# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Tunable Chiral Metasurfaces  
In addition to extensive research on enhancing the optical properties of chiral metasurfaces, tunability has emerged as the most recent advancement in this field of study. Tunable chiral metasurfaces offer improved approaches for the theoretical investigation of distinctive EM and optical features. Some tunable chiral metasurfaces are shown in Figures 4-6. Figure 4: Tunable chiral metasurfaces using phase change and electrical doping. **(a)** Dagger-like planar chiral metasurface along with the thermal monitoring system. **(b)** CD response for a switching method with VO\({}_{2}\) elements regarded as semiconductor and metal. **(c)** Switching in CD via thermal control showing “CD low” in the “OFF” state and “CD high” in the “ON” state (**(a–c)** reproduced from [91]). **(d)** Geometric model of the chiral 3D array BSRs in graphene for RCP and LCP and its unit cell. **(e)** Simulation of the difference in transmittance of the BSR with varied Fermi energies on the bottom layer of the split-ring chiral graphene (**(d,e)** reproduced from [58]). **(f)** Illustration of gate-controlled active graphene conjugated double Z-shaped metasurface (CDZM) diagram. **(g)** Active graphene of CDZM with a graphene of single layer, which is deposited on the CDZM top layer that is controlled by a gate and encased using ion gel layer. **(h)** CDZM with graphene layer connected to the G gate; B is a base coupled to the ion gel. **(i)** Experimental result of the coefficients of transmission for LCP (the dashed line), and RCP (the solid line) waves plotted for various gate voltages of \(\Delta\)V. **(j)** T\({}_{\rm LCP}\) (green) and T\({}_{\rm RCP}\) (orange) as functions of \(\Delta\)V at a 1.1 THz resonance frequency (**(f–j)** reproduced from [24]). The CD and OR can be tuned by creating building blocks from materials that undergo a phase shift [61]. Phase-change materials (PCMs), which have EM properties that change dramatically when heated, are widely used as thermally modulated materials. Recently, a phase-change material, vanadium dioxide (VO\({}_{2}\)), was utilized to create electro-optic switches, thermal switches, and all-absorber metasurfaces. The use of phase-change materials such as VO\({}_{2}\) allows for the tuning of chiroptical response in the metasurface by exploiting their reversible phase transition between metallic and insulating states in response to external stimuli such as temperature or light [61; 91]. This property, when integrated into the chiral metasurface, enables dynamic control of chiroptical response,  
Figure 5: Optical and magnetic tunable chiral metasurfaces. (**a**) Multi-layered graphene-based optically tunable chiral metasurface and geometrical parameters of the unit cell. (**b**) Simulated co-polarized spectra of transmission of the chiral metasurface in different intensities of optical pumping. (**c**) Spectra of the ellipticity angle of the graphene multi-layered chiral metasurface relying on optical pumping value obtained using simulation ((**a–c**) reproduced from [62]). (**d–e**) Schematic of the composite chiral nanohole array. (**f**) CDT spectra of the CCNA samples in the appearance of applied magnetic field of strength B = \(\pm\) 1.7 T externally ((**d–f**) reproduced from [66]). making them suitable for applications such as reconfigurable lenses, adaptive optics, and modulators, where real-time tuning of optical properties is critical for optimizing device performance in varying conditions or applications [140; 141; 142]. Figure 6: Different types of tunable chiral metasurfaces. (**a**) Chiral metasurface integrated with LCs. The LC molecules’ tilt depends on the applied voltage, resulting in a shift in the CD response. The right one demonstrates the placement of LC molecules at “ON” and “OFF” situations triggered by the voltage applied. (**b**) CD mapping with respect to the incident angle. (**c**) CD map using LC molecules that are tilted under incident CPL.