# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Single-Layered Chiral Metasurfaces
Unlike previous chiral metasurfaces, it employs intersubband transition coupling as well as chiral cavity modes. This method demonstrates excellent THG-CD and SHG-CD value in the mid-IR spectra from a nonlinear chiral metasurface [108]. A C4 metal-dielectric-metal planar plasmonic chiral metasurface with crescent meta-atoms arranged in a specific orientation also exhibited multiband CD at near-infrared (NIR) wavelengths (Figure 1i) [88]. In addition, the symmetric orientation of the metasurface affected the CD peak. The CD values identified as CD1, CD2, CD3, and CD4 were obtained in the 1565 nm, 999 nm, 878 nm, and 810 nm wavelengths, correspondingly, as shown in Figure 1j [88]. Designing a meta-atom at visible frequencies was a significant challenge [110]. To address the challenge, [111] proposed the use of a chiral-selective plasmonic metasurface consisting of an \(\eta\) shape operating in the visible frequency. This chiral metasurface generates a CD peak of around 0.5 due to the selective absorptive property of the plasmonic chiral metasurface [111]. Furthermore, a C4 symmetric chiral metasurface with nanoslits arranged on a layer of thin gold exhibited a potent controllable chiroptical resonant response from visible to NIR frequencies [83]. This metasurface emphasizes couplings in between surface-propagating and localized forms, allowing for an additional degree of freedom in manipulating the chiroptical response. As shown in Figure 1k, the CD spectra are comparable to the chirality strength provided by more sophisticated 3D structures, which require significantly more complicated fabrication methods. The structure with C2 symmetry comprised two identical parts. In C2 rotational symmetry, the element remains invariant after a \(\pi\) rotation in the z-axis (Figure 1d). Streptavidin tetramers, hemoglobin tetramers, and dimeric proteins are among the few biological structures with C2 symmetry [3, 99; 100; 99; 101]. Single-layered chiral metasurfaces with C2 rotational symmetry are suitable for enhancing the chiroptical responses of biomolecules with C2 symmetry. Five significant examples of C2 symmetric metasurfaces are presented in Figure 2.  
Figure 1: C4 single-layered chiral metasurfaces. The chiroptical response improvement using a gammadion configuration subjected to incident light of (**a**) LCP and (**b**) RCP at a 2.01 μm wavelength. ((**a**,**b**) reproduced from [82]). (**c**) Optical response of a gammadion metasurface composed of gammadion TiO\({}_{2}\) placed on thin film TiO\({}_{2}\) on a substrate made of glass. (**d**) Simulated CD response (the yellow dotted line) and experimental CD response (the black solid line). (**e**) I\({}_{\rm yz}\) cut plane demonstrating the magnetic field (the colored field plot) Hx and the corresponding distribution of current (the arrows that are black) J\({}_{\rm z}\)+J\({}_{\rm y}\) in a gammadion metasurface under RCP (right) and LCP (left) light. ((**c**–**e**) reproduced from [107]). (**f**) Pseudo-planar gammadion chiral metasurface, where the gammadion meta-atoms are deposited on a planar substrate with a squared trench and are deflected on the trench. ((**f**) reproduced from [109]). (**g.h**) Wavelength-based THG-CD from simulated (black) and measured (red) data of the C4 and C3 of the intersubband polaritonic metasurfaces, respectively. ((**g**,**h**) reproduced from [108]). (i) Three-dimensional (3D) view of the subwavelength crescent aperture (SCA) made in the MDM layer. (**j**) CD responses of the SCA. ((**i**,**j**) reproduced from [88]). (**k**) CD spectra comprising both-handed chiral metasurfaces, that result in an excellent mirror image. ((**j**,**k**) reproduced from [83]). be realized because these designs can achieve full-phase control. However, the chiroptical response obtained is insufficient for certain applications. Similarly, [PERSON] et al. introduced a low-loss all-dielectric chiral metasurface that has gigantic CD and AT values near unity [102].