Highly tunable mid-infrared perfect absorber based on unpatterned α-MoO₃ integrated with an inverted metasurface

α-phase molybdenum trioxide (α-MoO₃) is a promising van der Waals crystal for mid-infrared nanophotonics. However, direct patterning of this material typically induces severe lattice damage and edge scattering losses. Here, we propose a tunable mid-infrared perfect absorber that integrates a continuous, unpatterned α-MoO₃ film with an inverted dielectric-metal (SiO₂/Au) metasurface. This architecture leverages guided-mode resonances within the patterned SiO₂ layer to concentrate strong evanescent fields into the intact α-MoO₃ film. Full-wave simulations and temporal coupled-mode theory (TCMT) corroborate that the system satisfies the critical coupling condition, thereby achieving near-unity absorption (>98%). Owing to the pronounced in-plane optical anisotropy of α-MoO₃, the absorber exhibits a highly polarization-dependent response. Specifically, the transverse electric (TE) polarization shows relatively robust angular and polarization responses, whereas the transverse magnetic (TM) polarization exhibits stronger angular sensitivity with pronounced spectral evolution at large incidence angles. Furthermore, the resonant wavelength can be effectively tuned by adjusting the geometric parameters and the surrounding dielectric environment. By circumventing direct etching of the van der Waals crystal, this design offers a promising avenue for polarization-multiplexed optoelectronics, directional thermal emitters, and mid-infrared sensing.