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Low‐symmetry van der Waals materials are promising candidates for the next generation of polarization‐sensitive on‐chip photonics since they do not require lattice matching for growth and integration. Due to their low‐symmetry crystal behavior, such materials exhibit anisotropic and polarization‐dependent optical properties for a wide range of optical frequencies. Here, depolarization characteristics of orthorhombic α‐MoO₃is studied in the visible range. Using polarizers and analyzers, it is demonstrated that α‐MoO₃has negligible loss and that birefringence values as high as 0.15 and 0.12 at 532 nm and 633 nm, respectively, are achievable. With such a high birefringence, quarter‐ and half‐wave plate actions are demonstrated for a 1400 nm α‐MoO₃flake at green (532 nm) and red (633 nm) wavelengths, and polarizability as high as 90% is reported. Furthermore, a system of double α‐MoO₃heterostructure layer is investigated that provides the possibility of tuning polarization as a function of rotation angle between the α‐MoO₃layers. These findings pave the way to the promising future of on‐chip photonic heterostructures and twist‐optics that can dictate the polarization state of light.

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, α-MoO₃has 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 α-MoO₃to 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 α-MoO₃is reported without the need for nanoscale fabrication on the α-MoO₃. We observe coupling between the α-MoO₃optical 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.