UCLA

Optical microresonators provide an exceptional platform for confining and recirculating light within micron-scale volumes, enabling resonant enhancement and strong nonlinear interactions. Their ability to generate optical frequency combs through Kerr nonlinearity has led to breakthroughs in precision metrology, telecommunications, spectroscopy, and microwave photonics. Recent advances in complementary metal-oxide-semiconductor (CMOS) compatible platforms, particularly silicon nitride (Si3N4), have positioned integrated microresonators as leading candidates for scalable and low-loss nonlinear photonics. Si3N4 combines high optical nonlinearity, ultralow propagation loss, and broad transparency, supporting soliton microcomb generation and applications requiring long-term stability and low noise.In this thesis, we investigate Si3N4-based microresonators for both linear and nonlinear photonic applications, with a focus on soliton microcombs and ultra-low-noise microwave synthesis. First, we demonstrate polarization-diverse soliton transitions and deterministic switching in strongly coupled dispersion-managed microresonators allowing for stable, deterministic soliton generation. This is supported by rigorous numerical modeling using the coupled-mode Lugiato–Lefever equation. We further report spectrally purified dissipative Kerr solitons with high thermal stability and turnkey operation, enabling noise suppression without external filtering. These solitons exhibit high stability, turnkey generation with 99% success rate, and robustness against thermal fluctuations, remaining stable for day-long operation. We probe the hot-cavity response showing how differential thermorefractive effects between polarization modes can be harnessed to stabilize cross-polarized soliton state, advancing chip-scale ultrashort pulse generation. Expanding on this, we report the generation of free-running soliton microcombs with microwave repetition rate with exceptionally low phase noise reaching -137 dBc/Hz at 100-kHz offset, surpassing state-of-the-art electronic microwave oscillators at high frequency offsets. This is achieved using hybridized dual-mode operation for passive thermal stabilization and minimal Raman-induced noise transfer. To push optical frequency division in chip-scale mircorcomb further, we introduce a two-stage frequency division technique based on octave-spanning 1-THz microcombs and electro-optic combs, demonstrating a 57 dB phase-noise reduction and enabling widely tunable, low-noise RF signals from a chip-scale platform. Finally, we demonstrate wafer-scale fabrication of ultra-low-loss Si3N4 microresonators on 300-mm wafers using advanced immersion lithography. Post-fabrication annealing yields propagation losses as low as 0.2 dB/m in the C-band and as low as 0.7 dB/m at shorter near-infrared wavelengths. Using these resonators, we realize a stable laser system with an integrated linewidth of 4 kHz and sub-MHz frequency drifts over hour-long timescales.