A multimodal switchable terahertz metamaterial absorber based on the coupling of VO₂ and graphene

A switchable terahertz metamaterial absorber based on the phase transition of vanadium dioxide (VO₂) and a graphene resonant layer is presented. The absorber consists of a six-layer stacked structure comprising a VO₂ double-ring layer, a Topas dielectric layer, a graphene layer, a patterned Au layer, a second Topas dielectric layer, and an Au ground plane. Dynamic switching between two absorption modes is achieved through the thermally induced reversible phase transition of VO₂. When VO₂ is in the metallic state (σ = 2 × 10⁵ S m^-1), the absorber operates in a coexistence mode consisting of three low-frequency narrowband resonances at 0.534 THz, 0.995 THz, and 1.482 THz with peak absorptions of 91%, 97%, and 99%, respectively, together with a high-frequency broadband absorption response exceeding 90% over 2.16-3.68 THz. After VO₂ switches to the insulating state (σ = 200 S m^-1), the broadband response is replaced by five narrowband resonances located at 0.528 THz, 0.995 THz, 1.482 THz, 2.500 THz, and 3.000 THz, with corresponding peak absorptions of 91%, 97%, 99%, 97%, and 82%, respectively. The electric-field and surface-current distributions indicate that the absorption characteristics arise from distinct electromagnetic coupling mechanisms associated with different resonant layers. Owing to the fourfold rotational symmetry of the resonant structure, the absorber exhibits polarization-independent performance and maintains stable absorption for incident angles from 0° to 60°. The proposed absorber therefore shows considerable potential for terahertz imaging, sensing, material inspection, wireless communications, and electromagnetic stealth.