# Switchable Chiral Metasurface for Terahertz Anomalous Reflection Based on Phase Change Material
## 3 Results  
First, we simulate the reflection spectra of ASRR_1 and ASRR_2, which are mirror images of each other. Figure 2a-d shows the unit structure diagram and the reflection spectrum results of ASRR_1. As shown in Figure 2b, when the conductivity of GST \(\sigma\) is 0 S/m, ASRR_1 will efficiently absorb the incident LCPL at a frequency of 0.76 THz, while it will strongly reflect the incident RCPL. That is, \(R_{rr}\) = 0.76 and \(R_{ll}\) = 0. At this time, ASRR_1 is called RCP mirror. As \(\sigma\) rises to 3 \(\times\) 10\({}^{5}\) S/m, it can be seen from Figure 2c that ASRR_1 is switched to the LCP mirror. That is, it can efficiently absorb the incident RCPL and strongly reflect the incident LCPL. The reflectivity is \(R_{rr}\) = 0.02 and \(R_{ll}\) = 0.66. Since GST after the phase transition increases the electrical length of the whole structure, the working frequency of ASRR_1 shifts red to 0.66 THz. The quantitative calculation of circular dichroism is CD = \(R_{rr}\) - \(R_{ll}\). In Figure 2d, before the phase transition, the circular dichroism of ASRR_1 is CD = 76%. After the phase transition, the circular dichroism of ASRR_1 is CD = \(-\)64%. This also shows that switching between the RCP mirror and LCP mirror is realized by using the phase transition property of GST. Figure 2e-h shows the unit structure diagram and the reflection spectrum results of ASRR_2. As ASRR_2 and ASRR_1  
Figure 1: Schematic of (**a**) the electromagnetic responses of a gradient chiral metamirror and (**b**,**c**) the structure of a unit cell. are mirror image relationships in structure, their reflection spectrum and CD spectrum also show a reversal effect. That is, ASRR_2 is an LCP mirror before the GST phase transition and will switch to an RCP mirror after the GST phase transition. Its circular dichroism changes from CD = \(-\)76% to CD = 64%. Next, there are several reflection spectra of ASRR_1 with obvious changes under different conductivities in Figure 3. We have explained that the reflection effects of ASRR_1 and ASRR_2, which are mirror structures to each other, are reversed to each other in Figure 2. Therefore, only the simulation results of ASRR_1 will be discussed in future research. With increasing conductivity, the reflection valley of LCPL at 0.76 THz is gradually passivated, and the reflectivity increases. Moreover, the reflectivity difference between the RCPL and LCPL will also gradually decrease. As \(\sigma\) = 6 \(\times\) 10\({}^{4}\) S/m, the reflection valley of LCPL basically disappears. At this time, reflection valley of RCPL appeared at 0.66 THz. Moreover, the reflection valley of RCPL will gradually sharpen, and the reflectivity will decrease with increasing conductivity. At the same time, the reflectivity difference between the RCPL and LCPL also increases. When \(\sigma\) is 3 \(\times\) 10\({}^{5}\) S/m, ASRR_1 completes the switching from the RCP mirror to the LCP mirror. Figure 3: Reflection spectra of ASRR_1 with different conductivities of GST. Figure 2: Schematic diagram of structural units, reflection spectrum and circular dichroism curve spectrum of (**a–d**) ASRR_1 and (**e–h**) ASRR_2. To explain the physical mechanism of this switchable chiral metasurface, the surface current distribution of ASRR_1 under different conductivities is simulated, as shown in Figure 4. Here, we mark the current oscillation direction along the two arms of ASRR_1 with a white arrow. When the conductivity of GST is 0 S/m, ASRR_1 shows the effect of absorption of LCPL and reflection of RCPL at 0.76 THz. As seen from Figure 4a, the surface current oscillates in a single direction on the two arms, which shows that the incident LCPL excites a magnetic dipole resonance in ASRR_1. This resonant mode can bind the incident electromagnetic wave energy inside the structure, and the lossy PI layer will slowly absorb the energy. Therefore, LCPL is highly absorbed after incident on ASRR_1.