Electrically Controlled Optical Polarization Rotation on a Silicon Chip Using Berry’s Phase THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Qiang Xu, B. Sc. Graduate Program in Electrical and Computer Science The Ohio State University 2015 Master's Examination Committee: Prof. Ronald M. Reano, Advisor Prof. Fernando L. Teixeira Copyright by Qiang Xu 2015 Abstract Since the introduction of thin film integrated optics in 1969, the dominant light guiding paradigm has been based on planar optical waveguides. The continued convergence of electronics and photonics on the chip scale can benefit from the voltage control of optical polarization for applications in communications, signal processing, and sensing. It is challenging, however, to electrically manipulate the polarization state of light in planar optical waveguides. Here, we exploit all three physical dimensions by realizing optical waveguides that guide light out-of-plane. Three-dimensional photonic integrated circuits allow access to Berry's phase, a quantum mechanical phenomenon of purely topological origin, enabling electrically tunable optical polarization rotation on the chip-scale for the first time. Devices fabricated in the high-confinement silicon-on-insulator material platform are no longer limited to a single static polarization state. Rather, they can exhibit dynamic tuning of optical polarization between transverse electric and transverse magnetic fundamental modes at infrared wavelengths. Electrical tuning of optical polarization over a 19 dB range of polarization extinction ratio is demonstrated with less than 1 dB of conversion loss. Compact polarization diverse system architectures involving dynamic control of optical polarization in photonic integrated circuits are envisioned. ii Dedication To my family. iii Acknowledgments I would like to express my sincere gratitude to my advisor, Professor Ronald M. Reano, for this continuous support and guidance in the past three years. I would also like to thank Professor Fernando L. Teixeira for your valuable time serving on my committee. I would like to thank my team members in the research group: Dr. Li Chen, Dr. Peng Sun, Justin Burr, Michael Wood, for their helpful advices and discussions. I would also thank staff members at the Nanotech West Laboratory of the Ohio State University, in particular, Aimee Price, Paul Steffen and Derek Ditmer, for their help during the fabrication. Above all, I am grateful for my parents for their love and encouragement in every stage of my life. iv Vita 2008................................................................B.S. Electrical Engineering, Shanghai Jiao Tong University, China 2012 to present ..............................................Graduate Research Associate, The Ohio State University, Columbus, OH Publications Qiang Xu, Li Chen, Michael G. Wood, Peng Sun, and Ronald M. Reano, “Electrically tunable optical polarization rotation from Berry’s phase on a silicon chip,” Nat. Commun. 5 (2014). Li Chen, Qiang Xu, Michael G. Wood, and Ronald M. Reano, “Hybrid silicon and lithium niobate electro-optical ring modulator,” Optica 1 (2014). Fields of Study Major Field: Electrical and Computer Engineering v Table of Contents Abstract ............................................................................................................................... ii Dedication .......................................................................................................................... iii Acknowledgments.............................................................................................................. iv Vita ...................................................................................................................................... v List of Figures .................................................................................................................. viii Chapter 1: Introduction ...................................................................................................... 1 1.1 Background ............................................................................................................... 1 1.2 Conventional polarization conversion methods ........................................................ 2 1.3 Berry’s phase ............................................................................................................. 3 Chapter 2: Berry’s phase on a silicon chip: accessing and simulation .............................. 6 2.1 Accessing Berry’s phase ........................................................................................... 6 2.2 Numerical Simulation ............................................................................................... 8 2.2.1 Mode decomposition for extraction of polarization rotation angle .................... 8 2.2.2 Simulation results ............................................................................................... 9 vi 2.2.3 Considering sidewall angle ............................................................................... 11 Chapter 3: Device design and operation theoretical analysis .......................................... 14 3.1 Device design .......................................................................................................... 14 3.2 Device operation coupled mode theory description ................................................ 16 Chapter 4: Measurement results and analysis .................................................................. 20 4.1 Measurement setup .................................................................................................. 20 4.2 Experimental observation of polarization rotation due to Berry’s phase ................ 20 4.3 Comparison with calculation ................................................................................... 24 Chapter 5: Summary ........................................................................................................ 26 Bibliography ..................................................................................................................... 27 Appendix A: Fabrication processes ................................................................................. 32 A.1 Waveguide patterning............................................................................................. 32 A.2 ICP and PECVD ..................................................................................................... 33 A.3 Titanium micro-heater ............................................................................................ 34 A.4 Aluminum electrode pad ........................................................................................ 34 A.5 Releasing process ................................................................................................... 35 vii List of Figures Figure 1. Off-axis double core structure diagram of the polarization rotator ..................... 2 Figure 2. Cut corner triangle core structure diagram of the polarization rotator ................ 3 Figure 3. Momentum-space representation of Berry’s phase. In optics, Berry’s phase manifests as optical polarization rotation. (a) For planar paths, no significant optical polarization rotation is observed. (b) A two-dimensional momentum-space with non-zero (Gaussian) curvature. .......................................................................................................... 4 Figure 4. Berry’s phase observation where it is manifested as optical polarization rotation in optical fiber, (a) Helical optical fiber experiment, (b) Fiber helix in a ring experiment. 4 Figure 5. Concept to realize Berry’s phase in silicon photonic integrated circuits. (a) Physical space: Waveguide layout involving out-of-plane waveguides. (b) Momentum space: Non-zero solid angle subtended by the shaded surface corresponds to the Berry’s phase which manifests as polarization rotation. The numbers in parentheses correspond to paths in physical-space in (a). ......................................................................................... 7 Figure 6. FDTD modeling of optical polarization rotation from Berry’s phase. (a) Rotation angle versus deflection angle with bend radius as parameter for TE polarized input light at 1,550 nm wavelength. Inset: Schematic of out-of-plane waveguide in computational domain. (b) Rotation angle versus wavelength with deflection angle as viii parameter for TM polarized input light. (c) Waveguide mode at 1,550 nm wavelength for deflection angle equal to 0, 15, 30, and 45 in sequence. The silicon waveguide core cross-section is 300 nm square. The core is clad in silicon dioxide on all sides. ............ 10 Figure 7. FDTD modeling of polarization rotation from Berry’s phase with angled sidewalls. The waveguide cross-section full-width-half-height is designed to be equal to the waveguide height. The sidewall angle is denoted w. The out-of-plane computational domain is the same as the inset of Figure 6(a). ................................................................. 12 Figure 8. Electrically tunable polarization rotation. (a) Schematic of device, (b) Top- down optical micrograph of fabricated device, (c) Optical interferometric surface profilometer measurements of out-of-plane waveguide. The silicon waveguide core bottom width is 310 nm and the height is 300 nm. The sidewall angle is 86. ............... 15 Figure 9. Schematic of bus waveguide