clouds, and Trace Gasses Research Infrastructure Network 1. Investigation of the temporal (ACTRIS), FP7, Project No.262254. dynamics of fine and coarse aerosol 3. Aerosols and clouds. Long-term data components in the troposphere by a base from space-borne lidar two-wavelength lidar. measurements, CALIPSO Program, 2. Remote determination of some European Space Agency, ESA. statistical characteristics of non- uniform media by image processing. MEMBERSHIPS IN INTERNATIONAL 3. Lidar-radiometric probing of aerosol LIDAR NETWORKS characteristics in the PBL and the free atmosphere above a mountain valley. 1. European Aerosol Lidar Network 4. Laser sensing of tissue-like turbid (EARLINET). optical media for localizing 2. Global Aerosol Lidar network characteristic inhomogeneities. (GALION). 5. Laser system and methodology for optical probing of novel magnetic and COLLABORATIONS magnetoelectrical materials for the next generation of electronics 1. Advanced lidar technologies for components. tropospheric aerosol studies, 6. Lidar atmospheric system using high- Istituto di Metodologie per l’Analisi power diode lasers. Ambientale, CNR, Italy. 7. Error analysis of the lidar Thomson 2. Lidar investigation of aerosol fields scattering probing of the electron transformations in urban industrial temperature and density of zones, Institute of Physics, National thermonuclear plasma. Academy of Belarus, Minsk, Belarus. 8. Lidar monitoring of the aerosols in the free troposphere.

72 Laboratory Microwave Physics and Technologies

LABORATORY

MICROWAVE PHYSICS AND TECHNOLOGIES

HEAD: Assoc. Prof. O. Yordanov, Ph.D.

TOTAL STAFF: 17 RESEARCH SCIENTISTS: 14

Assoc. Prof. V. Atanassov, Ph.D.; Assoc. Prof. I. Sirkova, Ph.D.; Assoc. Prof. M. Taslakov, Ph.D.; E. Krasteva, Ph.D.; P. Zabov, Ph.D.; B. Simeonova Ph.D.; I. Atanasov, Ph.D.; K. Kostov; V. Ranev; L. Mladenov; L. Vulkova; V. Iordanova; K. Omar; S. Igov; K. Dimitrov; Y. Taneva

RESEARCH ACTIVITIES examples for our study was provided by the family of chaotic systems discovered 1. Emerging simple and extended by Sprott. In addition, we tackled cases chaotic systems. with piecewise linear nonlinearities, as well as some specific newly discovered 1.1. Simple electronic and model systems chaotic 3D systems. Our approach was We employed assorted concepts and based on classical single-point and two- methods of the Chaos theory illustrated on point statistical functions; however, we the example of Chua system, or Chua’s computed them using estimates specially chaotic oscillator. We built, tuned and tailored for chaotic systems and, in operated a Chua electronic circuit and particular, estimates suitable for a very compared its recorded time series of long data series. Such estimates should signals with the predictions of the theory prove to be appropiate metrics for large and numerical simulations. Our realization data sets recorded in economics and social of the circuit involves an important new sciences. Finally, in a number of modes aspect: the construction of the nonlinear determined by a wide range of parameter element – the so-called Chua’s diode – values, we described chaotic states using allows an asymmetric V-I characteristic. models based on approximate scaling at The latter renders new asymmetric chaotic the onset of chaos [2]. states and attractors. The pre-chaotic and the chaotic modes of the system were 1.2. Pattern formation studied experimentally and analyzed A new empirical model for the adhesion employing both qualitative and numerical of metal nanoparticles on a MgO{100} methods. The values of the parameters, at substrate was developed. The new model which bifurcations occur, were evaluated corrects the known bias of previous and measured. To ascertain the transition models toward structures with large to chaos we computed the Lyapunov interfaces with the substrate due to the exponents using an algorithm developed in failure to account for the so-called “metal- the Laboratory. The statistics of the on-top” effect, i.e., the enhancement of the chaotic states was estimated and described adhesion due to the presence of other using models based on approximate metal atoms on top of those that are scaling at the onset of chaos [1]. directly in contact with the substrate. The Extending further the scope of the new model is parameterized using density- study, we presented results related to the functional theory calculations on MgO- statistical properties of simple 3D supported Pd clusters with sizes up to 80 continuous chaotic systems. The core of atoms. The potential proposed is 73 Annual Report IE 2013

continuous with respect to the spatial shore. Characteristic profiles, determined coordinates and can, therefore, be for every cluster, may be used as typical used directly in molecular dynamics inputs to the electromagnetic models simulations [3]. applied for further characterization of the Molecular dynamics simulations of propagation channel for the given place vapor deposition of Pd and Rh on Au [5]. surfaces were carried out, which helped to gain insight into the factors that may have 3. Bibliometric indicators and influenced an unexpected interface distributions structure and chemical ordering in Pd/Au and Rh/Au nanorods observed using The interrelations were studied between aberration-corrected scanning transmission the most widely used scientometric electron microscopy. These structures indicators for several more or less realistic cannot be explained by the key structural citation-paper rank distributions. Analysis and energetic parameters of either system was provided for both continuous and in isolation, thus demonstrating the discrete representations and was illustrated advantage of taking a comparative further on examples for simultaneous time approach to the characterization of evolution (during a scientific career) and complex binary systems. This work computed using real life scientometric highlights the importance of achieving a data. The aim of the study was to fundamental understanding of reaction illuminate specific properties of the kinetics in realizing the atomically indicators, the pros and cons of their use in controlled synthesis of bimetallic various situations (citation-paper rank nanocatalysts [4]. distributions) and (hopefully) to contribute for a fair and better scientific assessment. 2. Tropospheric ducting We obtained relations between scientometric indicators, including total The performance of microwave radars number of citations, Hirsch’s, Egghe’s and and communications systems in coastal Zhang’s indexes for model citation-paper and maritime regions is often affected by rank distributions in continuous (uniform, the tropospheric ducting propagation linear negative-slope, three-parameter mechanism. Understanding this polynomial, three-parameter (positive mechanism requires application of exponent) power-law, exponential and sophisticated propagation prediction Pareto) and discrete (zeta, Zipf and models for which refractivity profiles are Kronecker-type) versions. In particular, we in essential input. In a previous stage, demonstrated that the Hirsch index ignores refractivity profiles for the Bulgarian a considerable amount of citations to the Black Sea coast were restored and metrics highly cited papers; this effect is more for their organization in clusters pronounced for distributions of convex introduced. The introduced metrics are type and/or where the ratio of the site-specific and depend not only on the maximum citation counts (number of peculiarities of the refractivity profiles, but citations gained by the most cited paper) also on the heights provided by the and the number of publications TL799L91 model of the European Center significantly deviates from unity. Further for Medium-Range Weather Forecasts on, Egghe’s g-index up to a certain limit used for profiles’ restoration. On the basis accounts best for the highly cited papers, of these metrics, four clusters, two with compared to the other two indexes; surface (or surface-based) and two with however, above this limit (where the total elevated inversions, were differentiated for number of citations exceeds the square of the northern part of Bulgarian Black Sea the total number of papers) the g-index 74 Laboratory Microwave Physics and Technologies

reaches its maximum value (equal to the AuRh and AuPd nanorods Nanoscale total number of papers). In such regime of 5 7452–7 saturation, the g-index accounts for all of 5. Csurgai-Horváth L, Frigyes I, Pontes the citations and will grow only if the total M S, Sirkova I 2013 Channel models number of papers (even of zero citation for fixed terrestrial communication count) is increased. This effect takes place systems in: Propagation tools and data for a finite number of papers only, and the for integrated Telecom, Navigation paper count where saturation occurs for and Earth Observation Systems convex distributions is greater than that for IC0802 Chap. 8 Final Report concave ones. Therefore, since the g- 6. Atanassov V and Detcheva E 2013 saturation occurrence might easily be Citation-paper rank distributions and overlooked, care is needed when associated scientometric indicators – a computing and comparing this index [6]. survey Int. J. Information Models and Analyses 2/1 46-61 PUBLICATIONS ONGOING RESEARCH PROJECTS 1. Taslakov M and Yordanov O 2013 Chaos at fifty on the example of 1. Action COST TD1210: Analyzing the Chua’s oscillator Proc. 2nd Nat. dynamics of information and Congress of Phys. (25-29 September knowledge landscapes 2013 Sofia, Bulgaria) pp 418-9 (KNOWeSCAPE), 2013/2017. 2. Yordanov O 2013 Analyzing the 2. Action COST MP0903: Nanoalloys dynamics of information and as advanced materials: from structure knowledge landscapes Chaos at fifty: to properties and applications A statistical perspective (First Annual (NANOALLOY) , 2011/2014.. Meeting of COST Action TD1210 18 - 20 November 2013 Espoo, LABORATORY VISITS Finland) http://knowescape.org/wp- content/uploads/2013/11/yordanov.pdf O. Yordanov 3. Atanasov I, Barcaro G, Negreiros F R, • COST TD1210 – Kickoff meeting, Fortunelli A and Johnston RL 2013 Brussels, Belgium, 06-08 June, 2013 Modelling the metal-on-top effect for • COST TD1210, First Annual Pd clusters on the MgO{100} meeting, Aalto University of Science substrate. J. Chem. Phys. 138 224703 and Technology, Finland, 20-23 4. Chantry R L, Atanasov I, November, 2013. Siriwatcharapiboon W, Khanal B P, Zubarev E R, Horswell S L, Johnston R L and Li Z Y 2013 An atomistic view of the interfacial structures of

75 Annual Report IE 2013

LABORATORY

MICROWAVE MAGNETICS

HEAD: Prof. I. Nedkov, Dr.Sc.

TOTAL STAFF: 8 RESEARCH SCIENTISTS: 7

Assoc. Prof. T. Koutzarova, Ph.D.; S. Kolev, Ph.D.; L. Slavov, Ph.D.; Ch. Ghelev M.S.; M. Uliverova M.S.; Tz. Biandova M.S. R. Angelova, Ph.D. student.

RESEARCH ACTIVITIES and M.S. students in the framework of ERASMUS European Program. Some of The Microwave Magnetics Laboratory Laboratory’s researchers are involved in (known also as Gyromagnetic Electronics experimental work in international Laboratory) at the Institute of Electronics scientific projects under the COST conducts research and training activities Program and bilateral cooperations. and dissemination of knowledge in the field of Magnetoelectronics. In 2013, a 1. New magnetic materials for predominant part of its team’s activities has electronics based on magnetic been aimed at preparing and studying new multiferroics nanostructured magnetic materials – bio- oxides, ferrite particles, hybrid structures, Single-domain nanostructured particles composites, thin films – and studying their of the Y-type hexagonal ferrites with interaction with electromagnetic radiation general formulas Ba2Mg2Fe12O22 and in a wide range of the radio- and Ba0.5Sr1.5Zn2Fe12O22 were produced by a microwave spectrum. The scientific soft chemical technology developed in the infrastructure and the potential of the Laboratory. The two magnetic phase laboratory staff have provided opportunities transitions were monitored by measuring for interdisciplinary research and the magnetization as a function of the capabilities of combining scientific studies temperature. The results show that the with technological approaches. changes in the magnetization behavior of Scientists from the Laboratory were this kind of hexaferrites are related to the actively involved in projects dealing with multiferroic effect. Traces of a second investigation of new materials for ferrite phase with a spinel structure were electronics and ecology based on biogenic discovered in the Y-type hexaferrites, iron oxides and with low-temperature which prompted us to study its influence multiferroics of nanostructured materials, on the electro-magnetic properties of the both financed by the Bulgarian National ferrites. The studies indicated that this Science Fund. The Laboratory staff closely second phase does not affect the collaborates with groups at the Faculties of multiferroic transition of the Y-type Physics and Biology of St. Kliment hexaferrites. This work was a part of a Ohridsky University of Sofia, groups at the collaboration with Josef Stefan Institute, University of Chemical Technology and Ljubljana, Slovenia, Project DHTC Metallurgy, Sofia, as well as with groups 01/4/2011 "Room Temperature at a large number of BAS institutes. The Multiferroics Based on Y-type Laboratory staff conduct high-level Hexaferrites"; contacts were also individual education courses for the B.S. established with colleagues of COST 76 Laboratory Microwave Magnetics

Action MP0904 "Single- and Multiphase IE. The preliminary results of the phase Ferroics and Multiferroics with Restricted and structure analyses of the films thus Geometries" (SIMUFER). In the year prepared pointed to the presence of a new reported, we started investigations on thin magnetic phase, which prompted us to multiferroic films of Ba2Mg2Fe12O22 renew our collaboration with the LCIS obtained by pulsed laser deposition in group at the University of Liege, Belgium. collaboration with our colleagues of the Table 1 summarizes the functional Micro- and Nano-photonics Laboratory of magnetic properties of these new materials.

