Greater Than One Billion Optical Q Factor for On-Chip Microresonators

Citation

Wu, Lue Leo (2024) Greater Than One Billion Optical Q Factor for On-Chip Microresonators. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n1cn-tn34. https://resolver.caltech.edu/CaltechTHESIS:06042024-003837900

Abstract

This thesis is focused on making ultra-high-Q optical microresonators on silicon chips based on design and constructing ultra-low-loss optical waveguides (with losses around 20 dB/km), their fabrication process development, and device applications in on-chip nonlinear optics, including frequency combs, low-noise microwave generation, and narrow-linewidth lasers.

First, using thermally grown oxide (thermal silica) and wedge microresonator structure, a record Q factor exceeding 1.1 billion is achieved. Then, the limitations of the Q-factor due to surface roughness scattering loss and OH absorption loss are investigated and identified. Absorption limited Q-factor of 8 billion mainly attributed to OH ions is measured. To further explore the potential of thick thermal silica as under cladding material, wedge resonator fabricated in 25-µm-thick thermal silica achieves a Q-factor of over 60 million, along with a sixfold improvement in thermal stability and a 5 billion absorption-limited Q-factor. Subsequently, low noise microwave signal generation is demonstrated using these devices in a fully optical packaged form, operating soliton microcomb to generate beatnote microwave signals. Noise limitations arising from dispersive waves induced by distinct transverse modes are identified. Additionally, a low-fundamental-linewidth microcavity Brillouin laser is demonstrated, benefiting from device high Q-factor. The noise limits stemming from thermal refractive fluctuation at low offset frequencies and laser output power at high offset frequencies are identified. To improve device integration level, an engineered reduction of interface scattering using TM mode enables a demonstration of 700 million Q factor in a fully-integrated high-aspect-ratio thin SiN platform fabricated in a CMOS foundry. To add one more thing, room temperature soliton microcomb generation is demonstrated for the first time in high-Q AlGaAs microresonators.

Thesis Files