Birefringence‐Driven Anisotropic α‐MoO ₃ Dual‐Wavelength Optical Cavities

ABSTRACT Many anisotropic layered materials (ALMs), 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 ‐ 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. Compared with the framework in absorptive ALMs, a birefringent dual‐wavelength cavity model that incorporates birefringence, separated cavity responses for incident/scattered photons with chromatic dispersion accurately reproduces the experimental data, elucidating how cavity resonances at both excitation and scattered wavelengths interact. Within this model, 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.