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

Abstract

Silicon nitride (Si ₃N 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 ₃N 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 ₃N 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.

Abstract

Abstract

Light can induce second order nonlinearity in microrings and can enable frequency conversion and comb generation afterwards.