# Quasi-phase-matched nonlinear optical frequency conversion in on-chip whispering galleries

## TL;DR

This paper introduces a novel on-chip whispering-gallery resonator platform with quasi-phase matching enabled by field-assisted domain engineering of lithium niobate, achieving efficient second-harmonic generation.

## Contribution

It demonstrates the first on-chip quasi-phase-matched nonlinear frequency conversion using engineered thin film substrates in whispering-gallery resonators.

## Key findings

- Q-factors up to one million achieved
- First demonstration of quasi-phase-matched second-harmonic generation on-chip
- Normalized conversion efficiency of 9×10^{-4} mW^{-1}

## Abstract

Chip-integrated whispering-gallery resonators enable compact and wavelength-agile nonlinear optical frequency synthesizers. So far, the most flexible phase-matching technique, i.e. quasi phase matching, has not been applied in this configuration. The reason is the lack of suitable thin films with alternating crystal structure on a low-refractive-index substrate. Here, we demonstrate an innovative method of realizing thin film substrates suitable for quasi phase matching by field-assisted domain engineering of lithium niobate, and subsequent direct bonding and polishing. We are able to fabricate high-Q on-chip WGRs with these substrates by using standard semiconductor manufacturing techniques. The Q-factors of the resonators are up to one million, which allows us to demonstrate quasi-phase-matched second-harmonic generation in on-chip WGRs for the first time. The normalized conversion efficiency is $9\times 10^{-4}\,\textrm{mW}^{-1}$. This method can also be transferred to other material systems.