Results and Discussions When the rotation angle φ_rot of the unit cell is 0°, 30°, 70°, 120°, and 180°, the variations of the simulated phase and normalized amplitude of the cross-polarized transmission coefficient T_RL with frequency under LCP incident waves show that the 3 dB bandwidth of T_RL of the dielectric unit cell is 0.1 THz, and the working frequency band is about 0.25-0.35 THz (with a relative bandwidth of 33.3%). In addition, as the rotation angle of the unit cell changes from 0° to 180°, the unit cell can cover the phase change of 0°-360° (Fig. 2). Therefore, the metasurface can be realized on the basis of the PB phase principle and the designed dielectric unit cell. When LCP plane waves with the frequency f₁ and angle θ_i(f₁) are incident on the metasurface in ±x and ±y directions separately, simulated far-field amplitude and phase patterns of RCP transmission waves show that four-channel beams are generated in the direction perpendicular to the metasurface. At the same time, according to the characteristics of OAM beam energy and phase distribution, the amplitude of one of the transmitted waves is in a solid distribution, with the phase unchanged, and thus, the topological charge l₁ equals 0. For the other three-channel beams, the amplitude and phases are distributed in circular and spiral shapes, respectively, and when observed along the -z-axis, the phase changes by +4π, -4π, and +8π in a clockwise direction. In other words, the three-channel OAM beams with topological charges of l₂=-2, l₃=+2, and l₄=-4 are generated. It can be seen that four-channel topologically orthogonal coaxial beams with frequency f₁ are generated in the direction perpendicular to the metasurface (Fig. 5). Similarly, for four-channel LCP plane waves with f₂ and θ_i(f₁), or f₃ and θ_i(f₃) incident on the metasurface along the ±x and ±y axes, four topologically orthogonal coaxial beams with frequency f₂ or f₃ in the direction perpendicular to the metasurface are generated (Figs. 6 and 7).

In recent years, the terahertz (THz) band has attracted extensive attention from researchers due to its potential of realizing high-speed and high-capacity wireless communication systems. The multiplexing technology has great research prospects in improving the communication rate and system capacity. The electromagnetic wave (EMW) carrying the orbital angular momentum (OAM) is called the OAM wave. OAM can be used as a new information carrier to provide an additional dimension for spatial multiplexing. The metasurface can effectively control the amplitude, phase, and polarization of EMW, and according to the main types of materials used, it can be divided into the metal and dielectric metasurfaces. Compared with the metal metasurface, the dielectric metasurface has the advantages of smaller ohmic loss, lower costs, easier processing and manufacturing, and higher transmission efficiency. Most previous research focused on generating an OAM beam or realizing OAM beam multiplexing by the metal metasurface, and hence, generating OAM beams and further realizing OAM beam multiplexing based on the dielectric metasurface have become the research hotspots. The methods of OAM beam multiplexing based on the dielectric metasurface have the disadvantages of OAM waves carrying the same information, the limited number of multiplexing channels, and the complexity and high cost of the communication system. Although the above problems can be effectively solved on the basis of the angle-multiplexed dielectric metasurface by converting multiple incident waves with different angles into orthogonal OAM coaxial beams, the current angle-multiplexed dielectric metasurface only works at a single frequency. Once the incident wave frequency changes, the generated OAM waves will deviate from the axis. In addition, the existing research focuses on the optical frequency band, and few studies combine two or more physical dimensions to achieve multiplexing. We need to study the realization of dual-dimensional or multi-dimensional multi-channel multiplexing in the terahertz band based on the dielectric metasurface and the expansion of the working bandwidth of the dielectric metasurface. Therefore, this paper proposes a dielectric metasurface, on the basis of which the dual-dimensional multi-channel multiplexing can be realized by the combination of OAM and frequency dimensions. Theoretically, the simultaneous transmission of 4×N-channel (N is any positive integer) orthogonal coaxial beams can be realized. The proposed dielectric metasurface has potential application value in the field of high-speed and high-capacity terahertz communication.

First, the designed dielectric unit cells of the metasurface are composed of silicon pillars and substrates, and unit cells with different rotation angles are simulated on CST Microwave Studio. Periodic boundaries are set in the x-axis and y-axis directions; two Floquet ports are set in the z-axis direction, and the excitation is set as the left circularly polarized (LCP) wave in the negative direction of the z-axis. Then, for topological charges l₁=0, l₂=-2, l₃=+2, and l₄=-4, according to the theoretical formula, the required phase distribution of the proposed metasurface is calculated. After that, the metasurface is designed on the basis of the Pancharatnam-Berry (PB) phase principle and the dielectric unit cell. Then, to verify the designed metasurface, we take frequencies f₁=0.35 THz, f₂=0.3 THz, and f₃=0.25 THz to calculate the corresponding incident angle simultaneously. Finally, far-field amplitude and phase patterns of right circularly polarized (RCP) transmission waves are simulated on CST Microwave Studio when three-channel circularly polarized (CP) plane waves with different frequencies and incident angles are obliquely incident on the metasurface in four directions.

In this paper, a dielectric metasurface working at 0.25-0.35 THz is proposed. When N-channel CP plane waves with different frequencies and incident angles are obliquely incident on the metasurface in four directions, in the direction perpendicular to the metasurface, 4×N-channel cross-polarized transmission waves are converted into coaxial beams that are orthogonal to each other in topology or frequency, namely that the 4×N channel multiplexing is realized. For simulation verification, we assume f₁=0.35 THz, f₂=0.3 THz, and f₃=0.25 THz. The simulations show that when four-channel LCP plane waves with frequency f₁, f₂, or f₃ are obliquely incident on the metasurface along the ±x and ±y axes, four topologically orthogonal coaxial beams with frequency f₁, f₂, or f₃ in the direction perpendicular to the metasurface are generated. At the same time, the generated three groups of beams are orthogonal to each other in frequency. It can be seen that on the basis of the designed dielectric metasurface, 12-channel incident waves are successfully converted into coaxial beams with topology or frequency orthogonality. In other words, 12-channel multiplexing is realized by the combination of OAM and frequency dimensions. The designed dielectric metasurface has potential application value in the field of high-speed high-capacity terahertz communication.