Terahertz (THz) vortex beams are electromagnetic waves characterized by helical phase structures and frequencies ranging from 0.1 to 10 THz (wavelengths from 30 to 3000 μm). These beams demonstrate significant potential in emerging applications, including broadband communication, military radar, high-resolution THz imaging, electron acceleration, and quantum state manipulation. While current research has achieved multimodal vortex beams, their elevation angles remain fixed. This paper introduces a 2-bit coding phase gradient metasurface designed to generate multimodal vortex beams with switchable elevation angles. The integration of two tunable materials enables dynamic switching of the elevation angle of multimodal vortex beams, presenting applications in radar detection, wireless communication, and stealth technology.

According to the Pancharatnam-Berry (PB) geometric phase principle, a phase gradient is introduced to design the coding elements, which are arranged following a specific coding sequence. Through the phase superposition principle, a multimodal vortex beams coding phase gradient metasurface is developed. The coding elements incorporate two tunable materials, photosensitive silicon and vanadium dioxide (VO₂). By modifying the control methods, the top-layer structure of the metasurface unit undergoes adjustments, introducing different phase gradients and forming three distinct sets of coding elements. State A represents an unregulated condition, where neither material is affected. During this state, VO₂ maintains a dielectric state, and the photosensitive silicon exhibits dielectric properties. State B implements optical control, regulating the photosensitive silicon while leaving VO₂ unaffected. In this state, the photosensitive silicon displays metallic properties, while VO₂ remains dielectric. State C employs both thermal and optical controls, regulating both materials, resulting in metallic properties for both VO₂ and photosensitive silicon.

Under linearly polarized (LP) wave excitation, the co-polarized reflection amplitude and phase difference of the designed coding phase gradient metasurface unit (Fig. 1) satisfy the requirements of the PB geometric phase principle (Fig. 3). In different states, the top structure of the metasurface unit changes, leading to variations in the phase gradient and the size of the coding elements, thereby realizing the control of the elevation angle of the emitted beam. In state A, the central open circle is active; in state B, the small open ring is active; in state C, the large open ring is active (Fig. 2). Subsequently, three sets of coding elements with different phase gradients were formed in states A, B, and C, with sizes of 4×4, 6×6, and 9×9, respectively (Figs. 4?6). After arrangement, the far-field scattering of the coding phase gradient metasurface was simulated using CST Microwave Studio. The results indicate that without regulation, when a 1.2 THz LP wave is incident perpendicularly, the metasurface generates multimodal vortex beams with an elevation angle of 16° (Fig. 7); with optical control alone, a 1.0 THz LP wave incident perpendicularly yields an elevation angle of 20° (Fig. 8); and when both optical and thermal controls are applied, a 0.54 THz LP wave incident perpendicularly results in an elevation angle of 28° (Fig. 9). The simulation results are basically consistent with the theoretical values calculated using the generalized Snell’s law.

This research presents a coding phase gradient metasurface capable of simultaneously generating vortex beams with topological charges of l=-1 and l=+1 in the x-direction, and l=+2 and l=-2 in the y-direction. The elevation angles can be switched by modifying the control methods of two tunable materials to activate different coding elements on a single metasurface, thereby altering the phase gradient. This approach provides an efficient method for flexible control of terahertz beams, offering significant potential in wireless communication, radar detection, and high-resolution imaging applications. The capability to switch angles of multimodal vortex beams enhances adaptability across various applications, establishing this metasurface as a promising component for advanced terahertz technologies.