Photo-induced cascaded harmonic and comb generation in silicon nitride microresonators.

Silicon nitride (SiN) is an ever-maturing integrated platform for nonlinear optics but mostly considered for third-order [χ] nonlinear interactions. Recently, second-order [χ] nonlinearity was introduced into SiN via the photogalvanic effect, resulting in the inscription of quasi-phase-matched χ gratings. However, the full potential of the photogalvanic effect in microresonators remains largely unexplored for cascaded effects. Here, we report combined χ and χ nonlinear effects in a normal dispersion SiN microresonator. We demonstrate that the photo-induced χ grating also provides phase-matching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs. In addition, the doubly resonant pump and second-harmonic fields allow for effective third-harmonic generation, where a secondary optically written χ grating is identified. Last, we reach a broadband microcomb state evolved from the sum-frequency-coupled primary comb. These results expand the scope of cascaded effects in microresonators.