A dual-mode ultrabroadband terahertz absorbing metasurface for linear and circular polarization

Terahertz metasurfaces are promising platforms for compact sensing, imaging, and communication systems; however, ultrathin absorbers that combine broadband absorption, circular-polarization selectivity, and active reconfigurability remain limited. Here, we propose a VO₂-controlled dual-mode terahertz absorbing metasurface comprising a VO₂ functional layer, a TOPAS spacer, an in-plane asymmetric chiral metallic/high-impedance-surface resonant layer, an MF₂ dielectric layer, and an Au ground plane. The chiral resonant layer produces handedness-dependent near-field coupling, whereas the VO₂ phase transition reconfigures the interlayer conductive pathways and impedance-matching conditions. Numerical simulations show that, with VO₂ in the insulating state, the metasurface selectively absorbs right-handed circularly polarized (RCP) waves, with an absorptance above 90% from 1.58 to 3.79 THz, while the absorptance for left-handed circularly polarized (LCP) waves remains low. The circular dichroism exceeds 0.8 from 1.60 to 3.65 THz. When VO₂ switches to the metallic state, the handedness-dependent contrast is reduced, and the metasurface exhibits broadband absorption for both circular polarizations. Under linearly polarized incidence, metallic VO₂ enables broadband absorption in the transverse-electric (TE) channel from 1.45 to 4.45 THz and maintains a strong transverse-magnetic (TM) response over a broad frequency range. This numerical design may provide a route towards reconfigurable terahertz absorbers for polarization-resolved detection and broadband functional devices.