With the rapid advancement of information technology and the increasing depth of research on metasurfaces, the static and uncontrollable nature of traditional metasurfaces has limited their further development. As a result, dynamically tunable chiral metasurfaces are in high demand across many application fields. Vanadium dioxide (VO₂), a novel two-dimensional material with dynamic tunability, offers significant potential in this regard. In recent years, the integration of metasurfaces with VO₂ has led to the design of various dynamically tunable metasurfaces. However, most of the proposed tunable chiral metasurfaces feature complex unit structures, suffer from low efficiency in practical applications, and are limited to single-band functionality. Therefore, when designing new chiral metasurface devices, factors such as material loss, structural simplicity, and the tunability of achievable functionalities must be considered. These challenges have become a critical topic in current research on chiral metasurfaces.

The metasurface we designed consists of a three-layer unit cell (Fig. 1). The bottom layer is a metallic reflective layer, the middle layer is a dielectric layer made of Topas (cyclic olefin copolymer), and the top layer is composed of a composite material of VO₂ and gold. The top-layer resonant pattern comprises two sets of semicircular arcs and rectangular patches. Each semicircular arc, with a width of w, is connected to a rectangular patch with a length of l₁ at its outer edge. Both sets of elements are made of gold and are arranged in a centrosymmetric configuration to form the overall resonant structure. The left gold rectangular patch is extended by a rectangular VO₂ patch with a length of l₂. Other parameters include period of P, representing the period of the unit cell. The bottom gold layer serves to suppress electromagnetic wave transmission, with its thickness set to 0.1 μm to ensure that all incident electromagnetic waves are reflected. The dielectric layer is made of Topas, which has a relative permittivity of 2.57. Characterized by transparency, thermally stable, and exhibits excellent optical properties, this material demonstrates negligible absorption coefficient in the terahertz range. The thickness of the dielectric layer is h₁. The top layer has a thickness of h₂, with the ring’s width w, outer radius R, gold rectangular patch length l₁, and VO₂ rectangular patch length l₂. Through simulation, the optimized geometric dimensions of the structure are as follows: l₁=10 μm, l₂=6 μm, R=14 μm, w=5 μm, h₁=15 μm, h₂=1.5 μm, P=50 μm. In the simulations, we use CST Microwave Studio software to calculate the optical properties of the structure through full-wave simulations in the frequency domain. Periodic boundary conditions are applied along the x-axis and y-axes for the basic unit cell, with the light source incident along the -z direction. Open boundary conditions are set along the z-axis.

We have proposed a dual-band terahertz chiral metasurface based on VO₂ material. The designed structure achieves remarkable circular dichroism (CD) responses of up to 0.91 and 0.82 at 1.93 THz and 3.83 THz, respectively, demonstrating excellent dual-band performance (Fig. 2). The dynamic tunability of the chiral response is enabled by the phase transition properties of VO₂ between its insulating and metallic states (Fig. 3). Additionally, leveraging the Fabry?Pérot resonance effect between the top layer and the metallic bottom layer, the circular dichroism response can be switched from dual-band to single-band by adjusting the thickness of the dielectric layer (Fig. 3). Based on this characteristic, we designed a multi-frequency circularly polarized wave detection and image encryption scheme. By combining single-band and dual-band metasurfaces, the system generates distinct imaging signals for different polarized waves at two frequencies (Fig. 7), enabling multiplexed digital imaging functionality.

In summary, we have proposed a design for a dynamically tunable dual-band chiral metasurface absorber based on VO₂ material. Theoretical results show that when VO₂ is in its metallic state, the metasurface achieves left-circularly polarized (LCP) absorption rates of up to 98.56% and 93.90% at 1.93 THz and 3.83 THz, respectively, while the absorption rate for right-circularly polarized (RCP) is less than 12% across the 1?4 THz. The CD values at the two resonant frequencies, 1.93 THz and 3.83 THz, are 0.91 and 0.82, respectively. When VO₂ is in its insulating state, the CD effect is significantly reduced. By adjusting the conductivity of VO₂, the optical response of the metasurface can be effectively controlled, enabling selective absorption of circularly polarized light in different states. Additionally, due to the Fabry?Pérot resonance, the CD response can be switched from dual-band to single-band by varying the thickness of the dielectric layer. Furthermore, based on terahertz near-field imaging, this structure shows potential applications in dual-frequency circularly polarized wave detection and image encryption.