Table 1. Magnetic properties of Ba2Mg2Fe12O22 and Ba0.5Sr1.5Zn2Fe12O22. T, M (60 kOe), M , H , Sample r c K emu/g emu/g Oe

Ba2Mg2Fe12O22 300 22.78 1.74 31.35 sol-gel self-combustion Ba2Mg2Fe12O22 4.2 30.47 9.69 545.87 sol-gel self-combustion Ba2Mg2Fe12O22 300 24.95 1.26 12.21 sonochemical synthesis Ba2Mg2Fe12O22 4.2 33.57 1.94 28.95 sonochemical synthesis Ba0.5Sr1.5Zn2Fe12O22 4.2 40.67 5.10 147.21 sol-gel self-combustion Ba0.5Sr1.5Zn2Fe12O22 4.2 56.49 6.61 313.39 sonochemical synthesis

2. New magnetic materials for conducted jointly with scientists of the electronics based on biogenic iron oxides Faculty of Biology of St. Kliment and hybrids Ohridsky University of Sofia working on the laboratory culturing of the iron The research performed under project oxidizing bacterial (FeOB) strains. The DID 02/38/2009 "New Materials for possibilities for application were explored Electronics and Ecology Based on together with researchers from the Institute Biogenic Iron Oxides of Nanosized Ferrite of Catalysis and the Institute of Particles", financed by the Bulgarian Electrochemistry and Energy Systems of National Science Fund and coordinated by the BAS. Three types of functional crystals the Institute of Electronics, yielded new were found in the biological subproducts: data on the structural, physico-chemical lepidocrocite (γ-FeO(OH)); magnetite and functional properties of bioferroxides (Fe3O4) and goethite (α-FeO(OH)) suitable produced by bacteria of the for application in electronics and ecology. Leptothrix/Sphaerotius group. The studies In 2013, biophysicist R. Angelova started are of interdisciplinary nature and were her research towards the Ph.D. degree in 77 Annual Report IE 2013

the area of bio-nanostructured magnetic particles due to the disturbances in the materials. The research was carried out magnetite particles’ octahedral sublattice. under the project cited above (DID More than 20 % of the structure of a 02/38/2009) and COST Action TD1105 non-shielded nanoparticle are characterized "New Sensing Technologies for Air- by a disturbed octahedral sublattice and are Pollution Control and Environmental in fact forming a different magnetic phase, Sustainability" (EuNetAir). The magnetic properties of nanostructured particles are highly 0,45 0,016 ACM (214.6; 0.432) dependent on the surface layer and the 0,014 0,40 10 Oe, 1000 Hz interpretation of this influence vacillates 0,012 between "finite size effects" and "surface 0,35 (124.8; 0.357) 0,010 spin disorder". The results on 0,30 nanostructured magnetite in the single- 0,008 0,25 PPMS

', emu/g 0,006 Fe O '',emu/g domain state obtained by biotechnology 3 4 M 0,20 covered with M raise questions concerning the current 0,004 β-cyclodextrin physical models used in explaining the 0,15 0,002 occurrence of the Verwey effect, which 0,10 0,000 became more complicated due to the 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 T, K particle's surface modification. We investigated hybrid (magnetite/ - β Figure 1. A.C. magnetization of shielded cyclodextrin) nanostructured superpara- magnetite. magnetic particles which are similar to the bioproduct; the findings were compared with results on non-shielded particles of which does not undergo the same changes the same size prepared by the co- at low temperatures as the particle’s core. precipitation technique. The structural This effect is sensitive to the external studies indicated that in the latter type of magnetic field applied, so that, while an particles there is a surface layer with a effective magnetic anisotropy exist in a depth of 3 nm wherein an exponential rise in the number of vacancies is observed in non-shielded particle, monoclinic the octahedral sublattice, so that the deviations are present in an encapsulated particle surface is highly defective. The magnetite particle, which is the reason structural deviations result in significant why one observes a well-expressed changes in the magnetic properties of such transition temperature. On the other hand, particles (the magnetic moment is 35 % of due to the particle’s small volume, one that of bulk magnetite), leading to the may relate this non-linearity to the absence of Verwey effect. For blocking temperature if В(Н) nanostructured superparamagnetic measurements were only performed. In the magnetite encapsulated in an organic a.c. measurements, this is a well-expressed matrix, the magnetic moment is more than effect that allows one to connect it with the 80 % of that of the bulk and distinct Verwey temperature. An empirical model changes are observed in their magnetic for spherical single-domain particle was properties at about 120 K, which we relate developed in an attempt to describe the to the appearance of the Verway transition. A non-linearity in the magnetic evolution of structural defects in the permeability near the Verwey transition in surface layer of nanostructured figure 1 was clearly observed for the ferrospinels. The results of this hybrid particles only; we assume that such investigation were reported at the a transition cannot be seen in non-shielded Congress of Physics in Sofia. 78 Laboratory Microwave Magnetics

PUBLICATIONS 9. Ghelev Ch and Guerassimov N ed 2013 Institute of Electronics 50th 1. Koutzarova T, Kolev S, Ghelev Ch, Jubilee Annual Report 2012 Nedkov I, Vertryuen B, Cloots R, Henrist C and Zaleski A 2013 Differences in the structural PATENTS and magnetic properties of nanosized barium hexaferrite 1. Nedkov I and Merodiiska T powders prepared by single and Patent № 65609 Method for double microemulsion techniques preparation of nanosized ferrite J. Alloys Comp. 579 174-80 particles 2. Ivanova T, Harizanova A, Koutzarova T and Vertruyen B 2013 Optical and structural characteri- ONGOING RESEARCH PROJECTS zation of TiO2 films doped with silver nanoparticles obtained by sol– Financed by the EU Programs gel method Opt. Mater. 36 207-13 3. Penchev S, Pencheva V, Nedkov I 1. COST TD1105 EuNetAir: Air- and Krezhov K 2013 Laser double- pollution control and enviromental heterodyne integrated microscopy of sustainability. magnetoelectric nanofilms Vacuum 2. COST MP0904 Single- and 96 7–11 multiphase ferroics and multiferroics 4. Nedkov I and Koutzarova T 2013 with restricted geometries Magnetoelectronics and the new (SIMUFER). challenges before nanotechnologies 3. The use of grazing incidence X-ray J. Bul. Acad. Sci. 6 49-54 diffractometry (Bruker D8) to 5. Koutzarova T, Nedkov I, Kolev S, characterize the crystallographic Ghelev Ch, Krezhov K, Kovacheva D structure of polycrystalline magnetic and Zubov P 2013 Y-type thin films, ERASMUS Lifelong hexaferrite Ba2Mg2Fe12O22 powders Learning Program. – new multiferroics Proc. 5th Int. 4 Investigation of polycrystalline Conf. Systematic Synthesis and multiferroic materials - training on Synergetic 70-6 (Russia 2013) using equipment and techniques, 6. Angelova R, Iliev M, Mitova M and ERASMUS Lifelong Learning Groudeva V 2013 Optimization of Program. the cultivation of neutrophilic iron bacteria from the group Sphaerotilus Financed by the National Science Fund – Leptothrix, Proc. Microbiologia Balkanica'13 45-6 (Bulgaria 2013) 1. Project DID 02/38/2009 New 7. Svetlana V, Angelova R, Stoyanov L, Materials for Electronics and Grudeva V, Kovacheva D, Ecology Based on Biogenic Iron Mladenov M and Raicheff R Oxides of Nanosized Ferrite 2013 Biogenic iron oxide-based Particles. nanocomposite electrodes for hybrid 2. Project DHTC 01/4/2011 Room battery-supercapacitor systems Proc. Temperature Multiferroics Based on 15th Int. Workshop Nanosci. and Y-type Hexaferrites. Nanotechnol. (Sofia 2013) 3. Project BG051ZD001-3.3.07-0002 8. Dimitrova M and Ghelev Ch ed „Student's practices", Human 2013 Proc. VEIT 2011 Resources Operational Program. 79 Annual Report IE 2013

COLLABORATIONS 2 T. Koutzarova – University of Liege, Belgium, Structural Contributions to 1. Structural Contributions to the the Ferroelectric and Ferromagnetic Ferroelectric and Ferromagnetic Effects of Y-type Multiferroics, 14 Effects of Y-type Multiferroics, days. University of Liege, Liege, Belgium. 3 S. Kolev – Novi Sad, Serbia, The 2. Cooperation in Education and Third Workshop for Early Stage Scientific Research with the Researchers - COST MP0904 Communications and Physical Single- and multiphase ferroics and Electronics Chair of the Faculty of multiferroics with restricted Physics of St. Kliment Ohridski geometries (SIMUFER), 6 days. University of Sofia. GUESTS LABORATORY VISITS 1. R. Closset, University of Liege, 1. I. Nedkov – Barcelona, Spain, Belgium, 8 days, Joint Research Management Committee Meeting of Project Structural Contributions to COST Action TD1105 (New the Ferroelectric and Ferromagnetic Sensing Technologies for Air- Effects of Y-type Multiferroics Pollution Control and Environmental between IE-BAS and University of Sustainability – EuNetAir), 4 days. Liege, Belgium.

80 Laboratory Physical Technologies

LABORATORY

PHYSICAL TECHNOLOGIES

HEAD: Prof. Petar Petrov, Dr.Sc.

TOTAL STAFF: 5 RESEARCH SCIENTISTS: 4

D. Dechev, Assistant Eng.; N. Ivanov, Assistant Eng.; Ch. Georgiev, Eng.; M. Ormanova, Ph.D. student.

RESEARCH ACTIVITIES Two groups of multilayer structures were thus prepared by succesively depositing 1. Fabrication and study of thin nanolayers of ТiN/WN and ТiN/МоN. aluminium coatings Using transmission electron microscopy and scanning electron diffraction, the A technology was developed of microstructure and crystal-phase content preparing a thin aluminium coating on a of a single film of MoN with a thickness of glass substrate. The coatings were d = 25 nm were studied. The analyses deposited by using the technique of DC demonstrated that the single MoN film magnetron sputtering on substrates of flat deposited on a NaCl monocrystal had a potassium glass (size of 30×30×4 mm); phase content corresponding to crystalline microscope-grade glass (30×20×1 mm); Мо2N with a cubic crystal lattice. The film and optical-grade glass (OPTIX Ltd., was homogeneous with a polycrystalline Panagyurishte, ∅25×6 mm). Two types of monodisperse structure. On a measurement coatings were formed with different scale of one micron, the size of the thicknesses. The first type were single- nanocrystals varied between 50 and layer aluminium coatings with thicknesses 100 nm. The phase content of a single TiN d1 = 300 nm and d2 = 1200 nm. The film was studied by means of XRD. The second type were two-layer coatings, the results showed that the predominant phase first layer being of pure aluminium, the was δ-ТiN along the (200) crystallographic second, of Аl2О3. The effect was studied axis with a wall-centered cubic lattice. For of using various techniques of preliminary this phase, the lattice parameter calculated surface cleaning, as well as that of the was a = 0,425 nm, which is practically main technological regime parameters, in identical with that of stoichiometric TiN view of improving the adhesion of the (а = 0,424 nm). aluminium coatings. The technology developed can find applications in fabricating single- and multilayer coatings 3. Preparation and study of on optical lenses, mirrors, reflectors, nanostructured coatings on polymers selective optical filters, protective screen glasses, and decorative coatings. Reactive magnetron sputtering was applied to produce thin metal films of pure 2. Multilayer nanostructured coatings for titanium and of the chromium-nickel alloy wear-resistance applications X18H9T on the following polymers: impact-resistant polystyrene PS BS 793, Reactive magnetron sputtering in crystalline polystyrene PS BS 190; and vacuum was used to form multilayer polyacetal POLIPOM–POM. The film nanostructured nitride-based thin films. deposition process parameters were 81 Annual Report IE 2013

established, namely, working pressure, PUBLICATIONS discharge current and voltage, target- substrate distance, together with the 1. Milusheva P, Nikolov N, Ivanov N preliminary substrate cleaning techniques and Dechev D 2013 Preparation of – mechanical and chemical cleaning and Ni-Cr coatings on polymer substrates high-energy ion bombardment were considered. by DC magnetron sputtering J. Balkan A considerable increase in the surface Tribological Association 3 272–5 hardness and rubbing resistance was found ISSN:1310-4772 for all film-substrate systems. The best 2. Dechev D, Ivanov N, Petrov P, results were obtained in the case of a Yordanov M and Uzunov Ts 2013 X18H9T coating on PS BS 190 Multilayer nitride-based nano- polystyrene (HV = 1586 MPa), with the structured materials Machine-building substrate hardness being HV = 141 MPa. and Machine Sci. 2 48–53 ISBN: The rubbing resistance was the highest for 1312-8612 a Ti coating on POLIPOM–POM (0.01 %) polyacetal. 3. Dechev D, Ivanov N and Petrov P A technology was developed for 2013 Obtaining and studying the forming copper coatings by using properties of aluminium thin films magnetron sputtering deposition on J. Technical University of Sofia polymer materials – BS 190 and BS 793 Plovdiv Branch 19 105-8 ISSN: 1310- polystyrene and polyethylene 8271 terephthalate; the technological parameters 4. Ivanov N, Dechev D, Petrov P and of the process were established. Milusheva P 2013 Studying the

mechanical properties of Ti and Cr-Ni 4. Study of nanosystems and new materials by neutron beams thin films J. Technical Universityof Sofia Plovdiv Branch 19 223-7 ISSN: A technology was developed for 1310-8271 reconstituting Charpy specimens of 5. Milusheva P, Nikolov N, Ivanov N nuclear reactor vessel steels (18MND5 и and Dechev D 2013 Formation of Cu 15Х2НМФА) by means of electric-arc, coatings on polymer surfaces by ion electron-beam and laser welding. Using sputtering Proc. Int. Sci. Conf. 3 pp neutron diffraction, the stress was studied 154–6 (Gabrovo Bulgaria) ISSN: arising due to electric-arc, electron-beam and laser welding of samples of reactor 1313-230Х vessel steel (18MND5). The studies were 6. Petrov P, Dechev D, Ivanov N and conducted on a Fourier diffractometer Krastev V 2013 Comparative study on during irradiation by fast neutrons in the carbon nitride thin films obtained by IBR-2 pulsed reactor at the Joint Institute CVD and PVD methods Proc. 5th Int. for Nuclear Research, Dubna, Russia. It Conf. FMNS 3 pp 65–71 was found that the stress within the (Blagoevgrad, Bulgaria) ISSN: 1314- welding beam zone and the thermally- 0272 affected zone in the case of laser welding exceeds that following electron-beam 7. Petrov P 2013 Physical vapor welding. The highest values of residual deposition of carbon nitride thin films stress were measured in samples welded Proc. Nat. Conf. Electronic, using the electric-arc technique. Infromational and Communication 82 Laboratory Physical Technologies

Systems (Sofia Bulgaria) ISSN: 1314- Financed by the Joint Institute for Nuclear 8605 Research, Dubna, Russian Federation

8. Petrov P 2013 Advanced welding Study of nanosystems and new materials processes for reconstitution of fracture by neutron beams. mechanics test specimens Proc. Int. Conf. Metallurgy of Welding and Financed by the Scientific Research Fund Welding Materials pp 263-73 (3–4 of Asen Zlatarov University, city of Burgas

June 2013 St Petersburg Russia) Using waste-water treatment end-products 9. Petrov P 2013 Hybrid technology for concentrates for metal coatings deposition hard coatings – electron and laser on polymer materials. beam surface treatment Proc. Int. Symp. Welding Technology pp 13-24 COLLABORATIONS (5-6 November 2013 Cottbus Chair of Welding and Joining Technology Germany) ISSN: 1867-4887 at the Technical University of Brandenburg – Cottbus, Germany – ONGOING RESEARCH PROJECTS Рroduction of new materials and nanotechnologies. Financed by the BAS

Nanostructured multilayer coatings for wear resistance applications.

