Blue-band frequency comb and photodarkening in silica whispering gallery microresonators

TL;DR

This paper demonstrates blue-band nonlinear optical processes in silica whispering gallery resonators, highlighting challenges like photodarkening and proposing methods to overcome them for visible wavelength applications.

Contribution

It introduces the first demonstration of Kerr frequency comb and Raman lasing at 462 nm in silica WGRs, addressing photodarkening issues and suggesting solutions for blue-region nonlinear optics.

Findings

Achieved stable stimulated Raman scattering at 462 nm

Demonstrated hyper-parametric oscillation despite photodarkening

Proposed in-situ thermal bleaching to mitigate photodarkening effects

Abstract

To date there are extensive studies of optical nonlinearities in whispering gallery resonators (WGRs) in the near and mid-infrared wavelengths. Pushing this research into the visible region is equally valuable. Here, we demonstrate a Kerr frequency comb and Raman lasing at 462 nm in an SiO2 WGR. Notably, due to the high optical intensities achieved, photodarkening is unavoidable and can quickly degrade the optical quality of both the coupling optical nanofiber and the microcavity even at very low pump powers. Nonetheless, stable stimulated Raman scattering (SRS) and hyper-parametric oscillation in normally dispersed WGRs is demonstrated in the presence of photodarkening by taking advantage of in-situ thermal bleaching. These observations highlight the challenges of silica-based, short wavelength nonlinear optics in high quality, small mode volume devices. We propose a method to overcome this apparent limitation and demonstrate blue-band nonlinear optical processes in silica WGRs, thus providing a baseline for optics research in the blue region for any optical devices fabricated from SiO2.