# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Multi-Layered Chiral Metasurface
(**j**) Geometrical representation of the twisted-layered chiral metasurface parameters. (**k**,**l**) CD spectra with different variables: (**k**) d = 50 nm, (**l**) d = 100 nm, with a specified twist angle ((**j**–**l**) reproduced from [70]). (**m**) The schematics of the bilayer chiral metasurface unit array and unit cell. (**n**) AT parameters \(\Delta_{\mathrm{lin}}(x)\) and \(\Delta_{\mathrm{lin}}(y)\) for the x and y linearly polarized lights. ((**m**,**n**) reproduced from [135]). (**o**) Schematics of the folded chiral metasurface. (**p**) CD spectra of the folded chiral metasurface at a folding angle \(\theta\) = 60\({}^{\circ}\). ((**o**,**p**) reproduced from [16]). It has been demonstrated that by adding a twist to the lattice orientation of the metasurface inclusions, a chiral metasurface with an exotic effect analogous to 3D metamaterials can be achieved [84]. The bandwidth of the multi-layered chiral metasurface was affected by the rotation angle (Figure 3i). In addition, the CD value increased rapidly when more layers were added in a cascade, and in the case of seven layers, the CD virtually encompassed the full visible spectrum (Figure 3a,b). These twisted arrays of chiral metasurfaces were fabricated by conventional lithographic techniques without the need for sophisticated inclusion shapes. It was also proved that the CD value can be enhanced with the addition of layers in a gradient C-shaped SRR array (Figure 3e) [85]. In contrast to the single-layered chiral metasurfaces (Figure 3d), the four-layer chiral metasurfaces (Figure 3f) show a higher transmission difference [85]. A high conversion coefficient was achieved over a relatively wide bandwidth with a fully controlled phase, and the CPL wave output purity was improved. Moreover, in a chiral gold ultrathin nanowire film designed using the Langmuir-Schaefer method, the chirality could be monitored using the orientation angle and the number of layers [86]. The maximum CD response of the two-layered gold nanowire assembly was obtained at 45\({}^{\circ}\) (Figure 3h), and no CD response was obtained at inter-layer angles of 0\({}^{\circ}\) or 90\({}^{\circ}\). In addition, at a fixed 45\({}^{\circ}\) inter-layer angle, adding a layer to the two layers almost doubles the CD response (Figure 3i). This multi-layered chiral metasurface was fabricated using a bottom-up assembly, as opposed to the top-down method used to fabricate the previously described chiral metasurfaces, which makes it suitable for a range of different applications, including chiral recognition, separation, and catalysis. Such multi-layered chiral metasurfaces have been used in chiral recognition and separation by optimizing their chiroptical responses to selectively detect and separate enantiomers in biomolecules and pharmaceuticals [86]. In catalysis, they have been used to improve the chiral selectivity in chemical reactions by controlling the phase and polarization of incident light, leading to enhanced reaction outcomes in asymmetric synthesis. Additionally, it exhibited the best performance of the bottom-up-fabricated chiral nanostructures, with a significant CD response and wavelength coverage spanning the visible and NIR ranges. In a twisted multi-layered chiral metasurface composed of helically organized oligomers, an ultrasharp CD can be obtained, which can be used in a chiral device that operates in the NIR region (Figure 3j) [70]. In these chiral metasurfaces, the chiroptical response originates from the coupling of three layers and was examined by evaluating the CD response and electrical field [70]. CD progressively grew, peaked, and finally declined as the angle raised from 0\({}^{\circ}\) to 45\({}^{\circ}\). The highest CD was obtained in the NIR region at an angle of 28\({}^{\circ}\) and inter-layer distance of 100 nm (Figure 3k,l). Unlike previously reported multi-layered chiral metasurfaces, the coupling of oligomers and the interactions between the layers were clearly examined.