# Switchable Chiral Metasurface for Terahertz Anomalous Reflection Based on Phase Change Material
## 1 Introduction  
Chirality is an inherent feature of chiral molecules. That is, an object cannot coincide with its mirror image through translation, turnover and other in-plane operations. Similar to our left and right hands, they mirror each other and cannot overlap. Chiral objects and their mirror images are called enantiomers. In the organic world, there are many chiral molecules, such as protein, DNA, sugar molecules, viruses, amino acids and liquid crystals, but there are few chiral structures in nature. Later, it is found that chiral materials can control the spin of electrons to a certain extent [1; 2; 3]. In addition, thanks to the development of metamaterials, the control of the spin of photons has been realized [4; 5; 6; 7; 8; 9; 10]. At present, the study of the interaction between circularly polarized light (CPL) and matter has been involved in many wavebands, such as microwave [11], terahertz [12], infrared [13] and visible [14]. It is well known that chiral metamaterials show different electromagnetic responses such as asymmetric transmission (AT) and circular dichroism (CD) when they interact with CPL with different spins. In other words, chiral metamaterials have selective transmission, reflection and absorption of incident CPL with different spins. In recent years, metamaterials with spin selectivity have played a very important role in many fields. For example, spin-selective beam deflectors [15], polarization-sensitive chiral circularly polarized light detectors [16], circularly polarized light multiplexed holograms [17] and spin- and wavelength-sensitive gray imaging [18]. However, the opticaleffect of metamaterials is difficult to change after manufacturing, which limits the practical application of functional metamaterials. Hence, actively tunable metamaterials are gaining increasing attention. At present, there are two design methods that can make metamaterials have the property of active regulation. The first method is to change the geometry of metamaterials. [PERSON] et al. proposed a reconfigurable anisotropic metamaterial at terahertz frequency, which can bend the structure through thermal stimulation to change the structural response [19]. [PERSON] et al. designed a micromechanically reconfigurable metamaterial. Dynamic tuning can be realized by changing the coupling distance between cells through bidirectional micromechanical actuators [20]. The second method is to add controllable materials such as silicon, graphene and phase change materials into metamaterials. [PERSON] et al. presented a nonlinear chiral meta-mirror based on silicon, which can be used for an all-optical picosecond optical polarization switch. The chiral optical response of the structure is dynamically adjusted by the change of carriers in silicon by pump light [21]. [PERSON] et al. proposed an absorber integrating a periodic Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) (GST) resonator. Through the phase transition of the GST resonator from an amorphous state to a crystalline state, a change in CD intensity in the range of 0.03 to 0.75 can be realized [22]. In contrast to the past, some researchers have shifted their attention from tuning the chiral response intensity to switching the chiral response. [PERSON] et al. took advantage of vanadium dioxide (VO\({}_{2}\)) to change the chirality of metamaterial resonators and realized a change in CD intensity from \(-\)75% to +55% [23]. [PERSON] et al. proposed a dynamically reversible chiral metamaterial by combining vanadium dioxide and [PERSON]'s principle, which can achieve a reversible CD tuning range of \(\pm\)0.5 at 0.97 THz [24]. [PERSON] et al. realized a reversible chiral response in the terahertz band by making use of achiral dielectric and graphene bands. Chiral response switching can be realized by reversing the Fermi level of the graphene band [25]. From the perspective of practical application, the design of a dynamically reversible metamaterial with strong circular dichroism is more in line with the application requirements.