VO2-Based Switchable Terahertz Chiral Metasurface for Circular Dichroism Sensing and Broadband Polarization Control

Terahertz (THz) chiral metasurfaces have attracted considerable attention for manipulating circular polarization. However, most reported designs offer either narrowband or broadband circular dichroism (CD) with fixed spectral characteristics, limiting their adaptability to multifunctional scenarios. This work proposes a switchable terahertz chiral metasurface integrating CD response and refractive-index sensing based on the thermally induced insulator-to-metal transition of vanadium dioxide (VO2) at approximately 68 °C. Without altering the geometric structure, the device can dynamically switch between narrowband and broadband CD responses via the VO2 phase transition. In the insulating state, the structure produces two sharp CD resonant peaks at 1.88 THz and 3.25 THz with CD values of 0.93 and 0.88, respectively. These narrowband resonances exhibit refractive-index sensing capability: as the ambient refractive index increases across an extended dynamic range from 1.00 to 1.60, the resonant frequencies undergo a continuous redshift, achieving sensitivities of 299.3 GHz/RIU and 258.0 GHz/RIU with figures of merit (FOMs) of 4.6 RIU−1 and 5.3 RIU−1, respectively. Upon the phase transition to the metallic state, the drastically enhanced VO2 conductivity redistributes the local current density and strengthens inter-resonator coupling, which broadens the localized narrowband resonances and triggers inter-mode coupling. Consequently, the discrete dual-narrowband CD response evolves into a broadband feature. In the metallic state, the maximum CD reaches 0.75 and remains above 0.5 across 1.56–3.48 THz, corresponding to an absolute bandwidth of 1.92 THz and a relative bandwidth of 76%—a substantial broadening compared with the insulating state. These results suggest that the proposed metasurface holds promise for reconfigurable terahertz chiral sensing, broadband polarization filtering, and multifunctional terahertz devices.