# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Single-Layered Chiral Metasurfaces  
We discuss the geometrical patterns of single-layered chiral metasurfaces and classify them based on their symmetry. To achieve giant chirality, a series of unit cell metas atoms, such as gammadions [30; 102], Z-shapes [21], L-shapes [103], split rings [92; 104], U-shapes [43], S-shapes [93], wind-mill structures [105], and other periodically arranged shapes [33; 34; 35; 36; 37; 38] have been used. The symmetry type that occurred in the geometrical patterns of chiral metasurface structures is an intriguing feature. Most chiral structures exhibited C4 or C2 symmetry. Chiral structures do not exhibit in-plane mirror symmetries and are pure chiral structures; however, several have C4 rotational symmetry. The C4 rotational symmetry prevents anisotropic effects that lead to polarization conversion [69; 106]. C4 symmetric single-layered chiral metasurfaces have potential applications in areas including circular polarizers, advanced optical components, and chiral sensing devices [82; 106; 107]. Some of the C4 symmetric single-layered chiral metasurfaces are shown in Figure 1.  
Gammadion structures are among the C4 rotationally symmetrical structures mentioned in several references [82; 107; 108]. The gold-gammadion structure enhanced the chiroptical response in terms of enantiomeric sensing [82]. The orthogonal orientation of the arms with one another and the associated conversion of linear polarization trigger chiral behavior in gammadion chiral structures [107]. [PERSON] et al. introduced a gold-gammadion chiral metasurface for enantiomeric sensing [82]. The gammadion structure was embedded in air, and chirality was locally improved mainly at the back and front of the arms (Figure 1a,b). However, the chiroptical response was small, and the regions with improved chiroptical responses were at the same location. An enhanced chiroptical response was achieved in a C4 planar dielectric gammadion nanostructure, in which the radiation patterns in the visible spectrum were surrounded by strong magnetic and electric higher-order multipole responses [107]. In contrast to the typical dipole approximation, which describes the chiral event, an improved chiroptical response was obtained by adjusting the multipole higher-order response, which is important for off-normal scattering. As shown in Figure 1d, three opposite-sign antinodes are observed in the in-plane component, where approximately 87% of the RCP wave is transmitted in zero order [107]. In addition, a C4 THz chiral metasurface that has robust OA under a normal incidence was introduced using a folded meta-atom with a gammadion shape to initiate an effective cross-coupling in between the magnetic and electric fields (Figure 1f) [82; 109]. This pseudo-planar metasurface (PPM) comprises a single-layer metasurface with each meta-atom deflected into a 3D configuration. Similar to 3D metamaterials, PPMs exhibit OA that is triggered by the chiral geometry of the meta-atoms, which produces potent and controllable optical chirality [109]. Single-layer chiral metasurfaces can demonstrate improved nonlinear optical responses compared to traditional nonlinear materials because of their engineered nanostructures that allow precise control over local field enhancement and phase matching [108]. This improved nonlinearity has advantages such as reduced power requirements for nonlinear effects, tailored nonlinear responses, and compact device designs, aiding applications in signal processing, frequency conversion, and efficient light generation for nonlinear optical processes. A single-layered chiral metasurface with a significantly improved nonlinear optical response and giant nonlinear CD was suggested [108]. It is the first nonlinear chiral metasurface framework that combines multiple quantum well (MQW) structures with massive second- and third-order nonlinear responses with a C3 Trisceli type and a C4 gammadion type plasmonic chiral nanoresonators for second harmonic generation (SHG) and third harmonic generation (THG), correspondingly (Figure 1g,h).