Turnkey generation of Kerr soliton microcombs on thin-film lithium niobate on insulator microresonators powered by the photorefractive effect

Generation of Kerr soliton microcombs on microresonators enables power-efficient, phase-coherent, and broadband frequency teeth generation, which has revolutionized a wide range of scientific areas such as astronomy, metrology, spectroscopy, communications, etc. However, compared with a conventional frequency scanning method that requires a complex start-up and feedback control, turnkey generation of soliton microcombs remains challenging and a more promising solution is desired. Here, we propose for the first time and numerically demonstrate that turnkey generation of soliton microcombs can be achieved on thin-film lithium niobate on insulator (LNOI) microresonators for polarization along the ordinary axis of lithium niobate (LN) for which the photorefractive (PR) effect dominates. The PR effect shows power-dependent refractive index change, which is strong and opposite to that of the Kerr effect and thermal effect, thus enables the self-routing and converge of the total pump-resonator detuning into the existence region of soliton. Our results show that initiated with a certain amount of initial pump-resonator detuning on either blue- or red-detuned side, generation of soliton microcombs can self-start, self-route, and finally get stable without any artificial frequency scanning. Moreover, we show that deterministic and turnkey generation of single soliton microcombs can be achieved by leveraging a phase-modulated pump laser. Thanks to the inherent electro-optic effect of LNOI, a lab-on-a-chip device with monolithically integrated high-speed phase modulators and high- Q microresonators is feasible.