Fine structure of second-harmonic resonances in \chi^{(2)} optical microresonators

TL;DR

This paper develops a model for second-harmonic generation in whispering gallery resonators considering frequency detunings, predicting resonance distortions and efficiency variations, validated by experiments on lithium niobate and cadmium silicon phosphide WGRs.

Contribution

The paper introduces a new model accounting for frequency detunings in SHG within WGRs, explaining resonance distortions and efficiency behavior, supported by experimental validation.

Findings

Model predicts resonance line shape distortions due to detuning.

Experimental results show excellent agreement with the model.

Distortions are absent in lithium niobate WGRs but present in cadmium silicon phosphide WGRs.

Abstract

Owing to the discrete frequency spectrum of whispering gallery resonators (WGRs), the resonance and phase-matching conditions for the interacting waves in the case of second-harmonic generation (SHG) cannot generally be fulfilled simultaneously. To account for this, we develop a model describing SHG in WGRs with non-zero frequency detunings at both the pump and second-harmonic frequencies. Our model predicts strong distortions of the line shape of pump and second-harmonic resonances for similar linewidths at both frequencies; for much larger linewidths at the second-harmonic frequency, this behavior is absent. Furthermore, it describes the SHG efficiency as a function of detuning. Experimentally, one can change the WGR eigenfrequencies, and thus the relative detuning between pump and second-harmonic waves by a number of means, for example electro-optically and thermally. Using a lithium niobate WGR, we show an excellent quantitative agreement for the SHG efficiency between our experimental results and the model. Also, we show the predicted distortions of the pump and second-harmonic resonances to be absent in the lithium niobate WGR, but present in a cadmium silicon phosphide WGR, as expected from the linewidths of the resonances involved.