Lithography-free, planar IR polarization filters and converters via biaxial phonons in a-MoO3 flakes integrated into Fabry-Perot cavities

Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical interactions and enabling inherent simplified device integration. Recently, a-MoO3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally anisotropic 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 by tailoring 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.