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 [chi(3)] nonlinear interactions. Recently, second-order [chi(2)] nonlinearity was introduced into Si3N4 via the photogalvanic effect, resulting in the inscription of quasi-phase-matched chi(2) gratings. However, the full potential of the photogalvanic effect in microresonators remains largely unexplored for cascaded effects. Here, we report combined chi(2) and chi(3) nonlinear effects in a normal dispersion Si3N4 microresonator. We dem-onstrate that the photo-induced chi(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 chi(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.