Design and Analysis of Quantum Dot Laser (InAsP) for Bio-photonic and Mode-Locking Applications IVAN BAHNAM KAROMI PhD Thesis School of Physics and Astronomy Cardiff University May 2018 DECLARATION This work has not been submitted in substance for any other degree or award at this or any other university or place of learning, nor is being submitted concurrently in candidature for any degree or other award. Signed ……………………………………………………… (candidate) Date ………21/05/2018………….…………….……… STATEMENT 1 This thesis is being submitted in partial fulfillment of the requirements for the degree of ………(insert MCh, MD, MPhil, PhD etc, as appropriate) Signed ………………………………………….…………… (candidate) Date …………21/05/2018………………….…………… STATEMENT 2 This thesis is the result of my own independent work/investigation, except where otherwise stated, and the thesis has not been edited by a third party beyond what is permitted by Cardiff University’s Policy on the Use of Third Party Editors by Research Degree Students. Other sources are acknowledged by explicit references. The views expressed are my own. Signed ……………………………………….……….…… (candidate) Date ………………21/05/2018…….………………… STATEMENT 3 I hereby give consent for my thesis, if accepted, to be available online in the University’s Open Access repository and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed ……………………………………………..…..….. (candidate) Date ………………21/05/2018………………………… STATEMENT 4: PREVIOUSLY APPROVED BAR ON ACCESS I hereby give consent for my thesis, if accepted, to be available online in the University’s Open Access repository and for inter-library loans after expiry of a bar on access previously approved by the Academic Standards & Quality Committee. Signed ……………………………………………..……… (candidate) Date ……………21/05/2018…………………….……… I ABSTRACT In this thesis, an original quantum dot material (InAsP) was introduced and characterised as a prospective laser material for applications in biophotonics and monolithic mode-locking. InAsP quantum dot material was grown in conditions that are appropriate for InP QD, which is the standard device in this study. The reasons for employing this material are to shift the emission to longer wavelengths than can be achieved with InP QD laser. In principle, by using both the InP and InAsP QDs in a single structure, a very wide gain spectrum can be produced that may be advantageous for passive mode-locking. The characteristics of the InAsP QD lasers were determined and compared with the standard device (InP QD laser) in this work, such as threshold current density, laser efficiency, lasing wavelength and temperature dependency of the threshold current density for different cavity lengths (1, 2, 3 and 4mm). The results show a shifting in the emission wavelengths by 55 nm toward longer wavelengths, while maintaining useable threshold current density and laser efficiency. For example, the 2mm long InP laser has a threshold current density of 170 A.cm-2 at room temperature, whereas for the same length, the InAsP QD laser has 260 A.cm-2. Moreover, both samples delivered optical powers of at least 250 mW. Edge-photo voltage spectroscopy (E-PVS measurements) confirmed deeper dot confinements for the InAsP materials by approximately 103 meV. The modal absorption spectra show a greater degree of inhomogeneous broadening for the InAsP QD materials, which was consistent with the dot size variation shown in TEM images for the InAsP wafer. This can support mode-locking in this material by broadening the optical gain spectra, which was also observed in this material. Gain-current measurements at different temperatures; specifically, 150, 200, 250, 300, 350, and 400 K, illustrate that InAsP QD material has a wider gain band-width at all studied temperatures. The carrier distribution study shows that the InAsP QD material tends to be non-thermally populated at 150 K. And also the recombination rate of this material is faster than the InP QD materials. Both of these points can be positive in relation to mode-locked performance. II ACKNOWLEDGEMENTS The past four years of my PhD studies, for me, have not just been a period of graduate study; it has been an invaluable journey full of new experiences and new learning. This journey would not have been successful without the numerous people supporting, accompanying and encouraging me. I would like to thank my supervisor, Prof. Peter Smowton for accepting me as a PhD student, for the endless support, in addition to the great suggestions and positive feedback throughout this project. Special thanks to my second supervisor Dr. Stephen Lynch for his beneficial advice and encouragement. I have been extremely fortunate to have such supervisors. I am particularly grateful to all my colleagues in the optoelectronic group at Cardiff