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

Dielectric perturbations: anomalous resonance frequency shifts in optical resonators