Mid infrared polarization engineering via sub-wavelength biaxial hyperbolic van der Waals crystals

Recently, in-plane biaxial hyperbolicity has been observed in $\alpha$-MoO${_3}$ --a van der Waal crystal-- in the mid-infrared frequency regime. Here, we present a comprehensive theoretical analysis of thin film $\alpha$-MoO${_3}$ for application to two mid-IR photonic devices -- a polarizer and a waveplate. We show the possibility of a significant reduction in the device footprint while maintaining an enormous extinction ratio from $\alpha$-MoO${_3}$ based polarizers in comparison with that of conventional polarizers. Secondly, we carry out device optimization of $\alpha$-MoO${_3}$ based waveplates with subwavelength thickness. We explain our results using natural in-plane hyperbolicity of $\alpha$-MoO${_3}$ via analytical and full wave simulations. This work will build a foundation for miniaturization of mid-infrared photonic devices by exploiting the optical anisotropy of $\alpha$-MoO${_3}$.