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

Silicon nitride (Si(3)N(4)) is an ever-maturing integrated platform for nonlinear optics but mostly considered for third-order [χ((3))] nonlinear interactions. Recently, second-order [χ((2))] nonlinearity was introduced into Si(3)N(4) via the photogalvanic effect, resulting in the inscription of quasi-phase–matched χ((2)) gratings. However, the full potential of the photogalvanic effect in microresonators remains largely unexplored for cascaded effects. Here, we report combined χ((2)) and χ((3)) nonlinear effects in a normal dispersion Si(3)N(4) microresonator. We demonstrate that the photo-induced χ((2)) 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((2)) 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.