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Novel Saturable Absorber Materials and Devices for Laser Modelocking

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Novel Saturable Absorber Materials and Devices for Laser Modelocking Novel Saturable Absorber Materials and Devices for Laser Modelocking by Igor P. Bilinsky B.S., Moscow Institute of Physics and Technology (1992) Submitted to the Department of Physics in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Physics at the Massachusetts Institute of Technology February 1999 © 1998 Massachusetts Institute of Technology All Rights Reserved 11 Signature of Author V '7 Deartmet of--ysic Department of Physics November 30, 1998 I Certified by James G. Fujimoto Professor of Electrical Engineering Thesis Supervisor Certified by % - t1 Daniel Kleppner Lester Wolfe Professor of Physics Thesis Co-Supervisor Accepted by OF TECHNOLOGY .jomas J. Greytak Professor of Physics, Associate Departmeg ead for Education LIBRARIES Novel Saturable Absorber Materials and Devices for Laser Modelocking by Igor P. Bilinsky Submitted to the Department of Physics on November 30, 1998 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics Abstract The recently developed semiconductor saturable absorbers have made a major impact on the field of ultrashort pulse generation. Most of the previously demonstrated saturable absorber struc- tures require epitaxial fabrication techniques which impose severe design constraints leading to performance limitations. In addition, the complexity and cost of epitaxial fabrication limit the widespread application of such devices. The goal of this thesis was to develop simpler and more versatile saturable absorber materials and devices, investigate their properties, and apply them to ultrashort pulse generation from solid-state lasers. Three types of saturable absorbers were stud- ied, namely semiconductor saturable Bragg reflectors, semiconductor-doped glasses, and thin sil- ica films doped with semiconductor quantum dots. Modeling of optical properties of epitaxially grown saturable Bragg reflectors was per- formed. Reflectivity, dispersion, and electric field distribution were calculated for various satura- ble Bragg reflector designs. The demonstrated techniques can be applied to analysis of arbitrary multilayer dielectric structures incorporating saturable absorbers. Using several different semiconductor-doped glasses, self-starting fast saturable absorber passive modelocking and saturable-absorber-assisted Kerr Lens Modelocking of a Ti:sapphire laser were demonstrated. Nonlinear absorption dynamics of the semiconductor-doped glasses were investigated using a pump-probe technique. Photodarkening effects in the glasses were observed and characterized on a femtosecond time scale. A new saturable-absorber materials system for solid-state laser modelocking was developed, based on thin, nonepitaxially grown, semiconductor-doped films. Films of InAs and GaSb semi- conductor quantum dots in silica were fabricated using rf sputtering. The properties of InAs- doped silica films were characterized by means of linear and nonlinear absorption spectroscopy, femtosecond pump-probe technique, x-ray photoemission spectroscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, and electron diffraction. The absorption satu- ration cross-section and recovery time could be adjusted by use of rapid thermal annealing. For the first time, an application of semiconductor-doped dielectric films to laser modelock- ing was demonstrated. Self-starting saturable-absorber-assisted Kerr Lens Modelocking opera- tion of a Ti:sapphire laser was achieved using InAs-doped silica films fabricated by rf sputtering. The concept of semiconductor-doped dielectric films can be extended to a number of dopants and host materials to obtain simple and versatile saturable absorbers for ultrashort pulse generation from various solid-state lasers. Thesis Supervisor: James G. Fujimoto Title: Professor of Electrical Engineering Thesis Co-Supervisor: Daniel Kleppner Title: Lester Wolfe Professor of Physics Acknowledgments The moment has come. The moment to which I have been looking forward for a long time. The moment to write the most frequently read part of this thesis. The work I have done over the past six years would not have been possible without the con- tributions, direct and indirect, of a large number of individuals. I would like to thank all of you. I would like to thank Professor James Fujimoto for giving me the opportunity to work in one of the world's leading femtosecond optics laboratories, for his ideas and support. My thanks go to Professors Erich Ippen and Hermann Haus for their help over the years. In addition, I would like to thank Professor Ippen for serving on my thesis committee. I am grateful to Professor Daniel Kleppner for helping me come to MIT, for being my