We theoretically investigate enhanced mid-infrared absorption in a one-dimensional zero-contrast grating structure incorporating a thermally tunable vanadium dioxide ( \(\hbox {VO}_{2}\) ) layer atop a metallic substrate. At lower temperatures, \(\hbox {VO}_{2}\) remains in its insulating phase with low optical loss, enabling it to function as a low-loss cavity spacer that supports Fabry-Pérot resonances under transverse magnetic polarization. When combined with the guided mode resonance effects induced by the zero-contrast grating, strong optical field confinement occurs within the \(\hbox {VO}_{2}\) layer, leading to pronounced absorption peaks at mid-infrared wavelengths (16–19 \(\mu \) m). We employ rigorous coupled-wave analysis to systematically analyze the optical responses, revealing that proper tuning of the grating period (9–12 \(\mu \) m), fill factor (0.2 \(-\) 0.8), and \(\hbox {VO}_{2}\) thickness (3 \(-\) 3.5 \(\mu \) m) results in narrowband absorptance exceeding 90% at resonance. The underlying molybdenum layer acts as a back reflector to suppress transmission, further enhancing light trapping. The absorption characteristics can be significantly modulated by the thermally induced phase transition of \(\hbox {VO}_{2}\) , offering dynamic control over resonant absorption. Additionally, the structure exhibits azimuthal angle-dependent behavior and supports enhanced absorption even under transverse electric polarization. The interplay between guided mode resonance and Fabry-Pérot cavity effects in the low-loss \(\hbox {VO}_{2}\) phase offers a passive route to achieve spectrally selective and thermally switchable absorption. These findings have potential applications in tunable infrared sensors, thermal emitters, and actively controllable photonic devices.
