PHOTODARKENING OF GERMANIUM-SELENIUM GLASSES INDUCED BY BELOW-BANDGAP LIGHT By CRAIG RUSSELL SCHARDT A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2000 Copyright 2000 by Craig Russell Schardt To my parents, Jean and George Schardt. ACKNOWLEDGMENTS It is my great pleasure to thank my advisor, Dr. Joseph Simmons, for his patience and guidance throughout my years as a graduate student. He gives his students the freedom to find their own research direction and the encouragement to produce quality over quantity. I believe that his supervision has helped me to become a better researcher and prepared me for the challenges I will face in my professional career. I also thank the members of my committee—Dr. Cammy Abernathy, Dr. Paul Holloway, Dr. Rolf Hummel, and Dr. David Reitze—for all of their assistance and thoughtful comments. I could not have completed this project without the excellent samples prepared by Pierre Lucas and Lydia LeNiendre. I must also thank Pierre for the many papers he sent me and the useful conversations we have had. My gratitude goes out to Dr. Li Wang for teaching me how to set up and operate the Ti:Sapphire laser and to Mrs. Catherine J. Simmons for all of her encouragement and advice. Gwain A. Davis has been a friend and mentor to me outside of the lab. I would like to thank him for all of his guidance in developing my leadership skills and for showing me that people live up to your expectations of them. Finally, I extend special thanks to my wife, Heather M. Mockler-Schardt, for her love and support during my years as a graduate student. She has been my unwavering companion from the beginning, and for this I am indebted to her. iv TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................. iv LIST OF TABLES........................................................................................................... viii LIST OF FIGURES ........................................................................................................... ix ABSTRACT...................................................................................................................... xii CHAPTERS 1 INTRODUCTION...........................................................................................................1 Chalcogenide Glasses and Light..................................................................................... 1 Electronic Properties................................................................................................... 2 Optical Properties........................................................................................................ 4 Nonlinear Index of Refraction and Absorption .......................................................... 5 Vibrational Properties ................................................................................................. 7 Photoinduced Effects in Chalcogenide Glass ................................................................. 7 Photodarkening ........................................................................................................... 8 Photoinduced Anisotropy............................................................................................ 9 Photoinduced Crystallization...................................................................................... 9 Photodiffusion and Photodoping............................................................................... 10 Photoexpansion and Photoinduced Fluidity.............................................................. 10 Summary of Photoinduced Effects ........................................................................... 11 Applications for Chalcogenide Glasses ........................................................................ 12 Optical Limiting........................................................................................................ 12 Infrared Fiber Optics................................................................................................. 14 Photolithography....................................................................................................... 19 Optical Switching...................................................................................................... 19 Optical Computing.................................................................................................... 20 Contribution of This Research...................................................................................... 22 Systematic Study of a Variety of Compositions....................................................... 23 Ti:Sapphire Laser...................................................................................................... 24 Measurement of Kinetics .......................................................................................... 25 Raman Spectroscopy................................................................................................. 27 Summary................................................................................................................... 27 v 2 BACKGROUND...........................................................................................................29 Optical Physics.............................................................................................................. 29 Absorption and Refraction........................................................................................ 29 Dielectric Response .................................................................................................. 34 Germanium-Selenium Glass ......................................................................................... 40 Amorphous Selenium................................................................................................ 43 Amorphous Germanium Diselenide ......................................................................... 46 Selenium-Rich Germanium-Selenium Glasses......................................................... 50 3 EXPERIMENTAL METHODS ....................................................................................53 Chalcogenide Glass Samples ........................................................................................ 53 Glass Preparation ...................................................................................................... 53 Spectroscopic Analysis ............................................................................................. 54 Photodarkening Measurements..................................................................................... 56 Ti:Sapphire Laser...................................................................................................... 56 Optical Arrangement................................................................................................. 58 Data Acquisition and Analysis.................................................................................. 64 Experiment Methodology ......................................................................................... 65 Raman Spectroscopy..................................................................................................... 67 Raman Apparatus...................................................................................................... 69 Experimental Raman Measurements ........................................................................ 72 4 MEASUREMENT OF OPTICAL PROPERTIES ........................................................75 Infrared Absorption Edge ............................................................................................. 75 Visible Absorption Edge............................................................................................... 78 Measured Data .......................................................................................................... 80 Extraction of Optical Properties................................................................................ 82 Discussion..................................................................................................................... 90 5 BELOW-BANDGAP PHOTODARKENING ..............................................................96 Changes in Optical Properties Induced by Below-Bandgap Light ............................... 96 Transmittance and Reflectance Changes ................................................................ 101 Real and Imaginary Dielectric Response................................................................ 105 Effect of Composition on Photodarkening ............................................................. 114 The Kinetics of Photodarkening ............................................................................. 115 Transient Darkening and Dark Recovery................................................................ 133 Mechanism of Permanent and Transient Below-Bandgap Photodarkening ............... 143 Kolobov’s Model of Dynamical Bond Formation.................................................. 147 Dynamical Bonds and Photodarkening of Germanium Selenium-Glasses............. 152 6 RAMAN STUDIES OF STRUCTURE AND TEMPERATURE ..............................156 Structure Changes During Photodarkening ................................................................ 157 vi Transmission Measurements..................................................................................