83 Annual Report IE 2013

PARTICIPATION OF THE INSTITUTE OF ELECTRONICS IN THE NATIONAL NETWORK OF REGIONAL ACADEMIC CENTERS

In 2013, the leadership of the Bulgarian integration of the BAS institutes with the Academy of Sciences initiated the creation Technical University of Sofia’s Sliven of a National Network of Regional Faculty and College, as well as with Academic Centers (RACs) in almost all manufacturing and trade companies active Bulgarian regions with the aim of in Sliven Region. This tripartite establishing the prerequisites and cooperation is expected to bring about conditions needed for adequate conditions for more efficient cultural, cooperation and intergration between the scientific and educational activities thus BAS institutes (and their divisions in the contributing to the overall development of respective Bulgarian regions) and the Sliven Region through its cultural, universities, municipalities and educational and business structures. industrialists located within these regions’ On the occasion of the Bulgarian territory. Academy of Sciences’ 145th anniversary, RACs were established in the main in March 2014, Bulgarian Science Days regional cities of Sofia, Plovdiv, Stara were held in the city of Sliven, organized Zagora, Sliven, Bourgas, Dobrich, by the Bulgarian Academy of Sciences, the Montana, Vratsa, Gabrovo, Shoumen, Sliven RAC, the Sliven Municipality, the Pleven, Veliko Turnovo, Rouse and Technical University of Sofia’s Sliven Blagoevgrad, all being large municipalities Faculty and College and Sliven Regional characterized by a strong academic Museum of History. presence – universities, higher schools, Prior to the above scientific event, a colleges – as well as large manufacturing press conference took place with the companies. participation of Academician Konstanin The RACs organized across the country Kosev, Academician Evgeni Golovinski, are meant to become hubs generating Corresponding Member Dimitar Dimitrov, opportunities for applying an Prof. Dr.Sci. Garabed Minasyan, Prof. interdisciplinary approach to the scientific Dr.Sci. Petar Petrov, Prof. Dr.Sci. Eng. research and education, but, most of all, Stanimir Karapetkov and Eng. Kolyo generating results that can be applied Milev, Mayor of Sliven Municipality. As efficiently in assisting national and Corresponding Member Dimitar Dimitrov municipal institutions and the industry. pointed out, Mr. Milev has shown more Amalgamated voluntarily, the RACs than once his openness to a dialog in view partners agree to cooperate concerning the of scientific research cooperation, and, activities performed and the goals set. being a champion of more efficient results At the beginning of the month of July of such cooperation, he has always sought 2013, a RAC was established in the city of ways of finding more investments and Sliven with the active participation and innovative solutions in the administrative support of the Institute of Electronics. and business activities. In October 2013, the Sliven RAC held In the framework of the Bulgarian the first scientific conference of the Science Days, Assoc. Prof. Dr. Rouzha National Academic Network of the Simeonova, Director of BAS’s Scientific Bulgarian Academy of Sciences whose Archives, opened in May Gallery the Dear topic was “Higher Education – Science – Motherland Documentary Exhibition Innovations – Public-Private Partnership”. dedicated to the life and work of Tsvetan During the conference, the possibilities Radoslavov. The exhibition presents were discussed for cooperation and documents, photographs and a part of the 84 Laboratory Physical Technologies

artistic heritage of Bulgaria’s National Innovations in Bulgaria”. It was Anthem author. Most of the exhibits are emphasized that the “Horizon 2020” largely unknown to the wide public. Program should be focused on Furthermore, a round table was held on transforming the scientific research the topic “Cultural-Histrorical Heritage achievements into innovation products and and Influence of Science and Education on services that would provide opportunities the Socio-Economic Development of the for the further progress of higher education City of Sliven and Sliven’s Region”. and scientific research institutions and of Lectures were delivered by Acad. the industry, thus bringing about positive Konstantin Kosev and Acad. Evgeni changes in the life of the entire population. Golovinski, as well as by Prof. Dr.Sci. Prof. Dr.Sci. Vasil Kavardjikov, Diana Gergova and Assoc. Prof. Dr. Director of the Institute of Mechanics of Georgi Nehrizov of the National Institute the BAS, presented projects financed by of Archeology with Museum of the BAS, the Bulgarian National Innovations Fund; Prof. Dr.Sci. Lozanka Peycheva and Assoc. Prof. Dr. Neli Koseva, Director of Assoc. Prof. Dr. Valya Raycheva of the the Institute of Polymers of the BAS, Institute of Ethnology and Folklore Studies presented this Institute’s innovation with Ethnographic Museum of the BAS, potential; Prof. Dr.Sci. Venko Beshkov, Assoc. Prof. Dr. Marin Dobrev, Director Director of the Institute of Engineering of Stara Zagora Fine Arts Gallery, and Dr. Chemistry of the BAS, acquainted the Vladimir Demirev and Nikolay Sirakov of audience with the possibilities of Sliven Regional Museum of History. producing energy by renewable energy During the following discussion, the sources; Assoc. Prof. Dr. Tanyu Bonev, prospects were considered of the Director of the Institute of Astronomy of archeological, cultural and art studies in the BAS, shared ideas on how to attract the region. young people to careers in natural In the framework of the “Horizon 2020 sciences. Prof. Dr.Sci. Latchezar Avramov – Science and Innovations” Conference, of the Institute of Electronics of the BAS the possibilities were presented for funding delivered the lecture “Biophotonics – a of projects by the EU. The EU program for Revolution on the Science-Technology scientific research, development and Interface”; and Assoc. Prof. Dr. Krasimir innovations for the 2014—2020 period is Spirov of the Pedagogy and Management among the BAS main priorities. Prof. Chair of the Technical University of Dr.Sci. Garabed Minasyan and Sofia’s Sliven Faculty and College Corresponding Member Filip Filipov elaborated on the place and role in the delivered lectures on the topics society of the pedagogical science and “Investments and Economic Growth” and education. “Problems of Technologies and

SELECTED PROJECTS

● Modeling and Simulation of Gyrotrons for ITER ● 3D Femtosecond Laser Microprocessing of Biomaterials for Application in Medicine ● Light-induced Atomic Desorption (LIAD) for All-optical Control of Light ● Velocity Distribution of Alkali Atoms in Micrometric Thin Cells ● Room-temperature Multiferroics Based on Y-type Hexaferrites

87 Annual Report IE 2013

EC–Project EURATOM of FP7

Contract № FU07-CT-2007-00059/Fusion CSA/EURATOM

MODELING AND SIMULATION OF GYROTRONS FOR ITER

S Sabchevski1, M Damyanova1, I Zhelyazkov2, P Dankov2, P Malinov2, E Balabanova1, E Vasileva1 and R Enikov1

1Emil Djakov Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria, Association EURATOM-INRNE 2Faculty of Physics, St. Kliment Ohridski University of Sofia, 5 James Bourchier Blvd., 1164 Sofia, Bulgaria Association EURATOM-INRNE

1. Motivation, scope and main activities than 0.96 MW at the matching optic unit of the project (MOU); (ii) output frequency of 170 ± 0.3 GHz; (iii) pulse length 3600 s; (iv) RF ef- High-power gyrotrons with megawatt ficiency not less than 50%; (v) Gaussian output power and frequencies ranging content of the wave beam greater than from 77 to 170 GHz are necessary for 95%; (vi) frequency of power modulation electron cyclotron resonance heating 3-5 kHz; (vii) reliability not less than 95%. (ECRH), electron cyclotron current drive For DEMO, however, gyrotrons operating (ECCD) [1, 2] and ECH-assisted start-up at 230–250 GHz, with step-tunability and of magnetically confined plasmas in vari- broadband window are considered in the ous reactors for controlled thermonuclear current design. A step-tunable 1 MW gyro- fusion (tokamaks and stellarators), as well tron with a frequency range 105–163 GHz as for plasma control and stabilization has been designed and tested by IHM-KIT. (e.g., NTM suppression and MHD con- For CTS on LHD a series of tubes (second trol). The gyrotrons are used also for harmonic 398 GHz/83 kW, and fundamen- plasma diagnostics based on collective tal 295 GHz/0.22 MW, 77 GHz/1.9 MW) Thomson scattering (CTS). Typical tubes are under development. The breakthrough that illustrate the state-of the-art in the de- achievements demonstrated recently by velopment of gyrotrons for fusion are: me- these gyrotrons are stipulated by advanced gawatt-class CW gyrotrons with frequen- designs concepts that are based on the use cies of 77 GHz for LHD, 110 GHz for JT- of: high performance electron-optical sys- 60SA and DIII-D, 140 GHz for W7-X, 170 tem (EOS); internal quasi-optical mode GHz for ITER developed in EU, Japan, converter; output window of synthetic Russia, and US [3]. For example, the CVD-diamond; and depressed collector for planned ECH and current drive energy recovery (recuperation). At the (ECH&CD) system of ITER requires 24 same time, despite the remarkable and MW installed power (20 MW delivered to promising achievements of the European 2 the tokamak in a CW mode for 60 min) at MW/170 GHz coaxial gyrotron, a number 170 GHz and additional 3 MW at 120 GHz of problems have been encountered in an (~3 s) for assisting the initial breakdown. attempt to demonstrate a stable long-pulse According to the technical specification of operation in a CW mode. Due to these the ITER project, the requirements to the problems, F4E and EGYC (European GY- gyrotrons are: (i) output power not less rotron Consortium, which includes CRPP, 88 Selected Projects

Switzerland; KIT, Germany; HELLAS, It is now commonly accepted and well Greece; CNR, Italy; ENEA, Italy, Thales, understood that the capabilities of the si- France) have decided to switch to a con- mulation tools (physical models and nu- ventional cavity 1 MW CW 170 GHz merical codes) are of crucial importance TE32,9 mode backup gyrotron replacing the for a successful computer-aided design EU 2 MW CW 170 GHz coaxial-cavity (CAD) and optimization of high perform- gyrotron. Currently, KIT is finalizing the ance gyrotrons with improved operational construction and testing (due to be com- characteristics (higher efficiency, stability pleted in 2014) of a novel 140 GHz, 1 MW of the operation, mode purity, high-quality CW gyrotron with an improved design for RF beam with a maximized Gaussian con- the W7-X stellarator in Greifswald, Ger- tent etc.). All this motivates the research- many [4]. In parallel, a feasibility study of ers involved in the development of gyro- a gyrotron operating at 240 GHz for future trons for fusion to work on the improve- plasma fusion devices is underway at KIT ment of the available simulation tools and as well. The main aim is to select an ap- on the development of novel more ade- propriate operating mode and to optimize quate physical models and software pack- the geometry of the cavity resonator in or- ages for numerical investigation and opti- der to achieve an output power of about mization of both the currently used and the 1.5 MW [4]. Tubes that generate at even future designs of these tubes. Our research higher frequencies (up to 300 GHz) and team has been involved in the maintenance allow frequency step-tunability are also and further development of a great number under consideration at present [4]. of standalone computer programs and According to the EFDA’s Work Pro- problem oriented software packages that gram 2014 (see WPHCD: H&CD Systems are being used in the course of the CAD of section of the Program) the planned activi- gyrotrons for fusion in the framework of ties for EC R&D include the design and the collaboration between the Institute for start of fabrication of a pre-prototype of a Pulsed Power and Microwave Technology step-tunable high frequency (250 GHz) at KIT Karlsruhe (Germany), Centre de gyrotron and an analysis of the integrated Recherches en Physique des Plasmas, gyrotron parts (subsystems), while the École Polytechnique Fédérale de Lausanne work planned in the EC Concept Design & (Switzerland), Institute of Electronics of Analysis section of the same Program is the Bulgarian Academy of Sciences and focused on a preliminary conceptual de- Faculty of Physics of Sofia University sign specifications of novel gyrotrons, HV (Bulgaria). The work on these topics is power supplies, main transmission lines, being carried out in accordance with a launchers etc. The Work Plan for the Im- novel concept for further improvements to plementation of the Fusion Roadmap in the available simulation tools as well as for 2014-2018 also includes tasks related to development of a new generation of nu- the development and analysis of gyrotrons merical codes based on more adequate for fusion. Among them are: (i) analysis of physical models, efficient numerical meth- the possible designs of broadband output ods and algorithms, advanced (state-of- windows (e.g. Brewster angle windows, the-art) computational platforms [5–8]. double-disc cavities, movable double disc The main results obtained so far have been arrangements etc.) and selection of appro- published in references [9–15]. It should priate technologies for their manufactur- be noted also that in recent years the inter- ing; (ii) study, design and testing (2015- national collaboration of the Bulgarian re- 2018) of multistage depressed collectors; search group has been extended to FIR FU (iii) cold testing of various gyrotron parts Research Center at the University of Fukui (2016-2018); (iv) prequalification of gyro- (Japan). Although the gyrotrons that are tron prototypes (2019). being developed at FIR FU (except these 89 Annual Report IE 2013

for CTS plasma diagnostics) are dedicated alization as well as comprehensive and to other applications, the fact that they op- detailed documentation. Since the whole erate on the same physical principles set of numerical tools is heterogeneous makes it possible to use common models (i.e., different codes are implemented in and numerical codes. Therefore, such col- different algorithmic languages and com- laboration leads to cross-fertilization of piled under different operating systems), ideas and approaches and is beneficial for its portability is ensured through the use of the development of gyrotrons for fusion server emulation and virtualization envi- ronments (e.g. DOSBox and VirtualBox). research.

