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Determining the Polarization State of an Extreme Ultraviolet Free-Electron Laser Beam Using Atomic Circular Dichroism

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Determining the Polarization State of an Extreme Ultraviolet Free-Electron Laser Beam Using Atomic Circular Dichroism ARTICLE Received 13 Feb 2014 | Accepted 14 Mar 2014 | Published 16 Apr 2014 DOI: 10.1038/ncomms4648 Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism T. Mazza1, M. Ilchen1, A.J. Rafipoor1, C. Callegari2, P. Finetti2, O. Plekan2, K.C. Prince2,3,4, R. Richter2, M.B. Danailov2, A. Demidovich2, G. De Ninno2,5, C. Grazioli2, R. Ivanov2,5, N. Mahne2, L. Raimondi2, C. Svetina2,6, L. Avaldi7, P. Bolognesi7, M. Coreno7, P. O’Keeffe7, M. Di Fraia8, M. Devetta9, Y. Ovcharenko10, Th. Mo¨ller10, V. Lyamayev11, F. Stienkemeier11,S.Du¨sterer12, K. Ueda13, J.T. Costello14, A.K. Kazansky15,16,17, N.M. Kabachnik1,13,17,18 &M.Meyer1 Ultrafast extreme ultraviolet and X-ray free-electron lasers are set to revolutionize many domains such as bio-photonics and materials science, in a manner similar to optical lasers over the past two decades. Although their number will grow steadily over the coming decade, their complete characterization remains an elusive goal. This represents a significant barrier to their wider adoption and hence to the full realization of their potential in modern photon sciences. Although a great deal of progress has been made on temporal characterization and wavefront measurements at ultrahigh extreme ultraviolet and X-ray intensities, only few, if any progress on accurately measuring other key parameters such as the state of polarization has emerged. Here we show that by combining ultra-short extreme ultraviolet free electron laser pulses from FERMI with near-infrared laser pulses, we can accurately measure the polarization state of a free electron laser beam in an elegant, non-invasive and straightforward manner using circular dichroism. 1 European XFEL GmbH, Albert-Einstein-Ring 19, D-22761 Hamburg, Germany. 2 Elettra-Sincrotrone Trieste, Basovizza I-34149, Italy. 3 Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, Area Science Park, Trieste I-34149, Italy. 4 eChemistry Laboratory, Faculty of Life and Social Sciences, Swinburne University of Technology, Melbourne, Victoria 3122, Australia. 5 Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica 5001, Slovenia. 6 University of Trieste, Graduate School of Nanotechnology, Piazzale Europa 1, Trieste 34127, Italy. 7 CNR Istituto di Metodologie Inorganiche e dei Plasmi, CP10, Monterotondo Scalo I-00016, Italy. 8 Department of Physics, University of Trieste, Trieste I-34128, Italy. 9 CIMAINA and Dipartimento di Fisica, Universita` di Milano, via Celoria 16, Milano I-20133, Italy. 10 Institut fu¨r Optik und Atomare Physik, Technische Universita¨t Berlin, Hardenbergstrasse 36, D-10623 Berlin, Germany. 11 Physikalisches Institut, Universita¨t Freiburg, D-79104 Freiburg, Germany. 12 Deutsches Elektronen-Synchrotron (DESY), Notkestrasse 85, D-22607 Hamburg, Germany. 13 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan. 14 National Centre for Plasma Science and Technology and School of Physical Sciences, Dublin City University, Dublin 9, Ireland. 15 Departamento de Fisica de Materiales, UPV/EHU, San Sebastian/Donostia E-20018, Spain. 16 IKERBASQUE, Basque Foundation for Science, Bilbao E-48011, Spain. 17 Donostia International Physics Center (DIPC), San Sebastian/Donostia E-20018, Spain. 18 Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow 119991, Russia. Correspondence and requests for materials should be addressed to M.M. (email: [email protected]). NATURE COMMUNICATIONS | 5:3648 | DOI: 10.1038/ncomms4648 | www.nature.com/naturecommunications 1 & 2014 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4648 he advent of short-wavelength free-electron lasers (FELs) Results has enabled unique investigations of photo-induced Two-colour photoionization. In the experiment, He atoms are Tprocesses. Non-linear phenomena and femtosecond ionized by XUV FEL radiation in the presence of an intense-near time-resolved spectroscopy can be explored using high infra red (NIR) laser field. In the resulting photoelectron spec- intensity extreme ultraviolet (XUV) and X-ray beams1–3.In trum a series of lines, so-called sidebands, are observed on both addition, the ultra-short pulse duration and the high degree of sides of the normal photoline27,28. Since the sidebands are very spatial coherence of the X-ray FELs have enabled imaging sensitive to the spatial and temporal overlap and to the physical experiments4–6 aiming to determine the structure of a single bio- characteristics of both the XUV and NIR pulses, this type of molecule. Nearly all experiments to date have been carried out experiment is widely used for studying