High-Q lithium niobate micro- /Nano-resonators

Lithium niobate (LN) exhibits unique material characteristics that have been used in many important applications. Scaling LN devices down to a nanoscopic scale can dramatically enhance light-matter interactions and would enable nonlinear and quantum photonic functionalities beyond the reach of conventional means. However, developing LN-based nano-photonic devices turns out to be nontrivial. Although significant efforts have been devoted in recent years, LN photonic crystal structures developed to date exhibit fairly low quality. This thesis focuses on the application and fabrication of high-quality microresonators (micro-ring) and nano-resonators (photonic crystal). High-quality LN photonic crystal resonators for both 1-D and 2-D geometries are demonstrated. For both case, the intrinsic optical quality factors are larger than 10^5, which is two orders of magnitude higher than other LN nano-cavities reported to date. The high optical quality together with tight mode confinement leads to an extremely strong nonlinear photorefractive effect, with a resonance tuning rate of ~0.64 GHz/aJ, or equivalently ~84 MHz/photon, three orders of magnitude greater than other LN resonators. In particular, intriguing quenching of photorefraction is observed, which has never been reported before. This phenomenon shows a new potential solution to the photorefractive damage problem. The demonstration of high optical quality LN photonic crystal nano-resonators paves a crucial step towards LN nano-photonics that could integrate the outstanding material properties with versatile nano-scale device engineering for diverse intriguing functionalities. On the other hand, this thesis also focuses on the engineering and application of LN micro-rings through theoretical and experimental investigations. High-quality LN micro-rings are demonstrated, with intrinsic optical Qs up to 7 million. Such a high-quality micro-ring succeeds in producing an optical Kerr frequency comb. The demonstrated broadband Kerr frequency comb in dispersionengineered LN micro-ring resonators has a loaded optical Q of 2.5 million. The comb exhibits a spectrum extending from 1450 nm to 1680 nm in the telecom band, with an on-chip pump power of only 33 mW. We also observed an upconverted second harmonic associated with the Kerr frequency comb on this platform. These demonstrations pave a crucial step towards the development of comb applications in this promising device platform.