Enhanced Mid-Infrared Light Trapping in Vanadium Dioxide-Loaded One-Dimensional Zero-Contrast Gratings on Metal Substrate

We theoretically investigate enhanced mid-infrared (mid-IR) absorption in a one-dimensional zero-contrast grating (ZCG) structure incorporating a thermally tunable vanadium dioxide (VO 2) layer atop a metallic substrate. At lower temperatures , VO 2 remains in its insulating phase with low optical loss, enabling it to function as a low-loss cavity spacer that supports Fabry-Perot (FP) resonances under transverse magnetic (TM) polarization. When combined with the guided mode resonance (GMR) effects induced by the ZCG, strong optical field confinement occurs within the VO 2 layer, leading to pronounced absorption peaks at mid-IR wavelengths (16-19 µm). We employ rigorous coupled-wave analysis (RCWA) to systematically analyze the optical responses, revealing that proper tuning of the grating period (p= 9-12 µm), fill factor (0.2-0.8), and VO 2 thickness (3-3.5 µm) results in narrowband absorptance exceeding 90% at resonance. The underlying molybdenum (Mo) 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 VO 2 , offering dynamic control over resonant absorption. Additionally, the structure exhibits azimuthal angle-dependent behavior and supports enhanced absorption even under transverse electric (TE) polarization. The interplay between GMR 1 and FP cavity effects in the low-loss 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.