Angle-robust and tunable mid-infrared absorption in a Ge grating/SrTiO ₃ /VO ₂ hybrid metastructure on a metallic substrate

We propose a dynamically tunable and angle-robust mid-infrared (mid-IR) absorber based on a hybrid metastructure composed of a top-layer Ge grating, an ultrathin SrTiO₃ polar dielectric layer, a thermochromic VO₂ film, and a metallic substrate. The optical response of the system is modeled using rigorous coupled-wave analysis (RCWA), revealing broadband and high-efficiency absorption across a wide range of incident angles (0°-80°) under transverse-magnetic (TM) polarization. The absorption behavior is governed by the interplay of multiple resonant mechanisms, including guided-mode resonance (GMR) in the Ge grating, phonon-polariton (PhP) excitation in the SrTiO₃ layer, and cavity-like modes facilitated by the insulating VO₂. By varying structural parameters such as the grating period (<i>p</i> = 6-10 μm), fill factor (<i>f</i> = 0.4-0.7), and the thicknesses of SrTiO₃ and VO₂, we map the design space for optimized performance. Numerical results confirm that strong field confinement and mode hybridization are responsible for the observed angular stability and tunability. This theoretical framework provides critical insights for the design of active mid-IR metastructures with application potential in thermal modulation, reconfigurable optics, and infrared photonic circuitry.