Low Noise, Low Power Cavity Optomechanical Oscillators Alejandro Grine Ming C. Wu, Ed. Electrical Engineering and Computer Sciences University of California at Berkeley Technical Report No. UCB/EECS-2016-180 http://www2.eecs.berkeley.edu/Pubs/TechRpts/2016/EECS-2016-180.html December 1, 2016 Copyright © 2016, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. Low Noise, Low Power Cavity Optomechanical Oscillators By Alejandro Joaquin Griñe A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy In Engineering - Electrical Engineering and Computer Sciences in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Ming C. Wu, Chair Professor Liwei Lin Professor Constance Chang-Hasnain Fall 2014 Copyright Alejandro Joaquin Griñe Abstract Low Noise, Low Power Cavity Optomechanical Oscillators by Alejandro Joaquin Grine Doctor of Philosophy in Electrical Engineering and Computer Sciences University of California, Berkeley Professor Ming C. Wu, Chair Cavity Optomechanical oscillators (OMOs) rely on photon radiation pressure to induce harmonic mechanical motion of a micron-scale light resonator. Unlike most oscillators, optomechanical oscillators require only CW input light without the need for electronic feedback and so hold promise for their novelty. In an optical cavity of sufficient quality factor, the transduction from photons to phonons can be quite efficient as we characterized optomechanical cavities which only required 17 microwatt input optical power to induce mechanical oscillation. The question then remains whether OMOs can be made low noise and of course better yet, low noise and low power. By characterizing various materials and designs, it is shown that indeed OMOs may be made low noise and low power through maximization of the mechanical quality factor – a common quest for MEMs designers. With an emphasis on wafer-scale processes on silicon substrates, OMOs constructed from reflowed phosphosilicate glass, silicon nitride, and silicon were characterized and modeled. Due to non-linear light-matter interactions, OMOs are also known to produce RF frequency combs with an optical carrier. These combs were investigated and a method to produce a frequency comb spanning more than 6GHz from a 52MHz carrier was found. As a demonstration for how an OMO may be utilized in a chip-scale atomic clock, the 9th harmonic of a voltage-tunable device was phase-locked to a low noise microwave reference resulting in an 85dB reduction in phase noise at 1Hz offset from the carrier. 1 Acknowledgements This thesis would not have been possible without the contributions of many, many people too numerous to mention all. My advisor, Ming Wu was always an honest source of knowledge and experience who always told me what I needed to hear. I also greatly appreciate his advice on all non-research matters which especially helped during the writing of this work. Clark Nguyen had never ending optimism and creativity in advising on all aspects of device design and testing. Karen Grutter was in many ways a partner in crime who diligently fabricated and designed most of the devices in this thesis. She helped immensely not only with fabrication but in testing and theory and was a great source of ideas. Turker Beyazoglu fabricated the multimaterial devices and also aided with design and testing. Over the years, he has become a steady friend who lives up to the meaning of his name in Turkey. Always genuine and always kind, I will miss my daily interactions with him. Tristan Rocheleau helped greatly in understanding RF measurements, fabrication and general understanding of optomechanics. He always had an expectation of perfection which was at times difficult to live up to but pushed me to better performance. Niels Quack became a dear friend who helped with initial setup and design and fabrication of silicon devices. Sangyoon Han fabricated the silicon devices and has been a great collaborator. Myung- Ki Kim initially setup the tapered microfiber setup. It was a pleasure to learn from someone who was born with micrometer fingers. Antoine Ramier came on board and tested samples with integrated waveguides. His contribution sparked our interest in the testing and was greatly appreciated. Many undergraduates came on board at different times each helping in different ways. In no particular order they are Inderjit Jutla who helped with automated testing, Eric Zheng who aided with optical Q automation and testing, Scott Li who helped with integrated device testing, and Jeremy Huang tested many high Q samples. Many members of the Nguyen group made contributions including Bobby Schneider, Thura Naing, and Jalal Naghsh. From the Wu group, Michael Eggleston was an excellent source of information in the lab and also provided shelter when I needed it. Philip Sandborn turned in the dissertation paperwork which was a huge favor. I owe a great deal to Sandia National Laboratories for supporting me through this work. Especially Bernadette Montano, who was a constant source of encouragement as the University Programs administrator and my supervisor, Charles Sullivan who pushed me to finish strong and gave me every available resource. Olga Blum-Spahn was an incredible mentor who always made time during her trips to Berkeley to check on progress and give advice. Darwin Serkland, Gary Patrizi, Tom Zipperian, Gil Herrera, and Kent Schubert all aided with the process of joining the DSP program. I must also thank my undergraduate advisor Majeed Hayat who patiently tutored me early on and encouraged me to continue my education. Leslie Kolodziejski and Gale Petrich were outstanding at MIT in teaching me fabrication and optoelectronics. Most of all, this wouldn’t have been possible without my amazing wife, Antonette who never wavered in her support. Fleas, moths, mice, and mold were minor inconveniences compared to being displaced and re-displaced from friends, family, and her business. Yet, she proved savvy, resourceful, and loving through it all. Antonette, I can only repay you with my eternal love. My two kids, Angelina, and Andres have made tremendous sacrifices that I am eternally grateful for. As promised, this thesis is dedicated to Andres. My mother and father, Frances and Alfredo sacrificed so much effort early in my education and continued their support to now. My in-laws Rick and Pauline have been gracious in their support along with Grandma Maggie, and Mickey. I thank the rest of my family and friends for their continued good will and i prayers as well as my past teachers. Without the friendship and prayers of Bay Farm Community Church and the young families group, California would have just been the place I went to school. Finally, thank you God for everyone above, and all the words below. ii Table of Contents 1 Motivation .................................................................................................................................... 1 1.1 Organization .......................................................................................................................... 2 2 Whispering Gallery Mode Optical Cavities ................................................................................. 4 2.1 Properties of WGM Cavities ................................................................................................. 4 2.1.1 Optical Quality Factor and Finesse ................................................................................ 8 2.2 Coupled Mode Theory for Ring and Disk Resonators ........................................................ 10 2.2.1 Steady State Solutions.................................................................................................. 12 2.3 Characterization Methods ................................................................................................... 15 2.3.1 RF Intensity Modulation Technique ............................................................................ 19 3 Cavity Optomechanics ............................................................................................................... 23 3.1 Radiation Pressure and the Solar Sail ................................................................................. 24 3.2 Cavity Enhanced Radiation Pressure .................................................................................. 25 3.2.1 Optomechanical Coupling ........................................................................................... 26 3.3 Dynamics of Cavity Optomechanics .................................................................................. 28 3.4 Coupled Mode Equations for Cavity Optomechanics ........................................................ 32 3.5 Large Signal Dynamics ....................................................................................................... 34 3.5.1 Frequency Comb Generation ....................................................................................... 36 3.5.2 Output Power in the hth Harmonic ............................................................................... 37 3.5.3 Carrier Power ..............................................................................................................