Broadband second harmonic generation in whispering gallery mode resonators

TL;DR

This paper introduces a broadband phase matching technique in whispering gallery mode resonators made of birefringent crystals, enabling efficient second harmonic generation across a wide wavelength range, including UV, with high efficiency.

Contribution

The authors demonstrate a novel broadband phase matching method in WGM resonators that allows efficient SHG over a broad spectrum, including UV, using a birefringent crystal.

Findings

Achieved SHG from 780 nm to 317 nm wavelengths.

Demonstrated continuous wave UV generation in a WGM resonator.

Observed SHG efficiency up to 4.6% (mW)^-1.

Abstract

Optical frequency conversion processes in nonlinear materials are limited in wavelength by the accessible phase matching and the required high pump powers. In this letter, we report a novel broadband phase matching (PM) technique in high quality factor (Q) whispering gallery mode (WGM) resonators made of birefringent crystalline materials. This technique relies on two interacting WGMs, one with constant and the other with spatially oscillating phase velocity. Thus, phase matching occurs cyclically. The technique can be implemented with a WGM resonator with its disk plane parallel to the optic axis of the crystal. With a single beta barium borate (BBO) resonator in that configuration, we experimentally demonstrated efficient second harmonic generation (SHG) to harmonic wavelengths from 780 nm in the near infrared to 317 nm in the ultraviolet (UV). The observed SHG conversion efficiency is as high as 4.6% (mW)-1. This broadband PM technique opens a new way for nonlinear optics applications in WGM resonators. This work is also the first reported continuous wave UV generation by direct SHG in a WGM resonator