academic advisor during these years, and for serving on my thesis committee. It was a pleasure meeting Professor June Matthews and I would like to thank her for being a member of my thesis committee. I was fortunate to work closely with a number of extraordinary individuals who not only helped me in my work and professional development, but also were great people to discuss life, Universe, and everything, to share long junky lunches, to go on ski and long CLEO trips, etc. Dr. Brett Bouma taught me a lot about modelocking and about the ways to make things work in the lab. We spent a lot of time working together, both at MIT and in such an unlikely place as Nash- ville, TN and I always enjoyed his advice and his company., I shared a lot of late nights and late mornings in the lab with Boris Golubovic, Costas Pitris, Gary Tearney, and Stephen Boppart, and I learned a lot from each of them. My thanks go to Rohit Prasankumar who helped me with the later stages of this research and I wish him success in the further development of saturable absorber films. I would like to thank Nick Ulman and Chi-Kuang Sun who taught me the basics of the 'femtosecond art'. I would like to thank Dave "Hey Guy" Jones, Nanda Ramanujam, Ravi Ghanta, Erik Thoen, Mike Hee, Lynn Nelson, Malini Paye, Siegfried Fleischer, Dave Dougherty, and Charlie Hultgren for their camaraderie. My special thanks go to Cindy Kopf for making the paperwork move, for resolving all those numerous administrative details, and for adding a touch of humor to the daily routine. 3 I am thankful to my collaborators at the MIT Lincoln Laboratory, Dr. Jim Walpole, Leo Mis- saggia, Bob Bailey, Paul Nitishin, and the other members of group 83. Without their help and support this work would not have been possible. The advice and insights of Dr. Walpole were critical for the success of this research and I greatly enjoyed our numerous conversations. Leo Missaggia taught me a lot about the clean room techniques and was always a fun person to talk to. I am thankful to Elizabeth Koontz for fabricating the SBRs and for sharing her film deposi- tion knowledge with me. I would like to thank Dr. Anthony Garratt-Reed for helping me with the TEM measurements and Libby Shaw for her assistance with the XPS characterization of my sam- ples. I would like to thank Dr. Eric McFarland and all the folks at Symyx Technologies for the opportunity to see and appreciate the small company world during the summer I spent there, and for being able to contribute to the company growth. I would like to thank all my friends for making graduate school so much fun for me. I would like to thank all my teachers, past and present, for helping me along the way. I would like to thank my parents and my brother Oleg for their faith in me, for their love and support. You are far away but I always feel your presence. Finally, in addition to doing all this physics in graduate school, I met my wife Elena. Or should I say that in addition to meeting Elena, I studied physics? The second sentence seems to more precisely reflect the significance of these events to my life as a whole. Jokes aside, I would like to thank Elena for her love, patience, and support over the years of my Ph.D. work. I acknowledge all those who helped me, for making my life easier, and all those who did not help, for not letting me relax too much. Thank you all. 4 Contents Chapter 1 17 Introduction Chapter 2 23 Laser modelocking 2 .1 In tro d u ctio n .......................................................................................................................... 2 3 2.2 Passive m odelocking ........................................................................................................ 24 2.3 Self-starting modelocking operation .............................................................................. 33 2.4 Kerr lens modelocking ..................................................................................................... 36 2.5 Saturable absorbers for laser modelocking ..................................................................... 40 2.5.1 Linear absorption................................................................................................. 4 1 2.5.2 N onsaturable losses ............................................................................................ 42 2.5.3 Absorption saturation intensity and fluence........................................................ 42 2.5.4 A bsorption recovery tim e................................................................................... 46 2.5.5 Bandwidth of operation and other characteristics ............................................... 49 2.6 Epitaxially-grown semiconductor saturable absorbers ................................................... 51 2.6.1 Semiconductor saturable absorber mirrors and
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