# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Tunable Chiral Metasurfaces
The BSR consists of two layers of rings labeled TOP\({}_{1}\), TOP\({}_{2}\), BOTTOM\({}_{1}\), and BOTTOM\({}_{2}\). When the rings are composed of graphene, BSRs with various Fermi energies for their formation can produce a CD effect. The tunability of the chiroptical response of BSR can be obtained by changing the gate voltage of the various parts of the BSR, given that the graphene's dielectric characteristics are contingent on its gate voltage. The CD of BSRs for V\({}_{\text{bottom1}}\) = 0.5 eV and V\({}_{\text{bottom2}}\) = 0.9 eV, and V\({}_{\text{bottom1}}\) = 0.9 eV and V\({}_{\text{bottom2}}\) = 0.5 eV are shown in Figure 4e. The chiroptical effect occurs at the resonant spectra. After exchanging the gate voltage at the wavelength related to the positive CD spectrum, a consequent negative CD response was observed. Whenever the gate voltage was changed, BSR transformed into its enantiomer. This condition demonstrates that by adjusting the Fermi level of graphene the chiroptical response of the BSR can be changed instead of restructuring the geometry [58]. Gate-controlled transmission difference and optical activity in gated graphene-integrated chiral metasurfaces were also demonstrated [24]. The coefficients of transmission for the LCP (T\({}_{\text{LCP}}\)) and RCP (T\({}_{\text{RCP}}\)) through the graphene-conjugated double Z-shaped metasurface at four different gate voltages are shown in Figure 4i. When voltage was applied to the gate, the T\({}_{\text{RCP}}\) was strongly modulated because of the significant transition arising from the shift in the optical conductivity of graphene; however, the T\({}_{\text{LCP}}\) was insensitive to the applied voltage (Figure 4f) [24]. Increasing the applied voltage significantly reduced the T\({}_{\text{RCP}}\) while maintaining the T\({}_{\text{LCP}}\) at the 1.1 THz resonance frequency (Figure 4j). This method of polarization control can be used in telecommunications and imaging devices as well as in ultrasensitive sensors to determine the chiroptical response and formation of biomolecules [24]. Optically tunable chiral metasurfaces are also available [60; 143], most of which use semiconductors for the optical regulation, because the semiconductor's conductivity changes significantly with the optical pumping at energies exceeding the band gap. Multi-layered graphene (MLG) with conjugated four-\"petal\" resonators was integrated to demonstrate that THz metasurfaces can offer dynamically tunable chiroptical responses utilizing optical pumping [62]. The metasurface was a sandwich structure with an MLG and metallic resonators arrayed alongside the dielectric structure (Figure 5a). Optical pumping with a 980 nm continuous wave laser was used to control the conductivity of the MLG resonator. As the linearly polarized wave traverses via the chiral metasurface, the linearly polarized wave changes into an elliptically polarized wave. As shown in Figure 5b, when the power of optical pumping increases, the amplitude of the cross-polarized spectra increases at frequencies exceeding 0.65 THz. The transmitted wave is converted into CPL near the frequencies of the observable CD, and with an increase in the optical pumping power, the resonant frequency of the ellipticity angle is shifted (Figure 5c). This proves that the ellipticity angle can be tuned very well by changing the optical pump. Recent studies have shown that chiral metasurfaces can be magnetically tuned [66]. The magnetic tuning of silver/cobalt (Ag/Co) composite chiral nanohole array (CCNA) [66] is shown in Figure 5d. CCNA contains different compositions of Ag and Co, and it breaks the mirror symmetry in-plane as well as in the light propagation direction, which results in a high chiroptical response. The Co component of the CCNAs enables a magnetically tunable chiroptical response, and Ag further increases the chiroptical response tunability. A magnetic field of B = 0 T, B = \(-\)1.7 T, and B = +1.7 T was used to switch the chiroptical response of CCNA (Figure 5e,f).