Polarization-selective out-coupling of whispering gallery modes

TL;DR

This paper presents a novel technique for polarization-selective out-coupling of whispering gallery modes in resonators, enabling independent control of different polarization modes for enhanced optical experiments.

Contribution

The authors introduce a birefringence-based scheme for selective out-coupling of specific polarization modes in WGM resonators, supported by a new coupling condition and experimental validation.

Findings

Achieved 22dB suppression of TM mode coupling compared to TE modes.

Demonstrated polarization-selective out-coupling in lithium niobate resonators.

Provided a theoretical framework for polarization-dependent coupling conditions.

Abstract

Whispering gallery mode (WGM) resonators are an important building block for linear, nonlinear and quantum optical experiments. In such experiments, independent control of coupling rates to different modes can lead to improved conversion efficiencies and greater flexibility in generation of non-classical states based on parametric down conversion. In this work, we introduce a scheme which enables selective out-coupling of WGMs belonging to a specific polarization family, while the orthogonally polarized modes remain largely unperturbed. Our technique utilizes material birefringence in both the resonator and coupler such that a negative (positive) birefringence allows selective coupling to TE (TM) polarized WGMs. We formulate a new coupling condition suitable for describing the case where the refractive indices of the resonator and the coupler are almost the same, from which we derive the criterion for polarization-selective coupling. We experimentally demonstrate our proposed method using a lithium niobate disk resonator coupled to a lithium niobate prism, where we show a 22dB suppression of coupling to TM modes relative to TE modes.