  Integration of conventional mid to long-wavelength infrared 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
non-lithographic 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 $\alpha$-MoO$_3$ is investigated for
application towards high temperature mid-IR transmission and reflection type
thin film polarizer. To our knowledge, this is the first report of above room
temperature mid-IR optical response of $\alpha$-MoO$_3$ to determine the
thermal stability of the proposed device. We find that our $\alpha$-MoO$_3$
based polarizer retains high extinction ratio with peak value exceeding 10 dB,
up to a temperature of 140$^{\circ}$C. We explain our experimental findings by
natural in-plane hyperbolic anisotropy of $\alpha$-MoO$_3$ 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.
