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.