Broadband spectral mapping of photo-induced second-harmonic generation in silicon nitride microresonators

By employing a pump-probe technique for enhanced spectral mapping of the dynamics in nonlinear frequency conversion, we demonstrate that photo-induced second-harmonic generation (SHG) in silicon nitride (Si₃N₄) microresonators can persist when transitioning from the preferred doubly resonant condition-where the resonances of the optical harmonics are required to be matched-to a highly detuned state where the generated second harmonic is significantly shifted away from its corresponding resonance. This results in an unconventionally broad conversion bandwidth. Other intriguing phenomena, such as detuning-dependent all-optical poling and nonlinear multi-mode interaction, are also presented, to the best of our knowledge, for the first time with direct experimental evidence. Our findings provide new insights into the physics of photo-induced second-order (χ⁽²⁾) nonlinearity, highlighting its potential applications for nonlinear χ⁽²⁾ photonics in an integrated Si₃N₄ platform.