We design a switchable terahertz metamaterial device based on an array of cross-shaped unit cell structures. By introducing vanadium dioxide (VO₂), we achieve cross-polarization conversion, linear-to-circular polarization conversion, and broadband absorption across different terahertz frequency bands, demonstrating multifunctionality.

We conduct electromagnetic simulations using CST MICROWAVE STUDIO software to analyze the device’s performance. The VO₂-based metamaterial consists of six layers: a VO₂ cross-shaped patch array, a polyimide (PI, ε_PI=3.5) layer, an elliptical metal strip, a VO₂ thin film, another PI layer, and a bottom metal layer. Geometrical dimensions are optimized through numerical calculations, resulting in a unit structure period P=70 μm, surface cross patch dimensions a=15 μm, b=4 μm, and dielectric layer thickness h=13 μm. The major and minor axes of the elliptical metal patch are m=44.8 μm and n=12 μm. The VO₂ thin film, elliptical metal strip, and bottom metal (σ_Au=4.56×10⁷ S/m) have a thickness of t=0.1 μm. The phase transition of VO₂ between its insulating and metallic states is triggered by external stimuli, such as temperature, electromagnetic fields, and optical fields. We assume conductivities of 2×10⁵ S/m and 20 S/m to represent the metallic and insulating states of VO₂, respectively. By changing the conductivity, the device can switch between absorption and polarization conversion functionalities.

We design a multifunctional terahertz metamaterial structure based on the phase transition characteristics of VO₂, which achieves broadband absorption, cross-polarization conversion, and linear-to-circular polarization conversion simultaneously. The structure has the following features: First, it adopts a symmetric design, making the metasurface absorber polarization-insensitive. Second, in terms of performance, when VO₂ is in its insulating state, the structure enables linear-to-circular polarization conversion and linear-to-linear cross-polarization conversion across multiple frequency bands. Specifically, linear-to-circular polarization conversion can be achieved at 0.68, 0.810?1.175, 2.225, 2.425?2.520, 3.13, 3.31, 3.570?3.670, 4.04, 4.152?4.395, and 4.495?4.625 THz frequency ranges or points. Cross-polarization conversion occurs within the 1.365?2.150, 2.579?3.065, and 3.770?3.981 THz frequency bands, with polarization conversion rates exceeding 90%. When VO₂ is in the metallic state, the absorption rate exceeds 90% within the 1.610?4.010 THz frequency range, demonstrating wide bandwidth and high efficiency. This is particularly important for applications requiring multi-wavelength signal processing, as high-efficiency terahertz wave absorption reduces loss and enhances overall system performance.

This study presents the design of a switchable terahertz metamaterial device based on a cross-shaped unit structure array. By incorporating VO₂, the device achieves multifunctionality, including cross-polarization conversion, linear-to-circular polarization conversion, and broadband absorption across different terahertz frequency bands. When VO₂ is in its insulating state, the structure enables linear-to-circular polarization conversion and cross-polarization conversion over multiple frequency bands. As VO₂ transitions from the insulating to the metallic state, the absorption rate exceeds 90% within the 1.610?4.010 THz range, offering broad bandwidth and high efficiency. Additionally, we study the influence of the incident angle and polarization angle of terahertz waves on the device’s polarization conversion and absorption characteristics, demonstrating its polarization-insensitive and wide-angle absorption capabilities. The results indicate that the proposed device’s unique multilayer stacked structure not only provides excellent absorption performance but also enables rapid switching between various polarization states, highlighting its great potential for terahertz imaging, communication, and security screening applications.