Dielectric perturbations: anomalous resonance frequency shifts in optical resonators

TL;DR

This paper reports experimental observations of anomalous resonance frequency shifts in optical whispering gallery mode resonators caused by dielectric perturbations, including shifts opposite to expectations and exceeding polarizability contributions.

Contribution

It demonstrates that dielectric perturbations can induce unexpected and large frequency shifts, supported by experimental data and a theoretical model with an intuitive explanation.

Findings

Observation of both blue- and red-shifts in mode frequencies

Frequency shifts can exceed the polarizability contribution

Increased mode linewidth with certain perturbations

Abstract

Small perturbations in the dielectric environment around a high quality whispering gallery mode resonator usually lead to a frequency shift of the resonator modes directly proportional to the polarizability of the perturbation. Here, we report experimental observations of strong frequency shifts that can be opposite and even exceed the contribution of the perturbations' polarizability. The mode frequencies of a lithium niobate whispering gallery mode resonator are shifted using substrates of refractive indices ranging from 1.50 to 4.22. Both blue- and red-shifts are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the whispering gallery mode. We compare the experimental results to a theoretical model by Foreman et al. and provide an additional intuitive explanation based on the Goos-H\"anchen shift for the optical domain.