University, especially Dr. Sam Shutts, Dr. Stella Elliott and Dr Angela Sobiesierski, my experimental research would have not been possible without their continuous support. I would like to acknowledge Andrey Krysa from Sheffield University for growing the samples and Richard Beanland from University of Warwick for TEM images. I would like to express my gratitude towards, Prof. Peter Blood for his invaluable advice and support through this work. Special thanks to all my friends for their encouragement. Most importantly, I would like to thank my family for their continuous support throughout this time, especially my wife Iras Oulo. Additionally, I would like to acknowledge the essential financial support, the Ministry of Higher Education and Scientific Research in Iraq and the Iraqi Cultural Attaché in London. Finally, I would like to thank all the academic staff in Mosul University Department of Physics. III PAPERS AND CONFERENCES Papers 1- Karomi, I., Smowton, P.M., Shutts, S., Krysa, A.B. and Beanland, R. 2015. “InAsP quantum dot lasers grown by MOVPE” Optics Express 23(21), pp. 27282-27291 2- Krysa, A.B., Roberts, J.S., Devenson, J., Beanland, R., Karomi, I., Shutts, S. and Smowton, P.M. 2016 “InAsP/AlGaInP/GaAs QD laser operating at ∼770 nm” Journal of Physics: Conference Series 740(1),012008. Conferences 1- Karomi, I., Shutts, S., Smowton, P.M. and Krysa, A.B. “Fabrication and characterisation of 770 nm (InPAs) quantum dot lasers” Semiconductor Integrated Optoelectronics, Cardiff, UK, 31March-2 April 2015. 2- Karomi, I., Shutts, S., Smowton, P.M. and Krysa, A.B. “Opening up spectrum with InPAs quantum dot lasers” CLEO: Science and Innovations, CLEO-SI pp. 2267, 2015. 3- Karomi, I., Shutts, S., Smowton, P.M., Krysa, A.B. and Beanland, R. “InAsP quantum dot lasers” 2015 IEEE Photonics Conference, IPC 7323595, pp. 573-574 (2015). 4- Karomi, I., Shutts, S., Smowton, P.M. and Krysa, A.B. “Opening up spectrum with InPAs quantum dot lasers” Conference on Lasers and Electro-Optics Europe - Technical Digest 2015-August,7184392. 5- Smowton, P.M., Shutts, S., Thomas, R., Beanland, R., Karomi, I., Gillgrass, S. and Krysa, A.B. “Quantum dot lasers for integrated photonics” Conference Digest - IEEE International Semiconductor Laser Conference7765729. IV 6- Krysa, A.B., Roberts, J.S., Devenson, J., Beanland, R., Karomi, I., Shutts, S. and Smowton, P.M. “Growth and characterisation of InAsP/AlGaInP QD laser structures” 2016 Compound Semiconductor Week, CSW 2016 - Includes 28th International Conference on Indium Phosphide and Related Materials, IPRM and 43rd International Symposium on Compound Semiconductors, ISCS 20167528566. 7- Karomi, I., Shutts, S., Smowton, P.M. and Krysa, A.B. “InAsP Quantum Dot Material Compatible with InP Quantum Dot Structure for Monolithic Passive Mode-Locked Laser” Semiconductor Integrated Optoelectronics, Cardiff, UK, 18-20 April 2017 V CONTENTS Chapter (1) Introduction .................................................................................. 1 1.1 Aims and motivation .................................................................................. 1 1.1.1 Toward longer wavelengths in InP quantum dot laser ................................ 1 1.1.2 Ultrafast pulse generation in quantum dot laser .......................................... 2 1.2 Literature review ......................................................................................... 6 1.2.1 Literature review for InP quantum dot laser ............................................... 6 1.2.2 Literature review of monolithic mode-locked lasers in QDs ...................... 8 1.3 Thesis structure ............................................................................................ 9 1.4 References .................................................................................................... 11 Chapter (2) Background and theory ....................................................... 15 2.1 Introduction ................................................................................................. 15 2.2 Fundamentals of lasers ............................................................................ 16 2.2.1 Components of a laser ............................................................................... 16 2.2.2 Optical transmission in a two-level system ............................................... 17 2.3 Semiconductor lasers ............................................................................... 18 2.3.1 Band structure and materials for semiconductor lasers............................. 18 2.3.2 Fermi-Dirac statistics and population inversion ....................................... 20 2.3.3 Semiconductor lasers ...............................................................................