# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Tunable Chiral Metasurfaces
(**d,e**) Simulation result of the CD response and reflection result of the LC-integrated chiroptical metasurface in the (**d**) “ON” and “OFF” states ((**a**–**e**) reproduced from [74]). (**f**) Schematic representation of the mechanical tuning techniques of the chiral metasurface. (**g**) Geometric parameters of the reconfigurable chiral metasurface, where the dashed lines and green solid denote the valley and the mountain creases of G-Mori, respectively. (**h**–**i**) Spin-selective transmission at an angle of (**h**) \(\theta=-60^{\circ}\) and (**i**) \(\theta=60^{\circ}\). (**j**) Linear to CPL conversion at an angle of \(\theta=60^{\circ}\). (**k**) CD response of the metasurface according to the folding angle from \(\theta=0^{\circ}\) to \(80^{\circ}\). (**l**) Transmission spectra and CD response of the chiral metasurface at \(\theta=60^{\circ}\) ((**f**–**l**) reproduced from [59]). In a study by [PERSON] et al. [91], a novel chiral plasmonic structure with a dagger-like shape composed of metal and metal-VO\({}_{2}\) alloys was proposed, as shown in Figure 4a. Using an exogenous thermal stimulus, the hybrid structure was tuned, displaying switching in the CD spectrum with minimal chirality in the OFF state (corresponding to the metallic phase of VO\({}_{2}\)) and conversely maximum chirality in the ON state (corresponding to the semiconducting phase of VO\({}_{2}\)). The phase of the VO\({}_{2}\) phase-change material switches from the state of semiconducting to metallic (Figure 4b). Fascinatingly, external temperature control of the hybrid system allows for the dynamic modulation of the VO\({}_{2}\) phase, effectively turning the device into a CD switch (Figure 4c). Using the metal-to-insulator shift characteristics of VO\({}_{2}\), thermal switching of perfect absorption and AT effects can be attained in the NIR [140]. When the VO\({}_{2}\) is in the metallic state, the metasurface device behaves as a chiral-selective plasmonic absorber that can result in a CD signal of around 0.7. When the state of VO\({}_{2}\) is in insulation, the proposed metasurface exhibits a dual-band AT effect [140]. Similarly, incorporating VO\({}_{2}\) into a metasurface device enabled the achievement of a switching effect between absorption and AT, attributed to the VO\({}_{2}\) phase-changing property [141]. When the VO\({}_{2}\) behaves as a metal, the metasurface becomes a selective absorber for linearly polarized light, and by changing the VO\({}_{2}\) phase, it achieves a broadband AT effect. The chiroptical response of this metasurface device can also be realized with the AT effect [141]. In addition, a switchable and tunable terahertz metasurface that results in the AT effect and dual polarization conversions can be achieved by integrating VO\({}_{2}\) with graphene [142]. Hence, using VO\({}_{2}\), a phase-change material known for its conductivity change in response to thermal, electrical, and optical stimuli, it becomes possible to tune a chiral metasurface [91; 140; 141; 142]. Meanwhile, recent studies have shown that the CD and OR can be tuned by electrical doping [58; 66]. Most of these devices use graphene owing to its high electrical conductivity, continuous chemical resistance, and wide electro-optical spectrum [59; 84; 85]. The Fermi energy level of the graphene could be tuned, and its optical properties can be altered by introducing an external voltage to the graphene layer [58]. Because the doping level can be controlled at a fine scale, the electrical doping with graphene enables precise tuning of the chiroptical response with high precision. The level of tunability is determined by factors such as the number of graphene layers, the applied voltage, and the design of the chiral metasurface [84; 85]. Higher voltage or adding graphene layers can result in better tunability, allowing for tailored chiroptical effects [58]. Furthermore, it has been demonstrated that by adjusting the Fermi energies of various bilayer split-ring (BSR) components, CD signals can be dynamically modified [58].