High-Q Lithium niobate Micro-/Nano-resonators

Hanxiao Liang

Supervised by Professor Qiang Lin

Computer Studies Building Room 601

Lithium niobate (LN) exhibits unique material characteristics that have found many important applications. Scaling LN devices down to a nanoscopic scale can dramatically enhance light-matter interaction that 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.

The thesis focus on the application and fabrication of high quality micro-resonator (micro-ring) and nano-resonator (photonic crystal). High quality LN photonic crystal resonators for both 1-D and 2-D are demonstrated. Both intrinsic optical Q is larger than 105, which is more than two orders of magnitude higher than other LN nano-cavities reported to date. The high optical quality together with tight mode confinement leads to 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 that has never been reported before. This phenomena shows a new potential solution to solve the photorefractive damage problem. The demonstration of high-Q 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, the thesis also focuses on theoretical and experimental investigation on engineering and application of LN micro-ring. High quality LN micro-rings are demonstrated, with intrinsic optical of 7 million. Such high quality micro-ring succeed in producing optical Kerr frequency combs, which has excited significant interest in recent years to develop optical frequency combs on a variety of device and material platforms. The demonstrated broadband Kerr frequency comb in dispersion-engineered LN micro-ring resonators has a loaded optical Q of 2.5 million and intrinsic optical Q of 7 million. The comb exhibits a spectrum extending from 1450 nm to 1680 nm in the telecoms band, with an on- chip pump power of only 33 mW. Up-converted second harmonic associated with the Kerr frequency comb are also observed in this platform. These demonstrations now pave an important step towards the development of comb applications in this promising device platform.