3-bit switchable terahertz coding metasurface based on Dirac semimetals

THz waves, located in the transition region between macroelectronics and microphotonics, have many unique advantages, such as low energy, large capacity, and strong penetration [1], [2]. In recent years, the manipulation of THz waves has become a major research hotspot [3], [4], [5]. However, most natural materials have a weak response to THz waves, so THz waves manipulation devices are still relatively scarce. The emergence of coding metasurfaces provides a new method for flexible regulation of THz waves. Coding metasurfaces have been widely used in many scenarios, including anomalous reflections [6], [7], RCS reduction [8], [9], [10], [11], focusing [12], and vortex beam generation [13], [14], [15], [16], etc. The coding metasurface combines digital coding and metasurface to truly realize digital regulation of THz waves. In 2014, Cui et al. [17] first proposed the concept of coding metasurfaces. The method is to discretize the phase space in the range of 0°–360°, design unit cells with corresponding phase responses. And coding unit cells with binary numbers. Then, by arranging these unit cells in a specific order, electromagnetic waves can be manipulated freely. For example, a 1-bit coding metasurface is coded with binary 0s and 1s, that is, ‘0’ and ‘1’ unit cells that are 180° out of phase between cells are required. Similarly, the 2-bit coding metasurface is coded with four binary digits, that is, four units of ‘00’, ‘01’, ‘10’, and ‘11’ with a phase difference of 90° between adjacent units. The design of the coding metasurface can also be extended to 3-bit (eight coding unit cells are required, and the phase difference between adjacent units satisfies 45°) or even higher [18].

Coding metasurfaces are also widely used in the THz band [19], [20], [21]. However, in the terahertz band, most coding metasurfaces are were made of metallic or all-dielectric materials. Moreover, the method of loading active devices cannot be directly applied to the terahertz band due to its large size [22], [23], [24]. The development of dynamic modulation for terahertz waves based on coding metasurfaces is restricted. In order to realize the dynamic tunability of coding metasurfaces in the terahertz band, active materials (such as graphene, liquid crystal, VO2, and DSMs) are often used to construct coding metasurfaces. In 2020, Feng et al. [25] proposed two graphene-based tunable coding metasurfaces, where the coding unit consists of an anisotropic rectangular graphene structure top layer, an intermediate dielectric layer, and a base plane. By changing the EF value of graphene, the designed coding metasurface can realize the mutual change between the steered beam and the unsteered beam. In 2021, Xu et al. [26] proposed a graphene-based coding metasurface. The introduction of liquid crystal materials into the coding metasurface can also realize real-time dynamic regulation of THz waves. In 2021, Liu et al. [27] proposed a liquid crystal-based 1-bit transmission digitally coding metasurface for programmable terahertz beam steering. In 2021, Pan et al. [28] designed a VO2-based coding metasurface that exploits the phase transition properties of VO2 to achieve real-time operation of THz switching.

However, graphene is a thin two-dimensional (2D) material whose chemistry is easily affected by the dielectric environment. Moreover, the preparation of graphene and metal hybrid structures is more complicated [29], [30], [31]. Under the same conditions, the carrier mobility of DSMs (9 × 106 cm2 v−1s −1, 5 K) [32] is higher than that of graphene (2 × 105 cm2 v−1 s−1, 5 K) [33], and the EF of DSMs can be dynamically tuned by applying voltage or alkali metal surface doping [34], [35], [36]. DSMs have the advantages of high stability, easy fabrication, strong coupling with electromagnetic waves, and fast response [37], [38], [39], providing a good platform for the study of tunable coding metasurfaces. Combining DSMs with the design of coding metasurfaces enables more effective real-time dynamic manipulation of THz waves.

In this study, we investigate a THz-band ‘#’-shaped 3-bit coding metasurface. When the EF value is set to 100 meV, the phase values of the 8 basic units of the 3-bit coding metasurface correspond to the ‘#’ of different lengths. And the phase difference between adjacent units at 1.1 THz is about 45°, which satisfies the design principles of 3-bit coding metasurfaces. When the EF value is set to 5 meV, the phase values between coding unit cells hardly change with the length. The coding metasurface can manipulate reflected THz waves to achieve multiple functions such as abnormal reflection, beam splitting, vortex beam generation, and RCS reduction. By changing the Fermi level of Dirac, the switching functions of four functions are respectively realized. In addition, in the range of 1.0–1.2 THz, the reduction of RCS is close to −10 dB, and the magnitude of the reduction of RCS is not affected by linearly polarized waves, right circularly polarized waves, and left circularly polarized waves, and has the characteristic of polarization insensitivity.

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