Optically reconfigurable quasi-phase-matching in silicon nitride microresonators

TL;DR

This paper demonstrates optically reconfigurable quasi-phase-matching in silicon nitride microresonators, enabling tunable second-harmonic generation with high efficiency and overcoming phase mismatch constraints in integrated photonics.

Contribution

It introduces a novel all-optical poling method in Si3N4 microresonators that achieves tunable second-harmonic generation independent of intermodal phase-matching.

Findings

Achieved mW level on-chip second-harmonic power.

Demonstrated widely tunable second-harmonic generation.

Confirmed all-optical poling via two-photon imaging and resonance tracking.

Abstract

Bringing efficient second-order nonlinear effects in integrated photonics is an important task motivated by the prospect of enabling all possible optical functionalities on chip. Such task has proved particularly challenging in silicon photonics, as materials best suited for photonic integration lack second-order susceptibility ($\chi^{(2)}$). Methods for inducing effective $\chi^{(2)}$ in such materials have recently opened new opportunities. Here, we present optically reconfigurable quasi-phase-matching in large radius Si$_3$N$_4$ microresonators resulting in mW level on-chip second-harmonic generated powers. Most importantly we show that such all-optical poling can occur unconstrained from intermodal phase-matching, leading to widely tunable second-harmonic generation. We confirm the phenomenon by two-photon imaging of the inscribed $\chi^{(2)}$ grating structures within the microresonators as well as by dynamic tracking of both the pump and second-harmonic mode resonances. These results unambiguously establish that the photogalvanic effect, responsible for all-optical poling, can overcome phase mismatch constraints even in resonant systems, and simultaneously allow for the combined record of output power and tunability.