# Switchable Chiral Metasurface for Terahertz Anomalous Reflection Based on Phase Change Material
## 3 Results
In Figure 4b, the surface current oscillates in different directions along the two arms of the resonant ring, indicating that the incident RCPL excites an electric dipole with low loss in ASRR_1. Thus, the metasurface at this time presents the performance of reflecting RCP incident light. As \(\sigma\) is 3 \(\times\) 10\({}^{5}\) S/m, ASRR_1 shows the effect of reflecting LCPL and absorbing RCPL at 0.66 THz. According to the current distribution in Figure 4c, the incident LCPL excited in the structure is an electric dipole; thus, LCPL is reflected. Similarly, by analyzing Figure 4d, it can be seen that the magnetic dipole that will cause high absorption is excited after the RCPL is incident on ASRR_1. Through the analysis, we can clearly understand that the different resonance modes excited by incidence lead to the different responses of the chiral metasurface to circularly polarized light with different spins under different conductivities. According to the Pancharatnam-Berry phase [26], when a structure rotates \(\varphi\) around the z-axis, the phase of the reflected light will change by 2\(\varphi\), while the amplitude of the reflected light remains unchanged. Then, the reflection spectrum of ASRR_1 with different orientations \(\varphi\) is simulated. As shown in Figure 5a, RCPL is incident to ASRR_1. The conductivity of GST is \(\sigma=0\) S/m. Regardless of the value of \(\varphi\), the reflection remains almost unchanged and approximately equals 0.76. When \(\varphi\) changes from 0 to \(\pi\) at an interval of \(\pi/18\), the phase of reflected RCPL changes between \(-\pi\) and \(\pi\), that is, the phase  
Figure 4: The physical mechanism of the switchable chiral metasurface. The surface current distribution of ASRR_1 with conductivities of (**a**,**b**) 0 S/m and (**c**,**d**) 3 \(\times\) 10\({}^{5}\) S/m when LCP and RCP are incident. change range is \(2\pi\). The operating frequency is 0.76 THz. Figure 5b shows that ASRR_1 with \(\sigma=3\times 10^{5}\) S/m is irradiated by LCPL. Similarly, the amplitude of reflected LCPL, which is approximately 0.66, will not change with different \(\varphi\). The phase of reflected LCPL depends on \(2\varphi\) and changes in the range of \(2\pi\). The operating frequency here is 0.66 THz. In the generalized Snell's law, we can control the direction of the emitted light by designing a linear varied phase shift \(\phi(x)\) along the surface of a metasurface. For reflected light, the equation of generalized [PERSON]'s law is defined as [27]:  
\[\mathrm{n}_{r}\sin\theta_{r}=\mathrm{n}_{i}\sin\theta_{i}+\frac{c}{2\pi f} \frac{d\phi(x)}{dx} \tag{2}\]  
where \(\theta_{r}\) and \(\theta_{i}\) represent the reflection angle and incident angle, respectively; \(\mathrm{n}_{r}\) and \(\mathrm{n}_{i}\) represent the refractive indices of the output and input media, which are equal to 1; \(c\) is the speed of light in vacuum; \(f\) corresponds to the operating frequency of the metasurface; and \(d\phi(x)/dx\) stands for a constant phase gradient. PB phase is a method to accurately control \(\phi(x)\). That is \(\phi(x)=\pm 2\varphi\)[28]. \(\varphi\) here is the rotation angle of ASRR_1 around the z-axis we mentioned above. The sign plus or minus is determined by RCPL and LCPL. As seen from Figure 5, ASRR 1 can change the phase of its reflected light by rotating around the z-axis, and the phase size can cover the whole phase period \(2\pi\). Therefore, we chose six different rotation angles: 0, \(\pi/6\), \(\pi/3\), \(\pi/2\), \(2\pi/3\) and \(5\pi/6\). The six small units are arranged in a row to form a superperiodic unit cell and thus a chiral metasurface with linear phase change that covers the whole \(2\pi\) range. Part of the structure of the chiral metasurface is shown in Figure 1c. The period in the x direction is P\({}_{\mathrm{x}}\) = 960 \(\upmu\)m, and the period in the y direction is P\({}_{\mathrm{y}}\) = 160 \(\upmu\)m. Similarly, we use the simulation software CST to simulate the reflected field distribution of the metasurface.