Birefringence-Driven Anisotropic $\alpha$-MoO3 Optical Cavities

TL;DR

This paper demonstrates a birefringence-driven optical cavity using ultralow-loss $eta$-MoO3 flakes, revealing how intrinsic phonon anisotropy and cavity resonances influence anisotropic optical phenomena.

Contribution

It introduces a novel birefringence-driven cavity with ultralow optical loss, supported by a unified model that explains mode-sensitive anisotropy and quantifies phonon anisotropy.

Findings

Enhanced anisotropy depends on flake thickness and Raman shift.

A unified model accurately reproduces experimental ARPR data.

Quantification of intrinsic phonon anisotropy provides new insights.

Abstract

Many anisotropic layered materials, despite their strong in-plane birefringence, exhibit substantial visible absorption, which severely restricts cavity lengths and hinders the observation of purely birefringence-governed optical phenomena. Here, we realize a birefringence-driven anisotropic optical cavity using $\alpha$-MoO3 flakes, capitalizing on their ultralow optical loss and pronounced in-plane birefringence. Using angle-resolved polarized Raman (ARPR) spectroscopy, we observe a mode-sensitive enhancement of anisotropy, dependent on both flake thickness and Raman shift. A unified model that incorporates the intrinsic Raman tensor, birefringence, and chromatic dispersion accurately reproduces the experimental data, elucidating how cavity resonances at both excitation and scattered wavelengths interact. Within this framework, the intrinsic phonon anisotropy is quantified, providing invaluable insights for accurately predicting ARPR responses and identifying crystallographic orientation. This work provides fundamental insights into birefringence-governed cavities and opens avenues for high-performance birefringent optics and cavity-enhanced anisotropic phenomena.