An ultra-broadband tunable chiral metasurface for circular dichroism and polarization conversion

An ultra-broadband tunable chiral metasurface (CMS) based on vanadium dioxide (VO₂) is proposed in the terahertz band. The designed CMS can be switched from exhibiting circular polarization conversion (CPC) functionality to possessing CPC and circular dichroism (CD) characteristics simultaneously by employing insulator-to-metal transition in VO₂, and the polarization conversion ratio (PCR) and CD value are dynamically manipulated by adjusting the conductivity of VO₂ (σ_VO2). When VO₂ is in the insulating phase, the CMS can realize left-handed circularly polarized (LCP) to right-handed circularly polarized (RCP) and RCP-to-LCP conversion. The polarization conversion ratios for LCP-to-RCP (PCR_LCP) and RCP-to-LCP (PCR_RCP) waves both exceed 90% in 4.80-10.84 THz, with a corresponding relative bandwidth (RB) of 77.24%. When VO₂ is in the metallic phase, the CMS simultaneously possesses an ultra-broadband LCP-to-RCP conversion function and a strong CD response, which can be applied in near-field imaging. Specifically, CD > 0.80 is achieved in the 4.63-10.51 THz range (RB = 77.68%). Meanwhile, the PCR_LCP above 90% is obtained in the 4.38-11.15 THz range (RB = 87.19%). The mechanisms of CPC and CD are revealed by analysing the impedance matching and the distributions of surface current, electric field and magnetic field at the resonant frequencies. In addition, the influences of σ_VO2, incident angle and polarization angle on the CPC and CD performances are systematically investigated. The proposed design features an ultra-compact profile while integrating ultra-broadband CPC and CD dual functionalities, thus holding enormous application potential in the fields of polarization imaging and polarization sensitive detection of electromagnetic waves.