Radar cross section (RCS) is an important physical quantity to measure the radar echo capability of the target. To reduce the RCS value of the target object is to achieve RCS reduction, which has important applications in the field of radar stealth. Terahertz (THz) waves refer to electromagnetic waves in the frequency range of 0.1-10 THz, which have broad application space in high-speed broadband communication, precision military radar, high-resolution imaging, and other fields. With the increasing complexity of the international situation and the rapid development of science and technology, RCS reduction in the THz band and its application in radar stealth has become a new research direction. Currently, there are two most effective and commonly used methods for RCS reduction in the THz band. The first one is to use a perfect absorber, which can absorb the incident THz waves to the surface and convert them into internal energy. The other is to use a metasurface to reshape the THz wave waveform in the space domain. The former has the disadvantages of narrow bandwidth and easy discovery by far-infrared detectors. The latter has become a hot research topic because of its simple structure, small size, and wide operating frequency band. In this paper, a coding metasurface with more degrees of freedom than the traditional metasurface is adopted. By combining it with a phase gradient metasurface, this paper proposes a coding phase gradient metasurface. Compared with the normal coding metasurface without phase gradient, it has a better RCS reduction effect. Moreover, the coding phase gradient metasurface has a wider working band. This is due to the introduction of a double Ω-shaped symmetrical structure at the top of the metasurface element. The metasurface will generate magnetic dipole resonance and electric dipole resonance in its interior, and the multiple resonance modes are conducive to broadening its working frequency band.

First, according to the Pancharatnam-Berry (PB) geometric phase principle, a number of double Ω-shaped reflective metasurface elements with different phase responses are designed. The conditions they meet are as follows. For the vertically incident x- and y-polarized waves, the amplitudes of the co-polarized reflection are almost the same, and their co-polarized reflection phase difference is 180°. Second, based on the designed metasurface elements, the coding elements are designed. The so-called coding element is the introduction of phase gradient on the basis of the supercell. Third, a genetic algorithm is written using Matlab to optimize the arrangement, so that the energy distribution of diffuse reflection is more uniform, and a better RCS reduction effect is obtained. Then, according to the optimized arrangement, the coding elements 0 and 1 are arranged to obtain the coding phase gradient metasurface. Finally, CST Microwave Studio is used to simulate the far-field scattering of the coding phase gradient metasurface at different frequencies. The RCS reduction value relative to a metal plate of the same size is calculated. In addition, the influence of x- and y-polarized incidence angles on the performance of the coding phase gradient metasurface is also analyzed.

When the x- and y-polarized waves are incident vertically to the metasurface element, the co-polarization amplitudes are larger than 0.8 in the frequency range from 1 THz to 1.5 THz, and their phase differences are close to 180°, which satisfy the PB geometric phase principle (Fig. 2). Then, the phase gradient is introduced on the basis of supercells, and 1 bit coding elements 0 and 1 are designed (Fig. 4). The optimal arrangement M1 of the coding elements is achieved with the help of the genetic algorithm [Fig. 6(b)]. The coding phase gradient metasurface is obtained by arranging the coding elements according to M1. CST Microwave Studio is used to calculate the far-field scattering of the coding phase gradient metasurface at different frequency points under the normal incidence of x- and y-polarized waves. The result shows that the diffusely reflected scattering waves will be further reflected in the direction of the two symmetrical main lobes, and the far-field beams have both diffuse reflection and abnormal reflection characteristics (Fig. 9). In addition, the results show that the designed 1 bit coding phase gradient metasurface can achieve RCS reduction of more than 10 dB (Fig. 10) in a wide frequency range (0.87-1.725 THz) with a relative bandwidth of 65.9%. The RCS reduction in the frequency range of 0.9-1.4 THz and 1.6-1.7 THz both reaches over 15 dB with a maximum RCS reduction value of 31.26 dB. The RCS reduction effect of the coding phase gradient metasurface (composed of coding elements 0 and 1 arranged according to M1) is compared with that of the normal coding metasurface without phase gradient (composed of supercells 0 and 1 arranged according to M1), and it is found that the RCS reduction effect of the former is better. Finally, when the incident angles of x- and y-polarized waves are both gradually increased from 0° to 30°, the RCS reduction is more than 10 dB in the frequency bands of 0.9-1.5 THz and 0.9-1.7 THz (Fig. 11). It indicates that good RCS reduction effect can be achieved over a wide frequency range with a certain degree of angular stability.

In this paper, a coding phase gradient metasurface is proposed, which can reduce the RCS in the THz band. The results show that the designed 1 bit coding phase gradient metasurface can achieve RCS reduction of more than 10 dB in a wide frequency band from 0.87 THz to 1.725 THz, and the maximum reduction value reaches 31.26 dB. Finally, the influence of the incident angles of the x- and the y-polarized waves on the performance of the coding phase gradient metasurface is analyzed. It was found that its performance is stable in the range of 0° to 30°. The above results show that this kind of metasurface has potential application value in radar stealth and other aspects.