With the continuous development of optical technology, there is an increasing demand for the precise manipulation of light beams and efficient utilization of energy. However, traditional optical devices often have single functions, which makes it difficult to meet the complex and diverse needs of modern applications. In recent years, the introduction of phase change materials has not only enabled dynamic control of metasurfaces but also significantly enhanced their functionality and flexibility, which better addresses the requirements of multifunctional application scenarios. Phase change materials are highly sensitive to environmental changes and can alter their lattice states under external stimuli, which exhibits rapid phase switching and phase retention capabilities. Among various phase change materials, vanadium dioxide (VO₂) has attracted attention due to its unique electrical and optical properties. Currently, most VO₂-based metasurface structures can only achieve single-function control within specific wavelength ranges. However, research on metasurfaces that can integrate perfect absorption and anomalous reflection dual functions, while also possessing switchable characteristics, has not yet been reported. We theoretically and numerically propose a switchable dual-function metasurface based on the phase change characteristics of vanadium dioxide.

The designed metasurface structure consists of three layers: gold (Au) as the substrate, silicon dioxide (SiO₂) as the dielectric middle layer, and a top layer made of a cross-shaped VO₂ structure (Fig. 1). The electromagnetic simulations are performed by utilizing the finite-difference time-domain (FDTD) method. Circularly polarized light is incident on the metasurface along the negative direction of z-axis. To ensure the convergence of calculations, the simulation time is set to 30000 fs.

The simulation results indicate that utilizing the reversible phase transition of vanadium dioxide between metallic and dielectric states, the metasurface can flexibly switch between a dual-band perfect absorber and a four-channel beam splitter. Specifically, when vanadium dioxide is in the metallic state, the designed metasurface structure can be regarded as a metal-insulator-metal (MIM) model, where the gold substrate and the top layer of metallic VO₂ form a selective perfect absorber, which achieves dual-peak perfect absorption at wavelengths of 490 and 798 nm (Fig. 2). When vanadium dioxide is in the dielectric state, the structure can reflect the incident circularly polarized light into four equal-intensity anomalous reflected beams in the 720?770 nm wavelength range. This results in a conversion efficiency exceeding 90%, thus functioning as a broadband four-channel anomalous reflector (Fig. 3). Furthermore, as the wavelength increases, the anomalous reflection angle changes from 46.05° to 50.35° (Fig. 4). The theoretical calculations are consistent with the numerical simulation results. Finally, since the geometric dimensions of the structural parameters cannot be ignored in practical applications, we discuss the effects of the geometric parameters, i.e., thickness (h), structural period (P), cross width (w), and cross length (L), on the performance of the four-channel beam splitter. The results reveal that the structural parameters h and P significantly influence the anomalous reflection characteristics (Fig. 5), while variations in parameters w and L have a smaller impact on the anomalous reflection performance (Fig. 6).

In summary, we present a functional switchable metasurface based on the phase change characteristics of vanadium dioxide. By changing the temperature to alter the phase change characteristics of VO_2, it is possible to switch between dual-band optical perfect absorption and beam splitting. When VO₂ is in the metallic state, dual-peak perfect absorption is achieved at wavelengths of 490 nm and 798 nm. When VO₂ is in the dielectric state, the structure can achieve a conversion efficiency exceeding 90% for four-channel broadband anomalous reflection in the 720?770 nm wavelength range. Ultimately, this switchable dual-function metasurface structure based on vanadium dioxide can achieve precise control over the direction, intensity, and wavelength of light beams, which facilitates dynamic optical path switching, multi-wavelength splitting, and non-traditional beam manipulation. This development provides new technological pathways for the integration of photonic chips, super-resolution imaging, and quantum optical devices, thus promoting the innovative advancement of multifunctional optical devices.