Objective Terahertz waves are electromagnetic waves between microwave and infrared wave with the frequency of 0. 1-10 THz and feature strong penetration, large information capacity, high security, and strong maneuverability. Additionally, they have extensive applications in remote communication, security imaging, radar detection, and other fields. With the increasing number of application scenarios, there is an urgent need for functional devices that can regulate terahertz waves in multiple frequency bands. As an important device to regulate terahertz waves, the coding metasurface is characterized by the phase response properties of the coding metasurface units by binary digital code and arranges the coding metasurface units according to the coding sequence to achieve flexible wave regulation. Meanwhile, it can generate various beam forms such as vortex waves, deflection waves, and focused waves. However, once a traditional coding metasurface is designed, it can only generate a beam form at a single frequency point, limiting the working frequency range of the coding metasurface. As a kind of phase change material, vanadium dioxide (VO2) can change its temperature by electricity, heat, and light to achieve phase change function, and is widely applied to metasurface design. Some studies implement the function of generating different beam forms by different coding sequences, but the working frequency band is single and cannot be switched. Another study adopts the PB phase principle combined with VO2 to design a frequency switchable coding metasurface, which achieves vortex wave generation at different frequency points. However, it only yields good results at three frequency points, limiting the working frequency range. Therefore, it is significant to broaden the working frequency range of the coding metasurface and achieve frequency band switching. Methods First, a new type of coding metasurface unit is designed by combining the PB phase principle with the VO2 phase transition characteristics. By rotating the unit at a certain angle and changing the phase transition state of VO2, the reflection amplitude and phase in different working frequency ranges are studied. The conditions are as follows. When VO2 is in an insulated state and a metallic state, it works in different frequency bands respectively and meets the conditions of 3-bit coding metasurface unit in the corresponding frequency band. Then, by taking the terahertz metasurface which can generate high-capacity vortex waves and high RCS reduced scattering waves as an example, a coding sequence is designed. Finally, the wave forms generated by the coding metasurface are simulated at different frequencies to study whether the beam form corresponding to the coding sequence can be generated. By changing the phase transition state of VO2, switching of the operating frequency band can be achieved. Results and Discussions By rotating the designed metasurface units (Fig. 1) counterclockwise in a step of 22. 5 degrees from 0 degrees to 157. 5 degrees, eight metasurface units can be obtained (Table 1). The unit analysis based on the two phase transition states of VO2 shows that when VO2 is in an insulated state, the unit maintains a large reflection amplitude between 1. 17 THz and 1. 37 THz, and the phase difference of the eight units strictly meets a 45 degrees phase difference. When VO2 is in a metallic state, it maintains a large amplitude and a phase difference of 45 degrees in sequence at 0. 87-0. 92 THz and 1. 4-1. 6 THz. Therefore, at all three frequency bands, the metasurface unit meets the design conditions for a 3-bit coding metasurface unit. The coding metasurface units are arranged according to a certain coding sequence, the coding metasurface formed by them can flexibly regulate terahertz waves, and its mechanism of regulating terahertz waves is similar to traditional phased array antenna theory (Formula 1). Therefore, the designed coding metasurface units are arranged according to the coding sequence that generates vortex waves with topological charge number 1 (Fig. 4) and scattered waves that can reduce RCS (Fig. 6). The results show that VO2 can generate the same beams with the same coding sequence at different operating frequency bands under different phase transition states, and the operating frequency band can change with the phase transition state of VO2. Conclusions Based on the PB phase principle and phase change material VO2, we design the 3-bit coding metasurface units. A variety of coding metasurfaces are formed by different coding sequence arrangements, which can regulate terahertz waves to generate beam forms corresponding to the coding sequences. Under an insulated state, VO2 works in a single frequency band of 1. 17-1. 37 THz, and it works in dual bands of 0. 87-0. 92 THz and 1. 4-1. 6 THz in a metallic state. The designed VO2 based on coding metasurface can switch the frequency bands without changing the wave forms, and provide important ideas for frequency modulation of terahertz waves.
