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

This is the authors' abstract. We don't add key points for this paper.

Silicon nitride (Si₃N₄) is an ever-maturing integrated platform for nonlinear optics. Yet, due to the absence of second-order (χ⁽²⁾) nonlinearity, Si₃N₄ is mostly considered for third-order (χ⁽³⁾) nonlinear interactions. Recently, this limitation was overcome by optical poling in both Si₃N₄ waveguides and microresonators via the photogalvanic effect, resulting in the inscription of quasi-phase-matched χ⁽²⁾ gratings. Here, we report cascaded nonlinear effects in a normal dispersion Si₃N₄ microresonator with combined χ⁽²⁾ and χ⁽³⁾ nonlinearities. 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 at pump wavelength. Additionally, the doubly resonant pump and second-harmonic fields allow for cascaded third-harmonic generation, where a secondary optically written χ⁽²⁾ grating is identified. Finally, we reach a low-noise, broadband microcomb state evolved from the sum-frequency coupled primary comb. These results expand the scope of cascaded effects in χ⁽²⁾ and χ⁽³⁾ microresonators.