The scope of the work on the develop- 2. Current status and functionality of ment of the simulation tools for numerical the problem-oriented software packages investigation, CAD and optimization of and the computational infrastructure gyrotrons includes: (i) formulation of ade- quate, self-consistent and informative Both the hierarchy and structure of the physical models; (ii) selection of efficient simulation tools are presented in figure 1 numerical methods and algorithms, pro- together with the computational platforms gramming libraries, integrated develop- on which the different packages are opera- ment environments (IDE), software for tional. The codes that are being maintained code optimization and debugging and their and developed are structured in several implementation and usage in the numerical problem-oriented software packages (most codes for simulation of gyrotrons; notably CAVITY, ESRAY, GYROSIM, (iii) development of pre-processing, proc- GYREOSS and DAPHNE) and are in- essing (computational), and post- stalled on the work-stations of the Bulgar- processing modules; (iv) maintenance, ian research team (see figure 1), except testing, benchmarking and improvements DAPHNE, which was available to us for of the codes; (v) planning and conducting remote execution and maintenance on the numerical experiments; (vi) analysis of the PLEIADES2 cluster from Sofia until the results and their use in the course of the end of March 2013, when this cluster was computer-aided design (CAD) of opti- de-commissioned. The most powerful of mized constructions of gyrotrons with im- our workstations have the following char- proved performance (e.g. increased effi- acteristics. ITER I has two CPU AMD Op- ciency, stability of the output parameters teronTM Dual Core 275, 2.2 GHz and in a CW mode of operation); (vii) integra- RAM 4 GB DDRAM with a MB Su- tion of all available simulation tools (new- permicro-Dual Opteron and SVGA Nvidia ly developed and various legacy codes) in GeForce 6600 TD. problem oriented software packages. As mentioned above, the work on all these aspects of both modelling and simulation of gyrotrons follows a well-defined novel concept [5–14] that imposes the following requirements on the codes under develop- ment: (i) portable to different computa- tional platforms and operating systems; (ii) extensible (possessing flexibility in adding new physics); (iii) efficient (using optimal numerical methods and algorithms and uti- lizing parallel calculations for minimiza- tion of the required computational re- sources); (iv) well-validated (being able to recover the results of the previously tested Figure 1. Structure of the simulation tools and 2-1/2 D codes; (v) user-friendly (offering computational infrastructure for numerical convenient pre- post-processing and visu- studies and CAD of gyrotrons. 90 Selected Projects

The workstation ITER II has 2 CPUs satile post-processing capabilities and Intel Xeon X5680, 3.33 GHz,12 MB visualization of all scalar and vector cache, 6 Cores; memory 4×4 GB DDR. On physical fields by color maps; (iv) fast both workstations the operating system is own solver for the boundary value prob- Ubuntu 10.04 (lucid), Kernel Linux lem by a finite difference method. The 2.6.32-41-generic. package consists of several modules: The installed packages and the underly- GRIDGEN (for geometry description and ing numerical libraries are undergoing mesh generation), MAGGEN (for calculat- constant adaptation and upgrade to the ev- ing the magnetic field of a system of sole- er-changing computational environments noids), ESRAYS (for iterative solution of (hardware, operating systems, and novel the self-consistent field problem), and versions of the compilers). Alongside with OVIS. The latter module serves as a GUI the maintenance of these codes and their and postprocessor that presents and visual- usage in numerical experiments, we are izes the results of the simulation. Similarly working on the further development of the to CAVITY, during 2013 this package was GYREOSS and GYROSIM packages. Bel- adapted and recompiled in order to be low we summarize briefly the current sta- compatible with the recent upgrade of the tus of the main simulation tools. software environments on the workstations The problem-oriented software package where it is being maintained. The capabili- CAVITY (IHM) [11, 12] consists of a hi- ties of the package have been demon- erarchy of codes that begins with simple strated by numerical experiments for programs (e.g., for an analysis of the mode analysis of EOS for powerful gyrotrons spectrum; cold cavity code, single mode [15]. self-consistent code) and culminates in the GYROSIM [13] is a problem-oriented most sophisticated self-consistent multi- software package which includes numeri- mode time-dependent code SELFT. The cal libraries and source codes of various codes are written in FORTRAN and are computational modules (standalone pro- invoked through a GUI. The GUI itself is grams, subroutines, pre-, post-processing, in fact a Tcl/Tk script for a Linux (Unix) and visualization codes) for solving a vari- bash shell that controls: (i) the interaction ety of problems pertinent to the simulation with the codes, (ii) the specification of the and CAD of gyrotrons using a rich set of input data, and (iii) the visualization of the adequate physical models. Its structure is results using a set of single commands in presented in figure 2. Unlike the packages the menu window. During 2013, the pack- described above, it is not, however, spe- age was modified (debugging, code re- cialized to only one subsystem of the gyro- finement) in order to improve its program tron tube. Rather, the individual compo- implementation and recompiled on the nents of GYROSIM are designed for simu- workstations using the latest versions of lation of all main subsystems of the gyro- the underlying development tools (numeri- tron tube, notably: (i) the electron-optical cal libraries and compilers). system (EOS), (ii) the magnetic system ESRAY (IHM) [8, 11, 12] is a problem- which includes the main magnet and an oriented package for trajectory analysis arrangement of additional coils, (iii) the (ray-tracing) of EOS based on a fully rela- electrodynamic system (resonant cavity), tivistic 2.5D electrostatic physical model. and (iv) the quasi-optical system for mode Its most characteristic distinguishing fea- conversion and transmission of the radia- tures are: (i) object-oriented program im- tion. It should be mentioned that the codes plementation in C++; (ii) advanced mesh for numerical modeling of the EOS (GUN- generator which discretizes the computa- MIG/CUSP) are based on a 2.5D physical tional domain with a great accuracy by model, which is analogous to the one im- structured boundary-fitted grids; (iii) ver- plemented in DAPHNE and ESRAY and, 91 Annual Report IE 2013

therefore, provide results that are consis- to add/replace easily different computa- tent and in a good agreement with each tional modules and in such a way to mod- other. Besides the differences in their pro- ify both the numerical algorithms and the gram implementation, GUN-MIG/CUSP, physical models implemented in the pro- however, allows magnetron injection guns grams. The latest upgrade of the GY- (MIG) with a reversal of the magnetic field ROSIM package was carried out in paral- (e.g., a magnetic cusp) that form axis- lel with the development of a novel mod- encircling (a.k.a. uniaxial) beams to be ule called GO&ART (which stands for simulated with an increased accuracy. Geometric Optics and Analytic Ray Trac- Similarly, the codes of GYROSIM for ing). It consists of several codes (RAYS, simulation of the electrodynamical system COMODES, and TRACE) for analysis of cover the same functionality as the CAV- quasi-optical components (Vlasov and ITY (KIT). At the same time, there are Denisov type launchers, reflectors and some notable differences between them. phase-correcting mirrors, and so on) as well as systems based on them (e.g., inter- nal mode converters and transmission lines). Recently, GYROSIM was used for development of a series of high-power sub-terahertz gyrotrons for a broad spec- trum of novel applications [16, 17]. The functionality of the latest version of the package was demonstrated by a series of numerical experiments presented in [15]. Initially, GYREOSS was conceived as a package of codes for simulation of EOS Figure 2. Structure of the package GYREOSS. using a physical model formulated in three space dimensions (3D) in order to take into For instance, the CAVITY (KIT) can account the departure from axial symmetry treat both conventional and coaxial resona- due to various misalignments (for instance tors but at fundamental operation while the of the electrodes, of the magnetic coils, cavity codes belonging to GYROSIM are etc.) and non-uniformities [10, 12, 14]. Its specialized only to cavities without an in- initial version was implemented using the sert (i.e., cylindrical resonators) but can gmsh package for meshing, pre- and post- simulate operation at the second (and in processing and GetDP as a solver. In re- the case of a large orbit gyrotron (LOG) cent years, however, GYREOSS has even higher) harmonics of the cyclotron evolved as a test bench for experimenting frequency. In its current form, the GY- with different numerical methods, solvers ROSIM is a heterogeneous package and and algorithms in 3D aiming at the final includes components written in different goal – a parallel 3D code for numerical languages (Fortran 77, Fortran 90, C, C++, simulation and CAD of EOS of gyrotrons. SciLab, and so on), operational and/or The latest version of GYREOSS is being portable to different computational plat- developed using the FreeFEM++ problem forms (ranging from laptops and worksta- solving environment. Recently, a novel tions to mainframe and supercomputers), field solver in both 2D and 3D was devel- and executable under different (genuine as oped. It provides the components of the well as emulated/virtualized) operating electromagnetic fields at the current parti- systems (e.g., Unix, Linux, Windows, cle locations for the relativistic particle Cygwin). Another characteristic feature of pusher in which the Boris–Buneman the package is that it is being built follow- scheme is implemented. During 2013, the ing a concept of extensibility which allows main focus of the work was the develop- 92 Selected Projects

ment of a fast charge-conserving algorithm pane shows the tetrahedral mesh used in for the space charge allocation (scattering 3D simulations and its cross-section. Maps or distribution) to the computational grid. of the electrostatic potential and the elec- tric field are plotted in figure 5 and figure 6, A characteristic distinguishing feature respectively. The electron trajectories in of GYREOSS is that it uses the advanced the studied EOS are traced by the relativis- visualisation tools provided by gmsh and tic particle pusher. Their projection in the FreeFEM++. These capabilities are illus- meridional cross-section of the electron trated in the following screenshots taken gun are plotted in figure 7. during different stages of the PIC simula- tions. Figure 3 shows the input of the ge- ometry description, which is used to pro- duce the meshes used in the computations.

Figure 5. Maps of the potential distribution Figure 3. Description of the geometry of a coaxial gyrotron in the CAD system gmsh.

Figure 6. Maps of the electrostatic field.

TRAJECTORIES Figure 4. Meshes generated for the geometry of figure 3.

In figure 4 the upper pane represents the boundary meshes at the electrodes (on which Dirichlet boundary conditions are Figure 7. Trajectories of the helical electron specified) and at the closing planes (with beam in coaxial magnetron injection gun Neumann boundary conditions). The lower (CMIG). 93 Annual Report IE 2013

3. Conclusions and outlook cyclotron H&CD system Fusion Eng. Des. 84 651-5 The outlined problem-oriented software [2] Omoria T, Henderson M A, Alba- packages are efficient tools for numerical jar F, Alberti S et al. 2011 Over- studies, CAD and optimization of gyro- view of the ITER EC H&CD sys- trons for fusion. Their codes are under tem and its capabilities Fusion continuous development and improve- Eng. Des. 86 951–4 ment. Recently they have been used in a [3] Thumm M 2013 State-of-the-art of series of numerical experiments carried out high power gyro-devices and free to study the designs of powerful gyrotrons electron masers (Update 2012) that are under consideration and/or devel- (KIT Scientific Reports) 7641 1– opment at present. The simulations con- 144 ducted provide a deeper physical insight [4] Jelonnek J, Avramidis K, Franck J, into the operation of high-performance gy- Gantenbein G et al. 2013 KIT gy- rotrons of megawatt class and demonstrate rotron development for future fu- the improved capabilities and functionality sion applications Proc. 38th Int. of the upgraded packages. Additionally, Conf. Infrared, Millimeter, and the results suggest some further experi- Terahertz Waves (IRMMW-THz) ments for more detailed study of the corre- [5] Sabchevski S, Zhelyazkov I, Be- lation between the beam-quality parame- nova E, Atanassov V, Dankov P, ters and efficiency, on one hand, and the Thumm M, Dammertz G, Piosczyk particular design (configuration of the B, Illy S, Tran M Q, Alberti S and electrodes, tailoring of the magnetic field, Hogge J-Ph 2006 Development of etc.), on the other. It is expected that the new generation software tools for novel and upgraded versions of the simula- simulation of electron beam forma- tion packages will contribute to the devel- tion in novel high power gyrotrons, opment of the next generation of high- J. Phys.: Conf. Series 44 96–101 power gyrotrons for fusion with improved [6] Sabchevski S, Zhelyazkov I, Be- performance. nova E, Atanassov V, Dankov P, Thumm M, Arnold A, Jin J and Acknowledgements Rzesnicki T 2006 Quasi-optical converters for high-power gyro- As a part of the scientific program of trons: a brief review of physical the Association-EURATOM-INRNE, the models, numerical methods and work on the Task 2.1.2 was supported by computer codes J. Phys.: Conf. the European Atomic Energy Agency Series 44 102–9 (EURATOM) and the Bulgarian Science [7] Sabchevski S and Zhelyazkov I Fund through the Association EURA- 2007 Modelling and simulation of TOM-INRNE”. The remote access to the new generation powerful gyrotrons resources of the Pleiades cluster at CRPP- for the fusion research J. Phys.: EPFL as well as the collaboration with the Conf. Series63 012003 gyrotron teams at CRPP-EPFL, Switzer- [8] Sabchevski S, Zhelyazkov I, land and IHM-KIT, Germany are greatly Thumm M, Illy S, Piosczyk B, appreciated. Tran T-M, Hogge J.-Ph and Pago- nakis J Gr 2007 Recent evolution References of the simulation tools for com- puter aided design of electron- [1] Darbos C, Henderson M, Albajar F optical systems for powerful gyro- et al. 2009 Progress in design and trons Comput. Model. Eng. Sci. 20 integration of the ITER electron 203–20 94 Selected Projects

