High Temperature Mid-IR Polarizer via Natural In-Plane Hyperbolic Van der Waals Crystals

Integration of conventional mid to long-wavelength infrared (IR) polarizers with chip-scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its nonlithographic fabrication, ease of integration with chip-scale platforms, and room temperature operation. In the present work, mid-IR optical response of the sub-wavelength thin films of alpha-phase molybdenum trioxide (alpha-MoO3 ) is investigated for application toward high temperature mid-IR transmission and reflection type thin film polarizer. To the authors' knowledge, this is the first report of above room temperature mid-IR optical response of alpha-MoO3 to determine the thermal stability of the proposed device. It is found that the alpha-MoO3 based polarizer retains high extinction ratio with peak value exceeding 10 dB, up to a temperature of 140 degrees C. The experimental findings are explained by natural in-plane hyperbolic anisotropy of alpha-MoO3 in the mid-IR, high temperature X-ray diffraction and Raman spectroscopic measurements. This work opens up new avenues for naturally in-plane hyperbolic van der Waals thin-films to realize sub-wavelength IR optical components without lithographic constraints.