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

Silicon nitride (Si₃N₄) 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 Si₃N₄ 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 Si₃N₄ 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.