Terahertz wavefront manipulation, particularly flexible switching of terahertz wavefronts through programmable metasurface, has good prospects for application. Electricity, temperature, and light control are the most common programmable metasurface control methods available today. In order to directly modulate an electronically controlled metasurface, diodes loaded on both ends of a particular metal structure are typically powered through an external power supply. However, due to the structural characteristics of the diodes, it is difficult to reduce the cell area for terahertz wave modulation. Temperature-controlled metasurfaces use phase-change materials to change the metasurface's reaction to electromagnetic waves. However, precise control of the ambient temperature is required. In addition, the phase-change material takes time to alter states, which induces the terahertz wave's low modulation efficiency. Optically controlled metasurfaces can integrate photosensitive semiconductors into elements to control the amplitude and phase characteristics of metasurface units by changing the properties of photosensitive materials. However, existing metasurfaces lack accurate optical control of the individual elements, which makes it impossible to modify the wavefront of incident terahertz waves by encoding the elements according to digital metamaterials theory. In this paper, a 2 bit phase-encoded metasurface unit is realized by controlling the conductivity of the embedded photosensitive semiconductor in the top metal split ring of the cell through the encoding of the structural light source to simulate different C-shaped rings. Subsequently, by encoding the spatial distribution of structured light, the developed metasurface units are built into arrays to generate angularly adjustable anomalous reflections and vortex beams of different orders. A new optical control method is proposed here. It is combined with spatially encoded structured light and can efficiently solve the problems of the existing optical control metasurface's single function and high processing difficulty.

In this paper, a metal split ring construction with an embedded photosensitive germanium material is used to imitate the reaction of a C-shaped ring to terahertz radiation. By changing the electrical conductivity of the embedded photosensitive germanium material with the different encoding of structural light, it switches between the metallic and insulated states, thus changing the connection state of the metal split ring on the top layer of the metasurface. The metal split ring with different connection states can polarize the incident terahertz waves while adjusting their phases. In this paper, we realize a metasurface unit with 2 bit phase encoding by finding a suitable encoded structured light. In the next step, we combine the designed 2 bit phase encoded units with digital encoding metamaterial theory to form different metasurface arrays to achieve anomalous reflections at different angles and vortex beams of different orders, thus verifying the correctness of the metasurface cell design.

The period of the optically controlled metasurface unit designed in this paper is 40 μm (Fig. 2). The 2 bit phase difference at 3.5 THz is achieved by changing the connection state of the metal split ring at the top of the metasurface with the different encoding of structured light. In this study, we digitally encode structured light and photosensitive materials in the form of least significant bit (LSB) (Fig. 3). Bit 0, Bit 1, Bit 2, and Bit 3 correspond to the codes 11000110, 00111000, 00011011, and 00001110 (Table. 1), respectively. The phases of Bit 0, Bit 1, Bit 2, and Bit 3 at 3.5 THz on the metasurface are 68.2°, 157.8°, 248.3°, and 337.8°, respectively. The amplitude of the cross-polarization of metasurface units is approximately the same while forming a 2 bit phase difference. The amplitudes of Bit 0, Bit 1, Bit 2, and Bit 3 at 3.5 THz are -0.91 dB, -1.196 dB, -0.91 dB, and -1.194 dB, respectively (Table. 2). In this paper, Bit 0 and Bit 2 are selected to form arrays with different lattice periods to achieve a double-beam splitting with beam splitting angles of 32°, 20°, and 15.3° (Fig. 6). Next, a single-beam anomalous reflection is achieved by selecting all bit units to form 8, 12, and 16 lattice metasurface arrays with reflection angles of 15.4°, 10.1°, and 7.4° (Fig. 7), respectively, and the angle decreases with increasing phase gradient encoding period. The vortex beam has a wide range of applications, including increasing system communication efficiency and capacity convenience. The 1st-order, 2nd-order, and 3rd-order vortex beams are realized by different spiral phase encoding methods (Fig. 8), and the beam center depressions are all more than 25 dB below the main flap, with good performance.

In this paper, a novel optically controlled metasurface method based on encoded structured light is presented. The combination of spatially encoded structured light and photosensitive semiconductors achieves independent amplitude phase tuning of the optically controlled metasurface unit. The problem of precise tuning of the optically controlled terahertz metasurface unit is solved. In this paper, the 2 bit phase encoded optically controlled terahertz metasurface is realized by combining the digital metamaterial theory. With the theory of digital metamaterials, anomalous reflections at different angles and vortex beams of different orders are achieved by different encoding methods, which realizes the flexible modulation of terahertz wavefront and enriches the application of optically controlled terahertz metasurfaces, and a new idea for developing optically controlled metasurfaces is provided.