[9] Sabchevski S, Illy S, Piosczyk B, [14] Damyanova M, Sabchevski S, Borie E and Zhelyazkov I 2008 Zhelyazkov I, Vasileva E, Bala- Towards the formulation of a real- banova E, Dankov P and Malinov istic 3D model for simulation of P 2013 Current status of the devel- magnetron injection guns for gyro- opment of the problem-oriented trons (A preliminary study) Wis- software package GYREOSS senschaftliche Berichte FZKA Proc. 18th Int. Summer School on 7409 1–34 Vacuum, Electron and Ion Tech- [10] Damyanova M, Sabchevski S and nologies (7−11 October, 2013, So- Zhelyazkov I 2010 Pre- and post- zopol, Bulgaria) processing of data for simulation [15] Sabchevski S 2013 Current status of gyrotrons by the GYREOSS and an outlook for further devel- software package J. Phys.: Conf. opment of the simulation tools for Series 207 012032 computer-aided design (CAD) of [11] Damyanova M, Kern S, Illy S, high-performance gyrotrons for Thumm M, Sabchevski S, Zhe- novel applications (Invited talk), lyazkov I and Vasileva E 2010 Int. Symp. Development of Tera- Modelling and simulation of gyro- hertz Gyrotrons and Applications trons for ITER Proc 4th Int. Work- (14-15 March, 2013, Fukui, Ja- shop & Summer School on Plasma pan). Available online at: Physics (5–10 July, 2010 Kiten, http://fir.u-ukui.ac.jp/IntSymTHz Bulgaria) Gyrotrons2013.pdf [12] Damyanova M, Balabanova E, [16] Idehara T and Sabchevski S 2012 Kern S, Illy S, Sabchevski S, Development and applications of Thumm M, Vasileva E and Zhe- high-frequency gyrotrons in FIR lyazkov I 2012 Simulation tools FU covering the sub-THz to THz for computer-aided design and range J. Infrared, Millimeter and numerical investigations of high Terahertz Waves 33/7 667-94 power gyrotrons J. Phys.: Conf. [17] Idehara T and Sabchevski S P Series 356 012044 2012 Development of high fre- [13] Damyanova M, Sabchevski S, quency gyrotrons in FIR FU cover- Zhelyazkov I, Vasileva E, Bala- ing sub-THz to THz range for ap- banova E, Dankov P and Malinov plications to high power THz spec- P 2012 Problem-oriented simula- troscopy Proc. Int. Kharkov Symp. tion packages and computational Millimeter and Submillimeter infrastructure for numerical studies Waves (MSMW) (23-28 June 2013) of powerful gyrotrons Proc. 5th p16 Available online at: Int. Workshop & Summer School http://dx.doi.org/10.1109/MSMW.- on Plasma Physics (25–30 June, 2013.6621990 2012, Kiten, Bulgaria)

95 Annual Report IE 2013

3D FEMTOSECOND LASER MICROPROCESSING OF BIOMATERIALS FOR APPLICATION IN MEDICINE

Project DMU 03/15 funded by the Bulgarian National Science Fund Project coordinator: Dr. Albena Daskalova Project period: 2011 – 2014

Laboratory of Biophotonics, Group Leader Prof. L. Avramov Acad. Emil Djakov Institute of Electronics Bulgarian Academy of Sciences, 72 Tsaridradsko Chaussee, 1784 Sofia, Bulgaria

Project goals Results

The principal goal of the regenerative Limited information is available on the medicine is to promote tissue regeneration interaction of high-energy ultra-short laser and healing after injury or disease that can pulses with thin collagen and gelatin films. be achieved through a delivery of cells The laser-induced formation of a and/or factors in tissue engineered nanometer-scale matrix of a biomaterial scaffolds designed to provide a biomimetic can be controlled by varying the number of microenvironment conducive to cell applied pulses, the pulse duration, and the adhesion, proliferation, differentiation, and laser fluence, which can be powerful tool host tissue integration. Scaffolds for controlling the cell proliferation. Our constructed from biocompatible polymers study showed that low and medium laser (gelatin, collagen and their blends) have fluence levels with a small number of laser been developed for the needs of skin tissue shots create the most suitable conditions replacement. The main demands to for producing nanoporous surface engineered scaffolds are: biocompatibility, modification (figure 1). biodegradability, high surface area for cell attachment, and a good mechanical integrity suitable for treatment handling. The surface topography has been shown to be a key issue in cell proliferation. The scaffold design should mimic the in vivo tissue microarchitecture and the cellular microenvironment. The biomaterials’ microstructure and mechanical properties influence the scaffold bioactivity. The goal of the present project was to demonstrate the effectiveness of applying femtosecond laser pulses to the modification of surfaces of natural biopolymers − gelatin, collagen and Figure 1. SEM images (2000×) of the surface collagen-elastin thin films − and the modification of a thin gelatine film irradiated formation of micro and nanoscale by 800-nm 30-fs laser pulses with laser fuence structures, as well as to study the F = 2.5 J/cm2: (a) N = 1, (b) N = 2, (c) N = 3, dependence of the thin-film structure (d) N = 4, (e) N = 5, (f) N = 10, (g) N = 25, evolution on the laser parameters. (h) N = 50, (i) N = 100.

96 Selected Projects

Figure 2 shows the structure evolution wave propagates radially from the LIOB as a function of the number of laser pulses center into the surrounding environment. applied. One can see that, as the number of The rapid energy deposition in a thin pulses is raised to N = 25 (figure 2d), the surface layer creates an extremely large structure of the porous matrix created loses energy density, leading to intense shock its porosity, the pores become occluded waves and a high pressure gradient in the and the modification is expressed mainly interaction zone. For a laser fluence of 14 2 in melting of the inner part of the area ≈10 W/cm a few microseconds after irradiated. In this respect, it seems possible irradiation, about 70 % of the absorbed to tune the structure to a specific need. energy is used by the expanding plasma to move the ambient gas, 20 % of the absorbed energy is lost as radiation to the environment, ultimately leaving less than about 3 % of the incident laser energy as heat inside the material. This thermal energy causes a thin layer of molten material to be formed immediately in the interaction zone. The lifetime of the molten layer (including both the melting and resolidification processes) depends on how quickly the energy is dissipated into the bulk. This in turn depends on the material’s thermal properties (heat conductivity, specific heat, density, etc). Controlling the morphology of the nanoporous matrix of biomaterial can be Higher SEM resolution images Figure 2. particularly powerful in controlling the (20 000×) of the surface modification of a thin gelatin film irradiated by 800-nm 30-fs laser cells’ behavior (cell alignment, densities, pulses with laser fuence F = 2.5 J/cm2: (a) orientation, migration, and localization). N = 1, (b) N = 2, (c) N = 5, (d) N = 25. By increasing the number of laser shots at 2 a constant laser fluence F = 6.4 J/cm and The interaction of the extremely high- a pulse duration of 30 fs the average size intensity ultra-short pulses with (diameter) of the surface nanopores transparent polymeric materials leads to created decreases from 400 nm to 100 nm. multiphoton absorption in the material. It was thus demonstrated that a control This results in ionization of some atoms over the pores dimensions of the and molecules thereby providing initial nanomatrix created, from 100 nm and up carriers. The free electrons and ions absorb to 500 nm, can be achieved by varying the energy from the electromagnetic field of number of pulses. the laser radiation by inverse Seeding of cells on functional bremsstrahlung resulting in their biocompatible scaffolds is a crucial step in acceleration. The subsequent avalanche- achieving the desired engineered tissue. In order to complete the cycle of like multiplication of free carriers finally experiments, human fibroblast cells were leads to a laser-induced optical breakdown seeded on a laser modified collagen-elastin (LIOB), rather than heat transfer, and to surface. Type I collagen is an extracellular the generation of microplasma and voxels matrix protein that is widely used as a in the bulk of the transparent material. In scaffold material since it provides the addition, owing to the expansion of the necessary adhesive properties and tensile heated plasma, a high-pressure transient strength. The elastin provides elasticity to 97 Annual Report IE 2013

tissues/organs and is crucial component of In conclusion, we performed a blood vessels. We cultured human systematic study of the influence of the fibroblast cells in a collagen-elastin porous laser pulse duration, pulse number, and matrix to examine the effect of fluence on the surface modification of thin proliferation. film of biopolymers − gelatine and We created a series of rows on a thin collagen. Irradiation in air of self-standing film of 9:1 collagen-elastin spaced by films of gelatin, collagen and collagen- 300 µm, the distance between each laser elastin with single and multiple laser spot being 50 µm. The matrix created was pulses at the wavelength of 800 nm irradiated by two pulses of 100 fs duration. produces a nanostructured layer with Fluorescent microscope observations submicrometric dimensions on the film showed the relation between the surface surface. topography and cells preferences during Femtosecond irradiation by different migration on the surface (figure 3). pulse temporal domains leads to the formation of the observed superficial fibrillar structures with different pores dimensions. Femtosecond laser modification is capable of producing a porous biopolymer matrix that mimics partially the structure and biological function of the extracellular matrix and can be potentially used for controlling cell behavior. It is tunable and can be used to design structures that affect cell proliferation, viability, and spreading. Figure 3. Fluorescence microscopy images of CFSE stained fibroblast cells cultured for 5 days on a 9:1 collagen-elastin porous matrix. Publications Magnification: 10× (A) and 40× (B). 1. Daskalova A, Nathala C, Bliznakova I, The cells in these images exhibited Stoyanova E, Zhelyazkova A, Ganz T, some migration after a 6-days incubation, Lueftenegger S and Husinsky W 2014 but they were still confined within the Controlling the porosity of collagen, patterns. The adhesion of the cells on the gelatin and elastin biomaterials by microstructured surfaces indicates that ultrashort laser pulses Appl. Surf. Sci. either the amount and/or orientation of the 292 367-377 (initial) protein adsorption are different on 2. Chandra N, Daskalova A, Bliznakova I, the topographically structured samples Lueftenegger S, Zhelyazkova A, than on the untreated ones. The Enikoe S, Ganz T and Husinsky W hydrophobicity or hydrophilicity of the 2013 Defined nano-structuring with surfaces are also a factor which influences ultrashort pulses in gelatine the proteins adsorption in their initial biopolymer films for tissue- conformation; they also may be prone to engineering 2013 MATEC Web of undergoing structural changes. The Conferences, EDP Sciences, 8 DOI: flexibility of the cell’s cytoskeleton 10.1051/matecconf/20130803009 depends on the rigidity of F-actin (major 3. Daskalova A, Selimis A, Manousaki A, component of the cytoskeleton essential Gray D, Ranella A and Fotakis C 2013 for such important cellular functions as the Surface modification of collagen- mobility), while their adaptation to the based biomaterial induced by pulse surface’s topography determines the width variable femtosecond laser alignment of the cells. pulses Proc. SPIE 8770 98 Selected Projects

Conference participation Summer School on Vacuum, Electron and Ion Technologies (7-11 October 1. Nathala C, Daskalova A, Bliznakova I, 2013 Sozopol Bulgaria) Lueftenegger S, Zhelyazkovа A, 3. Daskalova A 2013 Nano structuring Enikoe S, Ganz T and Husinsky W by ultrashort laser pulses of 2013 Defined nano-structuring with biomimetic materials for application ultrashort pulses in gelatin biopolymer in tissue engineering Second films for tissue-engineering Progress National Congress on Physics in Ultrafast Laser Modifications of (25-29 September 2013 Sofia Bulgaria) Materials (14-19 April 2013 Cargese France) 2. Daskalova A, Kasperski G, Rosseau P, Domaracka A and Lawicki A 2013 Interaction of slow highly charged ions with hard dental tissue: studies of fluoride uptake and remineralization efficacy Eighteenth International

99 Annual Report IE 2013

LIGHT-INDUCED ATOMIC DESORPTION (LIAD) FOR ALL-OPTICAL CONTROL OF LIGHT

M Taslakov, S Tsvetkov and S Gateva E. Djakov Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria

1. Introduction elements, LIAD-loaded atomic devices and their miniaturization, new methods for The light-induced atomic desorption surface and coating diagnostics, and (LIAD) is a non-thermal process whereby analysis of the quality of vacuum cells. atoms adsorbed at a surface are released under illumination. LIAD was reported for 2. Experimental setup the first time by Gozzini et al. [1], has been subsequently investigated extensively The experimental setup is presented in both experimentally and theoretically and figure 1. The frequency of a 780-nm diode has found various applications − vapor laser was tuned by varying the laser density stabilization, magneto-optical trap current and the transmission spectra were (MOT) loading, surface nanostructuring registered by a power meter. A special etc. [2-7]. However, there are still several sphere with a diameter of 150 mm aspects of the LIAD dynamics that remain provided homogeneous illumination for unexplained and questions that are open – LIAD activation. It consisted of two for example, the question on whether there hemispheres allowing easy mounting of is a common mechanism underlying all LIAD observations [8]. LIAD is influenced by many parameters, among which are the cell dimensions and geometry, the wavelength of the illuminating light, the atomic species, the cell’s history, the stem-opening area, etc. The models of LIAD are taking into account different processes for description of the LIAD dynamics [2-7]. Reducing the factors influencing LIAD will assist in Figure 1. Experimental setup. gaining a better understanding of these processes and their dynamics. the cells. The high-reflection coating on Under this project, a special system for the inner surface ensured near Lambertian homogeneous illumination of the cell was scattering. A 60-W light emitting diode developed. The Rb D2 line absorption (LED) (Luminus PT120 with a central spectra in uncoated and coated cells were wavelength of 460 nm and a FWHM of compared without illumination, with one- 20 nm) was mounted on the sphere. When side illumination and with homogeneous the sphere was closed, the efficiency of the illumination. The transmission was blue light illumination improved measured as a function of the illumination dramatically, with the inhomogeneity of intensity and the tuning direction. The the illumination of the sphere’s surface dynamics of the LIAD in an uncoated cell being less than 10 %. The blue LED was analyzed. The results demonstrated current was controlled by means of a the potential of the system for circuit designed in our laboratory. Two development of new optoelectronic photodetectors were placed on the inner 100 Selected Projects

