High-Q lithium niobate Fabry-Pérot microresonators for microwave-rate soliton microcomb generation

Lithium niobate on insulator (LNOI) offers exceptional electro-optic and nonlinear optical properties for nanophotonic applications. Nevertheless, its intrinsic optical anisotropy often constrains the full combination of these capabilities in conventional ring-shape resonators. Here, we demonstrate a Fabry-Pérot (FP) microresonator that comprises a straight waveguide section terminated by photonic crystal reflectors (PCRs) in both Z-cut and X-cut LNOI platforms. By combining an adiabatic PCR design with a high-fidelity dry-etching process, we achieved a high intrinsic quality factor of up to 2.4 million. Critically, the device was engineered to operate with near-zero anomalous dispersion, while featuring the suppression of adverse avoided mode crossing and stimulated Raman scattering. These attributes enable, for the first time, to our knowledge, the generation of Kerr soliton microcombs in both Z-cut and X-cut LNOI FP microresonators, with a measured repetition rate of ∼14 GHz and ∼17 GHz, respectively. Our results broaden the nonlinear optical capabilities in LNOI platforms.