Raman lasing and soliton modelocking in lithium-niobate microresonators

The recent advancement in lithium niobate on insulator (LNOI) technology is revolutionizing the optoelectronic industry as devices of higher performance, lower power consumption, and smaller footprint can be realized due to the high optical confinement in the structures. The LNOI platform offers both large \c{hi}(2) and \c{hi}(3) nonlinearities along with the power of dispersion engineering, enabling brand new nonlinear photonic devices and applications towards the next generation of integrated photonic circuits. However, the Raman scattering, one of the most important nonlinear phenomena, have not been extensively studied, neither was its influences in dispersion-engineered LNOI nano-devices. In this work, we characterize the Raman radiation spectra in a monolithic lithium niobate (LN) microresonator via selective excitation of Raman-active phonon modes. Remarkably, the dominant mode for Raman oscillation is observed in the backward direction for a continuous-wave pump threshold power of 20 mW with a reportedly highest differential quantum efficiency of 46 %. In addition, we explore the effects of Raman scattering on Kerr optical frequency combs generation. We achieve, for the first time, soliton modelocking on a X-cut LNOI chip through sufficient suppression of the Raman effect via cavity geometry control. Our analysis of the Raman effect provides guidance for the development of future chip-based photonic devices on the LNOI platform.