surface of the sphere, one of them is measured as κω = (1 − Tω)/L. providing negative feedback, the other monitoring the blue light intensity. All 3. LIAD at different illuminations measurements were performed at room temperature (25 oC). In order to minimize Figure 2 shows the transmission spectra the influence of the optical pumping, a at different illuminationa in an uncoated relatively low laser power (3 − 4 µW) was and and an SC-77 coated cell at 4 µW used at 780 nm. Table 1 presents some 780 nm laser power and 1 A blue LED parameters of the cells used, namely, an current. The spectrum of the 780 nm (Rb uncoated one and one coated by a silane- D2) line has four components. The optical based dry film (SC-77 [9]). spectrum of each Rb isotope consists of two sets of lines [10]: (i) 87Rb involves Table 1. Parameters of the uncoated and SC- two groups of hyperfine transitions starting 77 coated Rb cells: length L, diameter D, from Fg = 1 and Fg = 2 ground levels and volume V, surface S, and their ratio S/V. 85 (ii) Rb with two Fg = 2 and Fg = 3 sets as Cell L D V S S/V well. The spectrum is the sum of the 3 2 -1 [cm] [cm] [cm ] [cm ] [cm ] Doppler profiles (with FWHM ~500 MHz) unc. 4.8 3.2 38.3 64.3 1.68 of all Fg−Fe hyperfine transitions. SC77 6.0 2.6 31.6 59.6 1.88 Figure 2 presents the transmission spectra in an uncoated cell and in an SC- For an optically-thin medium, the 77 coated cell. The comparison of the change in the Rb density due to the LIAD absorption in the uncoated cell (Fig. 2a) is measured by the absorption coefficient and the SC-77 coated cell (Fig. 2b) at of Rb vapor. According to Beer’s law, κω different illumination shows that without the transmission T(ω) = T is: ω illumination (black lines) the absorptions Tω = I/I0=exp(−κωL) = exp(−σωNL) (1), are comparable (somewhat smaller in the coated cell). With homogeneous where I0 is the 780-nm laser power entering the vapor cell; I, the transmitted illumination (pink lines) in both cells, the absorption is the largest. As a result of the power; κω, the frequency-dependent different ratio of the desorption to the absorption coefficient; σω, the non- saturated, frequency-dependent absorption lateral diffusion [11] and the stem cross-section; N, the atomic density and L, influence, the one-side illumination the cell length. (blue lines) of the uncoated cell is not In the case of an optically-thick efficient, while it is efficient for medium, the average absorption coefficient LIAD in the case of the coated cell.

100 100

80 80

60 60

40 40 Transmission, % Transmission, % Transmission,

20 20 a b 0 0 0 2 4 6 8 10 0 2 4 6 8 10 Frequency, GHz Frequency, GHz

Figure 2. Influence of the illumination geometry in: (a) uncoated cell and (b) SC-77 coated cell without illumination (black), with one-side illumination (blue) and with homogeneous illumination (pink). 101 Annual Report IE 2013

4. LIAD dynamics our case, τ4 = 370 ms, which can be attributed to the stem influence and the Figure 3 illustrates the dynamic recovery of the alkali metal vapor density characteristics of the LIAD in the uncoated to the thermodynamic equilibrium value cell. There are four time constants and the initial Rb atom density on the cell characterizing the LIAD process: τ1 surface. characterizes the exponential growth after In coated cells (SC-77, PDMS, OTS, the blue light is switched on; τ2, the PCHS and paraffin coated cells), as a density decrease while the desorbing light result of the longer time scales of the is still on; τ3, the density decrease when processes, no modulation of the the light is switched off [3]; τ4, the transmission spectrum (alkali metal processes after the blue light is turned off. density) was detected at these frequencies The values measured of the first three time of modulation of the blue light intensity. constants are: τ1 = 2 ms, τ2 = 260 ms, Figure 4 illustrates the tuning of the τ3 = 4 ms. second and third components of the These constants are about three orders transmission amplitude when the blue light of magnitude smaller than those in a intensity is increased and decreased in the paraffin coated cell [2], which can be used uncoated cell. (The component’s number for a faster control of the alkali atom increases with the tuning frequency [10].) density and its modulation. Another As the light intensity was increased, the difference with the results obtained before typical saturation of the absorption was that when the blue light was switched coefficient was registered [3]. When the on and off, the changes in the Rb densities the current was decreased, the are comparable so that we could define transmission was nonlinear, too, but closer and measure the fourth time constant. In to a linear behavior.

Uncoated 1 Uncoated 1 Uncoated 1 5ms/div 50ms/div 500ms/div

τ 4 τ 4= 370 ms τ3=4ms τ1=2ms τ 3 τ1 τ 2 τ2= 260 ms

Figure 3. Time constants of the LIAD process dynamics.

80 80 0A raising slope 70 falling slope 70

60 60

50 50 Transmission, % Transmission, % Transmission,

0A raising slope 40 40 falling slope a b 0 1 2 3 4 5 0 1 2 3 4 5 Blue LED current, A Blue LED current, A Figure 4. Transmission as a function of the blue LED illumination intensity and the direction of its tuning at 0.2 Hz and 4-µW 780-nm laser light in an uncoated cell. 102 Selected Projects

5. Summary and conclusions References

A LED system for homogeneous [1] Gozzini A, Mango F, Xu J, Alzetta illumination of alkali metal cells was G, Maccarrone F and Bernheim R designed, where the illuminated cell was 1993 Il Nuovo Cimento D 15/5 709- mounted in a sphere with a high-reflection 22 coating on its inner surface, which [2] Karaulanov T, Graf M T, English D, functions as a Lambertian screen. A Rochester S M, Rosen Y J, special laboratory electronic circuit for Tsigutkin K, Budker D, Alexandrov controlling the blue LED current was used. E B, Balabas M V, Kimball D F J, The system increased the LIAD Narducci F A, Pustelny S and efficiency, prevented the formation of Yashchuk V V 2009 Phys. Rev. A clusters and enabled us to examine the 79/1 012902 LIAD without lateral diffusion. The [3] Villalba S, Failache H and Lezama reduction of the number of parameters and A 2010 Phys. Rev. A 81/3 032901 processes influencing the LIAD will be [4] Atutov S N, Biancalana V, Bicchi P, useful for a more detailed understanding of Marinelli C, Mariotti E, Meucci M, the LIAD process. Nagel A, Nasyrov K A, Rachini S As a result of the increased efficiency, and Moi L 1999 Phys. Rev. A 60/6 we were able to register the LIAD 4693-700 dynamics in an uncoated cell. The [5] Alexandrov E B, Balabas M V, measured time constants, (about three Budker D, English D, Kimball D F, orders of magnitude smaller than in a Li C H and Yashchuk V V 2002 coated cells) showed that this system can Phys. Rev. A 66/4 042903 be applied for optical modulation of the [6] Rebilas K and Kasprowicz M J 2009 alkali atom density. Moreover, such a cell Phys. Rev. A 79 /4 042903 has the potential for application in [7] Rebilas K 2009 Phys. Rev. A 80/1 experiments where magnetically silent 014901 modulation is needed [12]. [8] Hammers R J 2005 Surf. Sci. 583/1 This work is interesting in view of the 1-3 development of new optoelectronic [9] Stephens M, Rhodes R and Wieman elements, LIAD-loaded atomic devices C 1994 J. Appl. Phys. 76/6 3479-88 and their miniaturization, and new [10] Gateva S, Taslakov M, Sarova V, methods for surface and coating Mariotti E and Cartaleva S 2013 diagnostics. Proc. SPIE 8770 87700O [11] Vartanyan T, Przhibel’skii S, Acknowledgements Khromov V and Leonov N 2011 This work was supported by the Light-induced surface diffusion. In Bilateral Cooperation Program CNR/BAS Mass Transfer - Advanced Aspects Project “Coherent Ramsey resonance in H Nakajama ed (InTech 2011) pp coated cells for application in optical 415-36 magnetometry” and the Marie Curie [12] Grujić Z D and Weis A 2013 Phys. International Research Staff Exchange Rev. A 88/1 012508 Scheme Fellowship within the 7-th European Community Framework Program.

103 Annual Report IE 2013

VELOCITY DISTRIBUTION OF ALKALI ATOMS IN MICROMETRIC THIN CELLS

P Todorov1, N Petrov1, I Maurin2, S Saltiel2,3 and D Bloch2

Project DMU 02/17 funded by the Bulgarian National Science Fund Project coordinator: Dr. Petko Todorov Project period: 2009 – 2013

Partners

1Institute of Electronics, BAS, 72, Tsarigradsko Chaussee blvd., 1784 Sofia, Bulgaria 2Laboratoire de Physique des Lasers, Université Paris 13, Sorbonne Paris-Cité, UMR 7538 du CNRS, 99 Avenue J.-B. Clément, F-93430 Villetaneuse, France 3Faculty of Physics, University of Sofia, 5, J. Bourchier Blvd., 1164 Sofia, Bulgaria

1. Introduction nevertheless, it remains widely adopted when various averagings have to be When one considers a gas at thermal introduced in the modeling. The tradition equilibrium on the basis of the kinetic for using such a “cosθ” model is so strong theory of gasses as a part of that detailed desorption studies performed thermodynamics, it is natural that one under high vacuum, rather than in a should assume that the particles’ velocity thermal gas surrounding, often describe distribution is Maxwellian, and also that the angular behavior along a “cosθ” the velocity vector distribution is isotropic. expansion, i.e. a f(θ) law as f(θ) = These two points may not be perfectly n equivalent, as the anisotropic shape of the Σnan(cosθ) (see [1], and for examples of container may induce anisotropy in the high value of the exponent n, see e.g. [2]). vector distribution. The kinetic theory only Experimentally, it is not easy to considers ideal containers, i.e. ideal perform the corresponding measurements, surfaces at the boundary of the gas region. which demand a gas density low enough to It is by a flux equilibrium between atoms make genuine surface effects observable. incoming to and departing from the The high sensitivity of laser spectroscopy, surface that one derives the Knudsen law and its ability to resolve different atomic for a rarefied gas (“molecular regime”), velocities, make it an attractive tool for namely, a “cosθ” probability for a experimental tests of this law. At least two departing atom direction, with θ being the such experiments have found agreement angle between the departing atom with the “cosθ” law, one [3] with a thermal trajectory and the normal to the surface. gas cell (with not many details about the This “cosθ” law is well-known, but has no nature of the surface), the other one [4] connection with the microscopic more closely connected with desorption description (for a discussion on these processes on a specific surface. topics, see e.g. the review by Comsa and In all cases, the angular dependence has David [1]). It cannot be applied to not been specifically tested for atoms accommodation effects (surface and gas at departing from the surface following a different temperatures, as often occurs in grazing incidence. Such a situation of aeronautical studies), nor can it be used in grazing incidence appears to be of a a detailed study of the atomic desorption specific interest. Indeed, there is nowadays from a well-characterized surface; a sustained activity of using laser 104 Selected Projects

spectroscopy to study a gas confined close steady state (the medium is no longer to a surface [5-11]. absorbing), and the optical pumping is For the techniques relying on the hence a velocity-selective process. The confinement of the vapor, the contribution major assumption is that the atom of atoms with velocities nearly parallel to trajectories are wall-to-wall, as atom-atom the wall, i.e. atoms leaving the surface collisions are neglected owing to the dilute following a grazing incidence, is enhanced nature of the vapor. In principle [6,8], the and can even be made totally dominant, thicker the cell, the narrower the velocity notably in non-linear spectroscopic selection can be, provided the vapor schemes [6, 8-11], or when a FM remains sufficiently dilute. We have technique is coupled to linear spectroscopy already described in detail [8] the [7]. Relatively to the normal velocity effectiveness of this velocity selection. component, the atoms with flight nearly However, the natural width of the parallel to the wall are “slow atoms”. For a transition makes difficult the optical confinement close to a surface, one can observation of atoms slower than the expect that the roughness of the surface on “natural” velocity selection vnat, defined by the one hand, and the van der Waals vnat = uthγnat/ΓDopp with γnat being the attraction exerted by the surface on the natural width of the transition, ΓDopp, its other, would limit the existence of such Doppler width, and uth, the thermal “slow” atoms, notably when assuming velocity. To observe these slow atoms, one linear atomic trajectories. should rather consider a mechanical velocity selection, assuming a known (and 2. Principle of the experimental constant) thickness L, within which an measurement atom “marked” by a pump beam must propagate to a distance R in order to be In a dilute vapor cell of a micrometric probed [9]. thickness irradiated under normal Here, we consider a situation in which incidence by a single beam, the optical the pump beam is ring-shaped, and the pumping to a non-absorbing level, such as probe beam, sent under normal incidence, a ground state hyperfine level of an alkali is located precisely at the center of the vapor (e.g. the clock transition of Cs at pump beam ring (figure 1). 9.192 GHz), is governed by its transient evolution [6,8]: only slow atoms reach a

Figure 1. Principle of the experiment; a thin cell of thickness L much smaller than the overall diameter of the cell is irradiated by a ring-shaped pump beam. Only atoms with a slow normal velocity can leave the pump region and reach the axial probe region.

