Photo-induced on-chip second harmonic generation for Hz-level optical metrology

Self-referencing is the integral part of an optical frequency division, the process that bridging the gap between optical and RF domains. However, the most common material for octave-spanning microcomb generation, Silicon Nitride (SiN), lacks the intrinsic second-order non-linearity to produce the second-harmonic required for self-referencing. This issue can be overcome by employing a photo-induced effect, which has been successfully demonstrated in SiN microresonators, but this poses the question if the doubled signal comply to high metrological standards of optical clocks. In our work we show that it is possible to achieve a decent relative frequency stability at 1e-15 level in a SiN-based microring, which corresponds to the state-of-the-art miniaturized atomic frequency references requirements.