Tunability is an important requirement for metasurface application. In recent years, adjustable materials have been gradually used in the composite design of metasurface structures to achieve multi-functional switching. However, most of the existing relevant studies still have some shortcomings. First, the adjustable material is usually embedded in the metal pattern, which undoubtedly increases the complexity and manufacturing difficulty of the structure. Second, commonly used adjustable materials such as vanadium dioxide and graphene require particularly sensitive temperature environments or feeding conditions, and the temperature and bias voltage should be considered in the design and application, which not only complicates the production process but also brings a certain degree of difficulty to the practical application. Third, most of the reported metasurfaces only discuss the insulating and metallic states of adjustable materials, which limits the diversity of functions. Photosensitive silicon is a kind of light-adjustable material, and its conductivity changes with the change in pump light energy. It has attracted wide attention because of its simple regulation mode. Moreover, photosensitive silicon can continuously adjust the conductivity to generate a variety of coding states, so as to expand its functions. In this paper, a reconfigurable metasurface based on a photosensitive silicon pattern is designed. The metasurface does not need to change the shape, size, or direction of the unit but uses the optical control to continuously adjust the conductivity of photosensitive silicon, so as to realize several functions in the terahertz band, such as linear-to-linear polarization conversion, linear-to-circular polarization conversion, broadband absorption, near-field imaging, and beam splitting, which makes the regulation mode of multifunctional terahertz devices more convenient.

In this study, the pattern of the unit structure is completely composed of photosensitive silicon, and the processing technology of silicon-based metasurface is very mature, which will greatly reduce the production difficulty. The conductivity of photosensitive silicon varies with the light energy pumped. When the light energy increases, the carrier concentration in the semiconductor also increases. By adjusting the conductivity of the two photosensitive silicon rings, five coding states can be obtained, so as to encode metasurfaces with different functions. In this paper, the polarization conversion and absorption function can be realized by using the resonance between the photosensitive silicon and the metal plate or between the double rings. The amplitude difference of different state units can be used for imaging, and the phase difference can be used for beam splitting.

The designed metasurface can generate multiple coding states by continuously adjusting the conductivity of two photosensitive silicon rings (Table 1), so as to realize multifunctional switching in the terahertz band. When the conductivity of the large and small C-rings is 5.0×10⁵ S/m and 0 S/m, respectively, the designed metasurface is presented as a linear-to-linear polarization converter (Fig. 2), and the polarization conversion ratio (PCR) in the range of 2.10-3.15 THz is greater than 90%. When the conductivity of the large and small C-rings is changed to 0 S/m and 5.0×10⁵ S/m, respectively, the structure behaves as a linear-to-circular polarization converter (Fig. 4) in the range of 2.33-2.47 THz and 2.78-4.40 THz. When the conductivity of the large and small C-rings changes to 2.5×10⁵ S/m at the same time, the structure is transformed into an absorber (Fig. 7) with an absorption rate of more than 90% in the range of 2.40-4.60 THz. By encoding the cells with the conductivity of both large and small C-rings of 0 S/m and 2.5×10⁵ S/m, the structure achieves near-field imaging (Fig. 12) in the range of 2.80-3.00 THz. The cells with the conductivity of 5.0×10⁵ S/m and 0 S/m for the large and small C-rings and those with the conductivity of 0 S/m for the large and small C-rings are periodically coded, and this structure can realize two beam splitting (Fig. 14) and four beam splitting (Fig. 15) of the terahertz wave. The results show that the metasurface can be reconstructed by changing the external illumination conditions, and a variety of terahertz control functions can be obtained.

In this paper, the reconfigurable metasurface designed by the photosensitive silicon double C-rings structure realizes the switching of several functions, such as linear-to-linear polarization conversion, linear-to-circular polarization conversion, broadband absorption, near-field imaging, two beam splitting, and four beam splitting. Compared with the existing reports, the structure pattern designed in this paper is completely composed of photosensitive silicon, which greatly reduces the complexity of the pattern and the difficulty of device manufacturing. At the same time, this paper makes use of the continuously adjustable conductivity of photosensitive silicon to generate a variety of coding states, making the function of the metasurface more abundant. Compared with the single-function metasurface, it has greater advantages in integration and other aspects. In a word, the metasurface proposed in this paper is more flexible in switching functions and can realize a wider range of functions. It has excellent application prospects in terahertz modulation, stealth technology, communication system, and so on.