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

Chip-integrated whispering-gallery resonators (WGRs) 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-Ǫ on-chip WGRs with these substrates by using standard semiconductor manufacturing techniques. The Ǫ-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, to the best of our knowledge. The normalized conversion efficiency is 9×10−4  mW−1. This method can also be transferred to other material systems.