# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Tunable Chiral Metasurfaces
Circular differential transmission (CDT) changes were the greatest in the total sample of Co, and the amount of change decreased as the concentration of Co decreased, reaching zero in the whole sample of Ag CCNA. These magnetically tunable chiral metasurfaces are suitable for sensing and imaging applications. Magnetically tunable chiral metasurfaces, like Ag/Co composite nanohole arrays, have advantages, including the precise control of the chiroptical responses, which optimizes sensitivity in sensing applications [66]. However, they also have disadvantages, which may involve the demand for external magnetic fields, which limit the portability and create complexity, as well as the material constraints, like the need for particular magnetic materials such as cobalt [66]. Liquid crystals (LCs) have also been used to reconfigure chiral metasurfaces. LCs are used to reconfigure metasurfaces by changing the orientation angle of the LC molecule, which affects the chiroptical response. LC molecules are anisotropic, where their optical properties depend on the orientation of their long axes [74]. The alignment of the LC molecule on the chiral metasurface can be controlled by introducing an external electric field or temperature gradient. This change in alignment leads to changes in chiroptical responses such as OR and CD, enabling dynamic control and modulation in various applications, including beam steering, chiral sensors, and displays. In a study performed by [PERSON] et al. [74], a chiral metasurface, which is reconfigurable by a sandwiched structure of metal-insulator-metal, was numerically demonstrated (Figure 6a). The sign and magnitude of CD spectra were tuned by varying the CPL illumination angle or by incorporating the chiral metasurface with nematic LCs. The effective refractive index of the LC layer was changed by varying the orientation of the LC molecule using external voltage, allowing for the tunability of the chiral plasmonic metasurface (Figure 6a). The computed CD response along the LC molecule's tilt angle is shown in Figure 6c. Voltage applied from outside can change the angle of tilt of the molecules from 0\({}^{\circ}\) (\"ON\" state) to 90\({}^{\circ}\) (\"OFF\" state) (Figure 6c). When the angle changes from 90\({}^{\circ}\) to 0\({}^{\circ}\), the magnitude and sign of CD alter (Figure 6c). The CD spectra and reflections for the LCP and RCP light are shown in Figure 6d,e, correspondingly. The achieved CD still exceeded 50% in the \"OFF\" state because of the nonuniform refractive index (Figure 6d). Owing to the CPL oblique illumination, the light propagation time across different sections of the chiral metasurface meta-atom varies. The phase difference between LCP and LCP stimulated resonant modes causes strong mode interference. Extrinsic chirality is obtained from the difference in absorption of LCP and RCP incident light [74]. As demonstrated in Figure 6b, the CD response decreased as the illumination angle increased, and at 45\({}^{\circ}\), the CD spectra almost disappeared [74]. Such metasurfaces have many applications in spin-orbit communications, chirality sensing, and polarimetric imaging. Mechanically reconfigurable metasurfaces can also be used to continuously and reversibly tune chiroptical responses [9; 59]. Tunability is achieved through structural deformation in mechanically reconfigurable chiral metasurfaces, such as those using graphene Miura-origami (G-Mori). The geometry and periodicity of the chiral nanostructures are altered by physically stretching or bending the metasurface, which changes the chiroptical response of the chiral metasurface [59]. A mechanically reconfigurable chiral metasurface with high CD was demonstrated by combining U-shaped split-ring resonators (USRRs) with G-Mori [59]. Planar metasurfaces with four ordered counterclockwise or clockwise USRRs were drawn on a graphene film under a flexible substrate (Figure 6f-j).