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

Abstract

Silicon nitride (Si${}_3$N${}_4$) is an ever-maturing integrated platform for nonlinear optics. Yet, due to the absence of second-order (${\chi }^{(2)}$) nonlinearity, Si${}_3$N${}_4$ is mostly considered for third-order (${\chi }^{(3)}$) nonlinear interactions. Recently, this limitation was overcome by optical poling in both Si${}_3$N${}_4$ waveguides and microresonators via the photogalvanic effect, resulting in the inscription of quasi-phase-matched ${\chi }^{(2)}$ gratings. Here, we report cascaded nonlinear effects in a normal dispersion Si${}_3$N${}_4$ microresonator with combined ${\chi }^{(2)}$ and ${\chi }^{(3)}$ nonlinearities. We demonstrate 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 at pump wavelength. Additionally, the doubly resonant pump and second-harmonic fields allow for cascaded third-harmonic generation, where a secondary optically written ${\chi }^{(2)}$ 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 ${\chi }^{(2)}$ and ${\chi }^{(3)}$ microresonators.