The probe beam intensity should be absorption takes place in a steady-state non-saturating. To count the pumped mode: practically, as long as the cell atoms travelling across the probe beam, thickness largely exceeds the wavelength one should assume that the probe [7], the transient build-up of absorption 105 Annual Report IE 2013

can be neglected, especially for the slow beam internal diameter must be well atoms. When scanning the frequency of defined in order to avoid optical pumping the (normal incidence and minimum of atoms located in the dark region diameter avoiding saturation) probe beam, separating the pump and probe beams. the atoms pumped at a distance appear In an independent work [11], a only as a Doppler-free resonance. The calculation of the line-shape has been pump beam should provide an efficient performed as a function of the L and R transfer and must be strongly saturating. parameters, under the usual assumption For an alkali-metal resonance and a non- that the velocity distribution in the thin cell cycling transition, it is easy to reach these obeys a 3D-Maxwellian distribution. Here, strongly saturating intensities, owing to the we concentrate on the idea that some slow very long relaxation time constants of the atoms may be missing, and we analyze the optical pumping process. This standard influence of a missing “slice” in the hypothesis of a negligible relaxation of the velocity distribution of slow atoms. optical pumping requires that the pump

Figure 2. Velocity distribution of atoms reaching the probe region, after having travelled from the pump region. A Maxwellian distribution is assumed, with a thermal velocity uth=250 m/s. The R/L ratio is 10 (left) or 30 (right), assuming an initial velocity distribution in which atoms with

⏐vz⏐ < 3m/s do not exist. The pump intensity is assumed to be so strong that the calculation does not depend on the pump frequency detuning.

Figure 2 shows the relative velocity yields a sensitive indication of the missing distribution of pumped atoms in the velocities. situations R/L = 10, and R/L = 30. The probe spectrum is expected to be Doppler- free, or at least to exhibit only a weak residual Doppler broadening if the R/L ratio remains small. Assuming a modified velocity distribution f(vz) = 0 for⏐vz⏐ ≤ vmissing, 1/2 -1 and f(vz) = (uthπ ) exp-(vz²/uth²), for ⏐vz⏐ > vmissing [11], in which too “slow” (normal velocity) atoms are missing, one derives the distribution of the (normal) velocity of pumped atoms travelling to the probe region. Figure 3 shows that when Figure 3. Amplitude of the probe absorption varying R/L (e.g, by changing the pump as a function of R/L, for various distributions beam size), the amplitude of the probe of (normal) atomic velocities. The height is absorption, as measured on resonance, normalized, so that for a distribution with no loss of slow atoms, the expected reduction of 106 Selected Projects

signal when increasing the pump-probe The probe transmission is detected separation is already taken into account. One using a lock-in scheme synchronous with has taken uth = 250 m/s. the chopping of the pump. The most essential point in the set-up is the shaping of the pump beam, with the aim to convert 3. Experimental set up and results as much power as possible into a ring- shape geometry. For this purpose, we We are presently implementing the implemented a special transmission corresponding experiments on Cs vapor grating with circular grooves, which confined in a thin cell with sapphire converts the beam into (conically) windows, with the spectroscopic diverging diffraction rings. An axicon lens measurements to be performed on the Cs allows a reduction of the diverging rings D2 line. The cell is a cylindrical one, with (the half-angle is ~11° for the first order). a diameter of ~ 3cm, with Cs deposited in Figure 5 shows a spectrum of the probe a bottom reservoir (as shown beam transmission across the Cs D2 line, schematically in figure 1). The windows starting from the ground state Fg = 4, for a are made of standard high-quality polished pump-probe spatial separation ~1.5 mm. sapphire. The roughness was estimated at One distinctly observes the sub-Doppler ~5 nm. We measured interferometrically contribution of the separate hyperfine the thickness to be ~19 µm. When lines. We note that the Fg = 4 − Fe = 4 performing this thickness measurement at transition appears to be the larger one, various points, we found that the deviation while for a linear probe spectrum one from parallelism did not to exceed 100 nm expects the Fg = 4 − Fe = 5 transition to be over 1 mm. The cell (and the Cs reservoir) was generally heated up to ~60 − 70 °C to increase the Cs atomic density. From our experience with comparable cells, neither the thickness nor the parallelism should be thus affected. For the spectroscopic measurements, we used a single laser tunable around the D2 line (852 nm), namely, a DFB type diode laser. It is a narrow linewidth (< 3 MHz) semiconduc- tor laser delivering a relatively high power (150 mW) sufficient to provide an intense (highly saturating) pump beam. The laser Figure 5. Probe beam transmission spectrum beam is split into a standard weak probe across the Cs D2 line from Fg = 4 to (left-to- right) Fe = 3, 4, 5. The pump-probe separation beam and a pump beam modulated by a is ~1.5 mm. The Cs source temperature is 60 ºC.

larger than the Fg = 4 − Fe = 4. Actually, when using only a single laser, the efficiency of the pump beam on the non- cycling Fg = 4 − Fe = 5 transition is lower than for the Fg = 4 − Fe = 4 transition (and the Fg = 4 − Fe = 3 transition). We also note that there is a residual Doppler background, probably to be attributed to Figure 4. Experimental set-up. atoms which have collided with the wall mechanical chopper (typically at 2 kHz), without losing their excitation received and shaped into a diverging ring by optical from the pump beam. elements (see figure 4). 107 Annual Report IE 2013

Another notable point to be considered [2] Rettner C T, Michelsen H A, when discussing figure 5 is the presence of Auerbach D J and Mullins C B a weak quadrature signal that is indicative 1991 J. Chem. Phys. 94 7499 of the stronger and narrow contribution of the Fg = 4 − Fe = 4 transition. For our [3] Grischkowsky D 1980 Appl. Phys. pump-probe separation ( ~1.5 mm), an Lett. 36 711 atom’s time-of-flight between the pump [4] Bordo V and Rubahn H 1999 Phys. and the probe can be estimated to be Rev. A 60 1538 ~7.5 μs, a value in agreement with the [5] see e.g. Bloch D and Ducloy M phase shift of the signal (as can be 2005 Atom-wall interaction B measured in fig. 5) with respect to the Bederson and H Walther eds Adv. 2 kHz pump modulation. At. Mol. Opt. Phys. 50 91-156 (Elsevier Academic Press San 4. Conclusions Diego) [6] Briaudeau S, Bloch D and Ducloy M We were able to observe the response 1996 Europhys. Lett. 35 337 of atoms flying nearly parallel to the wall, [7] Briaudeau S, Saltiel S, Nienhuis G, with the level of angular selectivity Bloch D and Ducloy M 1998 Phys. obtained mechanically in fig. 5 (~1.5 mm Rev. A 57 R3169 over a 19 µm thickness) corresponding to [8] Briaudeau S, Bloch D and Ducloy M the observation of atoms with a normal 1999 Phys. Rev. A 59 3723 velocity ~3 m/s, smaller than the one just [9] Briaudeau S, Saltiel S, Leite J R R, expected from an optical selection. It is Oria M, Bramati A, Weis A, Bloch already closely comparable to the one D and Ducloy M 2000 J. Phys. IV reported in [10] in an experiment where France 10 145 the cell thickness had not been precisely [10] Beverini N and Izmailov A Ch controlled. 2009 Opt. Commun. 282 2527 [11] Izmailov A Ch 2006 Laser Phys. References Lett. 3 132–6

[1] Comsa G and David R 1985 Surf. Sci. Rep. 5 146-98

108 Selected Projects

ROOM-TEMPERATURE MULTIFERROICS BASED ON Y-TYPE HEXAFERRITES

Contract DNTS/SLOVENIA 01/4 /2011 financed by the Bulgarian National Science Fund Project coordinator S. Kolev S Kolev, T Koutzarova, Ch Ghelev and I Bliznakova

E. Djakov Institute of Electronics, Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee, 1784 Sofia, Bulgaria

In view of implementing the project’s homogeneous solution thus prepared is tasks, we developed two “soft chemistry”- heated to 80°С, which results in the based methodologies for preparing formation of a gel that self-ignites and complex oxides, such as Ba2Mg2Fe12O22 produces nano-sized precursor oxides and Ba1.5Sr0.5Zn2Fe12O22, namely, necessary for the subsequent high- sonochemical synthesis and sol-gel auto- temperature synthesis of Ba2Mg2Fe12O22 combustion. and Ba2Zn2Fe12O22. We studied the The sonochemical synthesis of complex influence of the temperature of synthesis oxides consists in the co-precipitation of in the 1080 − 1170 °С range and of the metal cations in the presence of a high- time of synthesis on the phase content of power ultrasound field. We will present the hexaferrites studied. below a brief description of the technology During the development of the two developed. The process of preparing a methodologies described above for precursor for the synthesis of Y-type preparation of Ba2Mg2Fe12O22 and hexaferrites has to do with the co- Ba1.5Sr0.5Zn2Fe12O22, the problem arose of precipitation of Ba2+, Mg2+ and Fe3+ or the existence of second phases in the final 2+ 2+ 2+ 3+ Ba , Sr , Zn and Fe cations by NaOH. product – BaFe2O4, MgFe2O4 and ZnFe2O4 The process takes place under the influence (figure 1). The main cause for the presence of a high-power ultrasound pulse. We of these phases is the complexity of the determined the metal cations’ concentration oxide systems of interest, and the fact that and ratios needed for the co-precipitation to several phase transformations occur during occur, together with the ultrasound pulse the process of synthesis. Thus, we power and duration. We further studied the conducted a series of experimental effect of the temperature in the 1080 − procedures in order to eliminate the spinel 1170 °С range, as well as of the time of phases and developed a methodology for synthesis, on the phase content of the removing the BaFe2O4 from the end hexaferrite studied. product. At present, we continue seeking a The sol-gel auto-combustion is a widely technique for avoding the formation of used technique for synthesis of oxides. It MgFe2O4 и ZnFe2O4. In addition, the makes use of an “in-situ” reaction between presence of second phases necessitated chelate agents and oxidants resulting in the that we study their structural and magnetic formation of nano-sized powders. The properties and their impact on the following raw materils were used: multiferroic characteristics of the samples. Ba(NO3)2, Sr(NO3)2, Fe(NO3)3, Mg(NO3)2, At this stage, we focused our efforts on Zn(NO3)2, and citric acid. The procedure exploring the properties of MgFe2O4 and is as follows: a stoichiometric amount developed a methodology based on sol-gel of these compounds is diluted in a auto-combustion for preparation of minimal amount of distilled water. The magnesium ferrite. 109 Annual Report IE 2013

35000 1000 30000 - MgFe O 25000 2 4 800

20000 600

15000 Intensity 400 Intensity(Counts) 10000

5000 200

0 20 30 40 50 60 70 80 0 20 30 40 50 60 70 80 2 Theta-Scale 2 Theta Scale a) b)

3000

* BaFe O 2500 2 4 800 o ZnFe O 2 4

2000

1500

400 ZnFe O 1000 2 4 * Intensity (Counts) Intensity(Arb.Units) * o

500 o

0 0 20 30 40 50 60 70 20 30 40 50 60 70 80 2 Theta-Scale 2θ (degrees) c) d)

Figure 1. XRD-spectra of Ba2Mg2Fe12O22 (a) sol-gel auto-combustion, (b) sonochemical synthesis, and Ba2Zn2Fe12O22 (c) sol-gel auto-combustion and (d) sonochemical synthesis.

Figure 1 presents the results of the predominantly amorphous and consisted of XRD analysis of the samples prepared by two phases – MgFe2O4 and Fe2O3 us, with the peaks assigned to the spinel (hematite). The high-temperature phases being indicated. treatment at 1100 °С resulted in a single- Applying the Rietveld method to the 2+ phase sample of MgFe2O4. above results, we found that the Mg Figure 2 presents SEM images of the cations occupy the octahedral and samples synthesized. One can see the tetrahedral vacancies in the extensive aggregation of the particles due to Ba Mg Fe O crystal lattice. We further 2 2 12 22 the strong magnetic attraction force. The determined the lattice parameters, namely, a = 5.8699 Å and c = 43.4989 Å; the latter particles have a plate-like shape. The parameter is reduced in comparison with particles produced by sonochemical that of a bulk single crystal synthesis have a perfectly built hexagonal (c = 43.6048 Å). The average particles shape, characteristic for the Y-type size was 100.5 nm. hexaferrite, and a size of several During the synthesis of magnesium microns. One can also observe the ferrite we found that, upon the sol-gel presence of small MgFe2O4 particles auto-combustion, the powder was with a size of about 20 nm. 110 Selected Projects

a) b) c)

d) e) f)

Figure 2. SEM images of Ba2Mg2Fe12O22 (a, b) prepared by sol-gel and (c) sonochemical synthesis; Ba2Zn2Fe12O22 prepared by sol-gel and (d) sonochemical synthesis (e, f).

Figure 3 presents an image of particles 60 kOe, the remanent magnetization Mr, of magnesium ferrite. The powder sample and the coercivity Hc. One can see that in consists of particles that are homegeneous what concerns the Ba2Mg2Fe12O22 samples, in size and shape, the latter being the values of the saturation magnetization spherical. of the samples prepared by sonochemical synthesis are somewhat higher that those of the samples produced by sol-gel auto- combustion. We attribute this to the fact that the particles in the samples prepared by sonochemical synthesis are of a perfectly built hexagonal shape and a size of a few microns. The values obtained for the coercivity are low, which is typical for hexaferrites with a planar magneto- crystalline anisotrpopy. The presence of small-size particles in a monodomain state Figure 3. Electron microscope image of in the samples prepared by sol-gel auto- MgFe2O4. combustion affects the magnetic properties Studies of the hexagonal ferrites’ at low temperatures, especially the magnetic properties coercivity values. The magnetic-phase transitions were The magnetic characteristics (figure 4) studied by measuring the magnetization as of the Y-type hexaferrites synthesized by us a function of the temperature as an ac were obtained at room temperature and at magnetic field was applied with a liquid helium temperature (4.2 K). Table 1 frequency of 9967 Hz and an amplitude of summarizes a part of the results obtained 10 Oe. for the values of the main magnetic In the case of Ba2Mg2Fe12O22 samples parameters – saturation magnetization M at (figure 5), one can observe changes in the 111 Annual Report IE 2013

20 30 30 10 30 15 20 0 20 0 -2 0 2

-20 -10 0 10 20 emu/g M, M, emu/g M, -10 -15 10 10 -20 -30 0 0 -60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60 H, kOe b) -10 H, kOe a) -10 15 -20 -20 0 -2 0 2 -30 -30 -15 30 15 40 15 10 30 30 5 0 M, emu/g M, -20 -10 0 10 20 20 0 20

-0,4 -0,2 0,0 0,2 0,4 emu/g M, -15 -5 10 10 -10 -30 0 -15 0 -60 -40 -20 0 20 40 60 20 -60 -40 -20 0 20 40 60 -10 H, kOe c) d) -10 H, kOe 10 -20 -20 0 -30 -2 -1 0 1 2 -10 -30 -40 -20

Figure 4. Magnetic measurements of Ba2Mg2Fe12O22 samples prepared by sol-gel auto combustion (a, b) and sonochemical synthesis (c, d) at 4.2 K (a, c) and 300 K (b, d).

