Asymmetric Fabry-Pérot cavities for thermally tunable multimode perfect THz absorption

Terahertz (THz) absorbers are attractive for emerging 6G links, imaging, sensing and electromagnetic protection; however, practical devices often face coupled trade-offs among absorption bandwidth, structural complexity, polarization/angle robustness and active tunability. Here, a dual-functional-layer metamaterial absorber is proposed. Unlike conventional single-layered designs, the synergy between a periodically patterned VO₂ layer and a continuous VO₂ film forms an asymmetric Fabry-Pérot (F-P) cavity that reconfigures the internal field distribution across different phase states. This design allows a continuous VO₂ film and a periodically patterned c layer to form an asymmetric F-P cavity, while a metallic backplane eliminates transmission. Finite-element simulations are carried out from 0.1 to 20 THz, and the conductivity evolution across the VO₂ insulator-to-metal transition is described with a Drude model. When both VO₂ layers are in the metallic state, the absorber provides ultrabroadband near-perfect absorption, exhibiting absorptance above 90% from 3.25-16.56 THz with an average absorptance of approximately 96.4%. By programming the phase states of the two VO₂ layers, the response can be switched among an ultrabroadband mode, a dual-band mode (2.15-6.17 THz and 11.75-16.52 THz), and a narrowband mode with a peak absorptance of about 99.98%. The design is essentially polarization-insensitive for rotation angles from 0° to 90° and preserves high absorption up to 60° incidence under both TE and TM polarizations. These results demonstrate a compact route to multifunctional THz absorbers combining ultrabroad bandwidth, wide-angle robustness and reconfigurable control for adaptive THz systems.