multi-photon ionization with linearly polarized short-wavelength FELs thereby precluding dynamics29 as well as for characterizing linearly polarized studies at high intensities and on systems where the result ionizing FEL pulses30. In the present study it is applied to the might depend on the target sensitivity to the helicity of the characterization of the polarization properties of the FEL beam. electric field. We are aware of only one FEL experiment with The CD is expected to be different in amplitude and sign for the circularly polarized light7 studying ferromagnetic, nanoscale central photoline and the sidebands24–26. The value of CD is spin order by imaging techniques. In this seminal work, linearly directly proportional to the degree of circular polarization of the polarized light from the Linac Coherent Light Source was used FEL beam and strongly depends on the intensity of the NIR field. together with a monochromator and transmission polarizer to The experimental results were obtained at the Low Density obtain 58% circular polarization, albeit at the expense of a Matter (LDM) end-station of FERMI. Circularly polarized FEL strong reduction in intensity. However, the FEL FERMI designed pulses (48.4 eV, 100 fs, 80 mJ) were spatially and temporally specifically to produce circularly polarized XUV radiation overlapped with intense NIR pulses (784 nm, 175 fs, 30 mJ) in the became available recently8,9. The use of these ultra-short interaction volume of a velocity map imaging (VMI) spectro- circularly polarized XUV pulses is expected to lead to a series meter (Fig. 1). The NIR beam is produced by optically splitting of breakthroughs in a number of associated domains, for the seeding laser beam and is introduced into the experimental example, spin-resolved features in atoms and molecules, chamber quasi-collinear with the XUV beam. For the measure- symmetry-dependent characteristics of chiral biomolecules, ment of the CD, the circular polarization of the ionizing XUV the dynamics of magnetic properties of solids and the radiation was kept fixed (left-handed circular) and the polariza- understanding of the secondary structure of proteins. Moreover, tion of the NIR-dressing field was switched from left- to right- the extremely high intensity of the FEL source opens up a handed by means of a rotating half wave plate—quarter wave completely unexplored class of dichroism phenomena involving plate combination. multi-photon processes at ultra-short wavelengths. Circular dichroism (CD) in photoemission is defined as the Angle-resolved electron spectra and circular dichroism. Typical difference of the photoelectron yield for left and right angle-resolved electron spectra derived from the VMI image (see circularly polarized ionizing radiation. For a non-zero CD to Fig. 1) are shown in Fig. 2a. The following points stand out in this exist, it is necessary that the target has intrinsic or induced photoelectron spectrum. About 11% of the total intensity is chiral properties, that is, physical properties that are not transferred from the main He 1 s photoline at 23.8 eV kinetic symmetric with respect to mirror reflection in one of the energy into the sidebands, which are separated by 1.58 eV from symmetry planes. Accordingly, CD in photoemission from the main line. In agreement with the theoretical results (Fig. 2b), solids has been widely used for the investigation of local maximal electron emission is observed at 90°, that is, perpendi- magnetization and other properties of solids and surfaces cular to the beam propagation axis. The angular distribution of (for example, refs 10,11). In atomic and molecular physics, circularly polarized photon beams have been applied for example in studies of electron spin-related phenomena and its polarization control12,13 as well as of symmetry breaking in chiral molecules14,15. With FERMI many experimental Electron image techniques, already well established for applications of CD using synchrotron radiation, high order harmonics or optical 16–21 lasers , can now be extended to studies at FEL sources. – VMI spectrometer e Measurements which are currently limited by the signal to noise ratio or detection efficiency, have now become feasible and precise characterization of all FEL beam parameters will be NIR laser crucial to making this powerful new source relevant, advantageous, effective and accessible to a wider scientific community. In this letter we report the experimental observation of CD in two-colour above threshold ionization of He atoms. The FEL angle-resolved photoelectron spectra provide direct confirmation Variable of the circular polarization of the FEL pulses and further enable polarization the characterization of their polarization properties. Generally, He gas jet CD in strong field single-colour multi-photon absorption
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