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 (χ (2) ) nonlinearity, Si 3 N 4 is mostly considered for third-order (χ (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 χ (2) gratings. Here, we report cascaded nonlinear effects in a normal dispersion Si 3 N 4 microresonator with combined χ (2) and χ (3) nonlinearities. We demonstrate that the photo-induced χ (2) grating also provides phasematching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs at pump wavelength. Additionally, the doubly resonant pump and secondharmonic fields allow for cascaded third-harmonic generation, where a secondary optically written χ (2) grating is identified. Finally, we reach a lownoise, 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.

Dynamics of Dark Pulse Affected by Higher-Order Effects in Microresonators

“…This field are currently studied in two branches, namely bright and dark solitons generated in the anomalous group velocity dispersion (GVD) and the normal group velocity dispersion regime, respectively. In particular, bright solitons have been explored actively in recent years and have been observed in many platforms like MgF2 [12,13], diamond [14,15], silica [16,17], aluminum nitride [18], silicon nitride [19,20,21,22,23,24,25,26], and silicon [27,28] microresonators, achieved by elaborate waveguide crosssection engineering. Actually, many optical materials present normal GVD in the visible and near-infrared range due to ultraviolet absorption, which makes it still challenging to obtain anomalous dispersion at arbitrary center wavelengths [26].…”