Table 1. Magnetic properties of Ba2Mg2Fe12O22 and Ba0.5Sr1.5Zn2Fe12O22.

Sample T, K M (60 kOe), emu/g Mr, emu/g Hc, Oe Ba Mg Fe O 2 2 12 22 300 22.78 1.74 31.35 sol-gel auto-combustion Ba Mg Fe O 2 2 12 22 4.2 30.47 9.69 545.87 sol-gel auto-combustion Ba Mg Fe O 2 2 12 22 300 24.95 1.26 12.21 sonochemical synthesis Ba Mg Fe O 2 2 12 22 4.2 33.57 1.94 28.95 sonochemical synthesis Ba Sr Zn Fe O 0.5 1.5 2 12 22 4.2 40.67 5.10 147.21 sol-gel auto-combustion Ba Sr Zn Fe O 0.5 1.5 2 12 22 4.2 56.49 6.61 313.39 sonochemical synthesis magnetization curve behavior at prerequisite for the appearance of temperatures of about 190 K and 50 K that multiferroic properties in Ba2Mg2Fe12O22. are related to magnetic-phase transitions. We assume that the transitions at 40 K and The changes at 183 K for the sample 30 K for the sample prepared by sol-gel prepared by sol-gel auto-combustion and at auto-combustion and that produced by 196 K for the sample produced by sono- sonochemical synthesis, respectively, chemical synthesis is related to a phase are due to a reorientation of the spins transition from a ferromagnetic to a spiral to a longitudinal conical state along spin ordering. This transition is a the с axis. The difference in the transition 112 Selected Projects

0,6 a) 0,04 b)

0,5 0,03 0,4

0,3 0,02 M', emu/g M', M", emu/g M",

0,2 0,01 0,1

0,0 0,00 0 50 100 150 200 250 0 50 100 150 200 250 T, K T, K

0,35 0,010 c) d) 0,30 0,008 0,25

0,20 0,006

0,15 0,004 M', emu/g M', M", emu/g M",

0,10 0,002 0,05

0,00 0,000 0 50 100 150 200 250 0 50 100 150 200 250 T, K T, K

Figure 5. Magnetic measurements of Ba2Mg2Fe12O22 samples prepared by sol- gel auto-combustion (a, b) sonochemical synthesis (c, d) in an ac magnetic field; (а) and (c) M'(T) – the real part, and (b) and (d) M"(T) – the imaginary part of the differential magnetization. temperatures in the two samples is the the variation of the magnetization with the result of the difference in the particles’ temperature when an ас magnetic field was shape and size in the samples, and of their applied with a frequency of 9967 Hz and an orientation in the magnetix field. amplitude of 10 Oe. The results are presented in figure 6. As is seen, no 0,32 changes in the magnetization curve

0,31 behavior are observed, which demonstrates that in this temperature interval magnesium 0,30 ferrite does not undergo phase transitions, 0,29 so that its presence should not affect the 0,28 multiferroic properties of the ', emu/g M 0,27 Ba2Mg2Fe12O22 samples examined by us.

0,26

0,25 Main results

0,24 0 50 100 150 200 250 300 1. A novel original technology was T, K Figure 6. Magnetic measurement of a developed, namely, sonochemical synthesis, for preparation of complex MgFe2O4 sample in an ac magnetic field. magnetic oxides – Ba2−хМхMe2Fe12O22, In order to clarify the effect of the where М is Sr, and Ме, Zn, Mg, Cu. presence of second phases in the samples The conditions were optimized for studied of Ba2Mg2Fe12O22, we carried out preparation of Ba2Mg2Fe12O22 and measurements on a MgFe2O4 sample in Ba0.5Sr1.5Zn2Fe12O22. view of determining the existence of 2. Based on the sol-gel auto- magnetic-phase transitions by following combustion technique, a 113 Annual Report IE 2013

methodology was developed for Publications preparation of multiferroics. The conditions were optimized for 1. Koutzarova T, Kolev S, Ghelev Ch, preparation of Ba2Mg2Fe12O22 and Nedkov I, Vertruen B, Cloots R, Ba0.5Sr1.5Zn2Fe12O22. Henrist C and Zaleski A 2013 The methodologies developed – Differences in the structural and sonochemical synthesis and sol-gel magnetic properties of nanosized auto-combustion – can be applied to barium hexaferrite powders prepared the preparation of other complex by single and double microemulsion magnetic oxides and multiferroics. techniques J. Alloys and Compounds 3. Based on the sol-gel auto- 579 174–180 combustion technique, a methodology was developed for 2. Koutzarova T, Kolev S, Ghelev Ch, Krezhov K, Kovacheva D and Subov preparation of MgFe2O4. The conditions of the process were P 2013 Y-type hexaferrite Ba2Mg2Fe12O22 powders – new optimized. The methodology th developed allows one to prepare multiferroics, Proc. V Int. Sci. Conf. nano-sized magnesium ferrite with System Analysis and Applied particles that are homogeneous with Synergetics 1 (Pyatigorsk, Russian respect to their size and shape; it can Federation) be applied to the preparation of other 3. Kolev S and Koutzarova T 2014 The ferrites with a spinel structure. influence of the agglomeration in the 4. The structural and magnetic ferrofluid on the thick film properties obtained in a magnetic properties of Ba2Mg2Fe12O22, field J. Phys.: Conf. Series Ba0.5Sr1.5Zn2Fe12O22 and MgFe2O4 514 were explored. 012021 5. The temperatures were determined of the magnetic-phase transitions in Ba2Mg2Fe12O22 and Ba0.5Sr1.5Zn2Fe12O22 that are responsible for the multiferroic properties of these materials.

SCIENTIFIC EVENTS

● Eighteenth International Summer School on Vacuum, Electron and Ion Technologies (VEIT 2013)

117 Annual Report IE 2013

EIGHTEENTH INTERNATIONAL SUMMER SCHOOL ON VACUUM, ELECTRON AND ION TECHNOLOGIES (VEIT 2013)

The International Summer School on attended by 91 participants from 15 Vacuum, Electron and Ion Technologies countries: Bulgaria, Czech Republic, (VEIT) has been organized biennially France, Germany, Greece, The since 1977, when the series of VEIT Netherlands, Portugal, Romania, Russia, School was launched by the Institute of Serbia, Slovakia, Slovenia, Sweden, Electronics, Bulgarian Academy of Ukraine and UK. Sciences, with the aim to act as a forum for Following the tradition of publishing exchange and dissemination of knowledge the VEIT Proceedings, a selection of and ideas on the latest developments in papers presented at the event is published electron-, ion-, and plasma-assisted in a special issue of Journal of Physics: technologies. The organizers of the 2013 Conference Series, under the originality edition of the event were the Institute of and quality criteria of acceptance by the Electronics, Bulgarian Academy of journal, including peer reviewing. Sciences, Sofia, Bulgaria and the Dutch The school comprised fifteen plenary Institute for Fundamental Energy and three poster sessions. At the plenary Research, Nieuwegein, The Netherlands. sessions, twenty-one invited talks of While the school has initially been general interest were presented by highly providing a meeting place for researchers renowned international experts in the field, mainly from Eastern and Central European as well as ten progress reports by young countries, its importance has grown issue scientists. In total, 66 contributed papers by issue. The school is now a major were presented during the poster sessions. scientific event and a meeting place for There were several scientific highlights young scientists from Eastern and Western covering fundamentals of interaction of Europe involved in research and fast particles with solids and challenging development associated with high-tech practical applications ranging from novel industries. Many former school techniques for creating hard coatings, participants have gone on to become optical/protective layers, biocompatible leading scientists in research materials to nanosized structures produced establishments and companies throughout by evaporation, sputtering or external the world. Leading international irradiation. Latest results were presented companies, such as High Voltage on ion-beam synthesis and modification in Engineering, Balzers, Varian, and Hauzer both low-energy (deposition and film have used the VEIT forum to present their growth) and high-energy (sputtering, products through oral presentations, poster implantation) regimes, and processing of contributions or exhibits. The School solid materials aiming at patterning the Proceedings have been published in surface or at creating nanophase systems special issues of the international journals for electronic or tribological/wear resistant Vacuum, Plasma Processes and Polymers, applications. Journal of Physics: Conference Series. Despite the busy scientific program, the The Eighteenth edition of VEIT was atmosphere was relaxed and informal. The held in the Black Sea resort of Sozopol, early afternoons of most conference days Bulgaria, on 7–11 October 2013. It was were free to stimulate both scientific and 118 Scientific events

social interaction between participants, conference site, etc. We are also grateful to which often took place on the beach. The the members of the International Advisory social program included a welcome party, Committee - Th. Czerwiec, M. Dimitrova, an official dinner, and an outing to G. Dinescu, A. Ehiasarian, N. historical landmarks in old Sozopol. Guerassimov, I. Katardjiev, H. Kersten, VEIT 2013 owes its success to many D. Mataras, W. Möller, I. Petrov, Z. people. The International Advisory Petrovic, B. Rauschenbach, M.C.M. Committee shaped the scientific program (Richard) van de Sanden, M. Ürgen, for and ensured high-quality plenary their continuous concern about the high presentations by careful selection of scientific level and the future of the event, as well as for their contribution to the invited speakers. The Local Committee formation of a new generation of scientists bore the brunt of the organization both at and experts in the field. And, last but not the conference site and in dealing with least, we would also like to thank all correspondence, abstracts, and manuscripts authors for their valuable contributions to for the proceedings. We are grateful to our the proceedings and to the school, and all sponsors, the Dutch Institute for reviewers for their hard and tedious, but Fundamental Energy Research, very important, work. Nieuwegein, The Netherlands, the The next conference in the series will companies AllData and Astel, and the be held in September 2015. Ministry of Education and Science of Republic of Bulgaria for there generosity Dr. Miglena Dimitrova, Member of the International Advisory that enabled us to support the attendance Committee and Chairman of the Local of students and provided support to deal Organizing Committee with mailing, printing, renting the

HONORARY AWARDS

● Assoc. Prof. Dr. M. E. Koleva Assoc. Prof. Dr. A. Og. Dikovska Assoc. Prof. Dr.Sc. N. N. Nedyalkov Prof. Dr.Sc. P. A. Atanasov Dr. I. A. Bliznakova

● Prof. Dr.Sc. S. S. Tinchev

● Assoc. Prof. Dr. S. Gateva ● Assoc. Prof. Dr. M. E. Koleva ● Assoc. Prof. Dr. E. G. Koleva ● Doctoral student A. Zhelyazkova ● Doctoral student A. Krasteva ● Mr. A. Gisbrecht ● Mathematician Veliko Donchev

121 Annual Report IE 2013

In recognition of the results of ACAD. E. DJAKOV INSTITUTE OF studies in the field of radiophysics, ELECTRONICS physical and quantum electronics, the Scientific Council of the Acad. Emil Djakov Institute of Electronics presented the 2013 LAUREATE Academician Emil Djakov Annual ACAD. E. DJAKOV AWARD 2013 Award to:

Assoc. Prof. Dr. M. E. Koleva Prof. Dr.Sc. S. S. Tinchev Assoc. Prof. Dr. A. Og. Dikovska for the papers 1. Surface modification of Assoc. Prof. Dr.Sc. N. N. Nedyalkov diamond-like carbon films to Prof. Dr.Sc. P. A. Atanasov graphene under low energy ion Dr. I. A. Bliznakova beam irradiation for the paper entitled published in Applied Surface Science 2012 258 Enhancement of ZnO photo- 2931; luminescence by laser nano- 2. Ion modified high-Tc-Josephson structuring of Ag underlayer junctions and SQUIDs published in published in Superconductivity: Theory, Materials Applied Surface Science 2012 and Applications 2012 (Nova 258 9181. Science Publishers Inc).

Assoc. Prof. Dr. S. Gateva was elected a Honorary Member of the Council of European Scientific and Cultural Community and her name was added to the GOLDEN BOOK in recognition of her contribution to Bulgarian science.

122 Awards

Assoc. Prof. Dr. M. E. Koleva was awarded the International Fellowship of Matsumae International Foundation, Japan. The Fellowship was successfully completed and the researcher was awarded a medal and issued a Certificate.

Assoc. Prof. Dr. E. G. Koleva was awarded the Honorary Diploma of the Minister of Education and Science of Ukraine and of the President of Academy of Pedagogical Sciences of Ukraine for her contribution to the development of thorough scientific education materials in the field of nanoelectronics.

Ms. A. Zhelyazkova, a doctoral student at the Institute of Electronics, won the Third SPIE Prize for a poster presentation at the First International Biophotonics Conference, 25 - 31 August 2013, Riga, Latvia.

123 Annual Report IE 2013

Ms. A. Krasteva, a doctoral student at the Institute of Electronics, was awarded the 2013 Ivan Evstratiev Geshov Competition First Prize of the Bulgarian Academy of Sciences for her work in the field of Nanosciences and New Materials and Technologies.

Mr. A. Gisbrecht was awarded the 2013 Inventor of the Year Honorary Diploma of the Bulgarian Patent Office for his Invention No. 66257 “System for Photodynamic Therapy of Oncological Diseases”.

Mr. V. D. Donchev was awarded the Gold Medal and Honorary Diploma of the Second National Student Olympics in Computer Mathematics.

124 Awards

Mr. V. D. Donchev was conferred the St. Kliment Ohridski University of Sofia 2013 Annual Award for significant scientific research results in mathematics.