LOW LOSS ORIENTATION-PATTERNED GALLIUM ARSENIDE (OPGaAs) WAVEGUIDES FOR NONLINEAR INFRARED FREQUENCY CONVERSION Dissertation Submitted to The School of Engineering of the UNIVERSITY OF DAYTON In Partial Fulfillment of the Requirements for The Degree of Doctor of Philosophy in Electrical and Computer Engineering By Izaak Vincent Kemp Dayton, Ohio December 2012 LOW LOSS ORIENTATION-PATTERNED GALLIUM ARSENIDE (OPGaAs) WAVEGUIDES FOR NONLINEAR INFRARED FREQUENCY CONVERSION Name: Kemp, Izaak Vincent APPROVED BY: Dr. Andrew Sarangan Dr. Peter Powers Advisory Committee Chairman Committee Member Professor Professor Electrical and Computer Engineering Department of Physics Dr. Partha Banerjee Dr. Guru Subramanyam Committee Member Committee Member Professor Department Chair Electrical and Computer Engineering Electrical and Computer Engineering Dr. Rita Peterson Dr. Kenneth Schepler Committee Member Committee Member Adjunct Professor Adjunct Professor Electro-Optics Electro-Optics John G. Weber, Ph.D. Tony E. Saliba, Ph.D. Associate Dean Dean, School of Engineering School of Engineering &Wilke Distinguished Professor ii © Copyright by Izaak Vincent Kemp All rights reserved 2012 iii Distribution Statement A: Approved for public release, distribution is unlimited. This dissertation contains information regarding currently ongoing U.S. Department of Defense (DoD) research that has been approved for public release. Distribution of this dissertation is unlimited pursuant to DoD Directive 5230.24 subsection A4. Requests for further information may be referred to the author, Izaak V. Kemp AFRL/RYMWA. iv ABSTRACT LOW LOSS ORIENTATION-PATTERNED GALLIUM ARSENIDE (OPGaAs) WAVEGUIDES FOR NONLINEAR INFRARED FREQUENCY CONVERSION Name: Kemp, Izaak Vincent University of Dayton Advisor: Dr. Andrew Sarangan The mid-IR frequency band (λ = 2-5 μm) contains several atmospheric transmission windows making it a region of interest for a variety of medical, scientific, commercial, and military applications. Recently there has been a growing interest in using orientation-patterned semiconductors such as orientation-patterned gallium arsenide (OPGaAs) to achieve frequency conversion in this region. GaAs has a large nonlinear coefficient, broad transparency range, and a well-developed fabrication technology allowing for the manufacture of nonlinear integrated optical devices such as waveguides. By confining the nonlinear pump beam throughout the length of a waveguide one can achieve very high field intensities while at the same time overcoming the diffraction normally associated with tight focusing of Gaussian beams. However, v device performance in OPGaAs waveguides has been limited by large propagation losses resulting from the unique nature of the material. In this dissertation I report an OPGaAs/OPAlxGa1-xAs embedded ridge waveguide design which, combined with a new growth process using alternating MOCVD growth and chemical mechanical polishing (CMP), is capable of achieving the low losses necessary to function effectively as a nonlinear gain material. Our waveguide was designed to be single mode from 2-10 μm, based upon a series of numerical simulations to determine the effective indices of the waveguide across the mid-IR band, which was crucial for accurate calculation of the grating period Λ needed for quasi-phasematching. Record low RMS surface roughness values of 5 nm were obtained using this new growth process. In addition, record low waveguide losses of 1.0 dB/cm were measured, validating our theoretical predictions, and nonlinear gain was investigated in an optical parametric amplifier (OPA) experiment. vi PREFACE This work summarizes an effort to develop guided wave devices in orientation patterned gallium arsenide (OPGaAs) for nonlinear frequency conversion in the infrared. This work was conducted at Wright-Patterson Air Force Base under the direction of the Air Force Research Laboratory, and resulted in several original contributions to the field of nonlinear optical waveguide devices. We have established a working recipe for growing guided wave devices in OPGaAs with low loss, and constructed a theoretical model for designing such devices. We have thoroughly investigated this model by placing our finished waveguide samples in an OPA experiment, and laid the groundwork for future OPGaAs waveguide development. In addition we have constructed a temperature-tunable bulk OPGaAs OPO and shown that the tunability matches very closely what is predicted by theory. vii TABLE OF CONTENTS ABSTRACT ..................................................................................................................................... v PREFACE ...................................................................................................................................... vii TABLE OF CONTENTS .............................................................................................................. viii LIST OF TABLES ......................................................................................................................... xv CHAPTER 1 INTRODUCTION ..................................................................................................... 1 1.1 Motivations ........................................................................................................................ 1 1.2 Current infrared sources ...................................................................................................... 4 1.3 Nonlinear infrared sources ................................................................................................. 6 1.4 Infrared nonlinear optical materials .................................................................................... 7 1.5 Orientation patterned (OP) semiconductors ........................................................................ 9 1.6 Waveguides and integrated optics..................................................................................... 13 1.7 Organization ...................................................................................................................... 15 CHAPTER 2 THEORETICAL BACKGROUND ......................................................................... 16 2.1 The nonlinear susceptibility .............................................................................................. 16 2.2 Nonlinear interactions (plane waves) ................................................................................ 17 viii 2.3 Phasematching and conversion efficiency ........................................................................ 21 2.4 Optical parametric oscillation ........................................................................................... 28 2.5 Waveguide theory ............................................................................................................. 30 2.6 Frequency conversion in waveguides ............................................................................... 32 CHAPTER 3 WAVEGUIDE MODELING AND DESIGN ......................................................... 34 3.1 2D computational mode solving .................................................................................... 35 3.2 OPGaAs waveguide tuning curves ................................................................................. 42 3.3 Template design .............................................................................................................. 46 3.4 Other design considerations ............................................................................................ 48 CHAPTER 4 WAVEGUIDE FABRICATION, PREPARATION, AND ANALYSIS ................ 51 4.1 Waveguide MOCVD growth .......................................................................................... 52 4.2 Template results and analysis ......................................................................................... 56 4.3 Sample preparation and surface metrology ..................................................................... 60 4.4 Polishing and cleaving .................................................................................................... 63 CHAPTER 5 EXPERIMENTAL PROCEDURE AND RESULTS .............................................. 68 5.1 Experimental setup .......................................................................................................... 68 5.2 Waveguide loss measurements ....................................................................................... 72 5.3 Temperature tuning ......................................................................................................... 76 ix 5.4 OPA results ..................................................................................................................... 78 5.5 Future Work ......................................................................................................................... 81 REFERENCES .............................................................................................................................. 83 x LIST OF FIGURES Figure 1: Diagram depicting a nonlinear optical parametric generation (OPG) process ................. 1 Figure 2: Plot of horizontal atmospheric transmission (10 km) as a function of wavelength .......... 4 Figure 3: 3D illustration of the zincblende structure and two 2D cross sections. ............................ 9 Figure 4: Illustration depicting how an orientation template is fabricated in the MBE process .... 11 Figure 5: Microscope image of a bulk OPGaAs sample,