Thermally Tunable Ultra-Broadband Terahertz Metasurface Absorber Based on VO₂ Phase Transition

Vanadium dioxide (VO₂), owing to its reversible insulator-to-metal phase transition, is a promising material for tunable terahertz (THz) devices. In this work, a compact VO₂-based terahertz absorber is numerically proposed to achieve thermally tunable ultra-broadband absorption. The proposed single-layer metasurface employs a fourfold-symmetric integration of cross-shaped, circular-ring, and square-frame elements to induce multiple coupled resonances and their spectral overlap, providing a design strategy for broadening the absorption bandwidth while maintaining polarization-insensitive operation. The optimized structure exhibits five near-perfect absorption resonances at 3.23, 5.27, 7.34, 8.82, and 9.73 THz. At a VO₂ conductivity of 2.0 × 10⁵ S/m, the absorber achieves absorptance exceeding 90% from 2.95 to 10.36 THz, corresponding to a bandwidth of 7.41 THz and a relative bandwidth of 111.35%. The absorption mechanism is attributed to the hybridization of multiple resonant modes and effective impedance matching with free space, as confirmed by electromagnetic field and impedance analyses. Furthermore, the absorption response can be dynamically tuned through the phase transition of VO₂. The absorber also exhibits polarization-insensitive behavior and maintains high absorptance over a wide range of incident angles. These results demonstrate a compact and thermally reconfigurable platform for terahertz imaging, sensing, spectroscopy, and adaptive electromagnetic stealth.