  Exploiting polaritons in natural vdW materials has been successful in
achieving extreme light confinement and low-loss optical devices and enabling
simplified device integration. Recently, a-MoO3 has been reported as a
semiconducting biaxial vdW material capable of sustaining naturally orthogonal
in-plane phonon polariton modes in IR. In this study, we investigate the
polarization-dependent optical characteristics of cavities formed using a-MoO3
to extend the degrees of freedom in the design of IR photonic components
exploiting the in-plane anisotropy of this material. Polarization-dependent
absorption over 80% in a multilayer Fabry-Perot structure with a-MoO3 is
reported without the need for nanoscale fabrication on the a-MoO3. We observe
coupling between the a-MoO3 optical phonons and the Fabry-Perot cavity
resonances. Using cross-polarized reflectance spectroscopy we show that the
strong birefringence results in 15% of the total power converted into the
orthogonal polarization with respect to incident wave. These findings can open
new avenues in the quest for polarization filters and low-loss, integrated
planar IR photonics and in dictating polarization control.
