# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Multi-Layered Chiral Metasurface  
Despite significant progress in single-layered chiral metasurfaces, the metasurface design frequently requires significant computational time and intensive heavy optimization techniques for a better chiroptical response. In single-layer chiral metasurfaces, the interactions between light and ultrathin meta-atoms are usually constrained, yielding a reduced efficiency and constrained controllability for practical applications [128, 129, 130, 131, 132, 133]. 3D metamaterials have been used to solve controllability issues; however, they also have a complex fabrication process and a loss that reduces their efficiency [134]. Multi-layered chiral metasurfaces are more controllable and efficient compared to 3D metamaterials due to their planar geometry, which enables precise control of the chiroptical responses at multiple wavelengths and ease of fabrication [71, 72, 73]. Multi-layered chiral metasurfaces, on the other hand, comprehensively manipulate the phase whilst keeping a substantial level of conversion efficiency. They can also provide a large chiroptical response, AT, and full-phase manipulation of EM waves [71, 72, 73, 135]. The computational difficulties in multi-layer chiral metasurfaces arise because of the increased interaction and complexity between the layers, leading to higher computational resources and longer simulation times required. These challenges were addressed by offering design flexibility to fine-tune the chiroptical responses while reducing the computational demands by layer decoupling, making them manageable for optimization and simulations [134, 136, 137]. The presence of near-field coupling and interference within the layers in a multi-layered chiral metasurface plays a crucial role in substantially redistributing energy and facilitates interaction with the meta-atoms. The key factors considered when customizing the multi-layered chiral metasurfaces for a broadband chiroptical response include adjusting the magnetoelectric couplings in between layers to control the overall response, carefully spacing layers to obtain phase matching along a broad wavelength range and optimizing the inter-layer interactions to enhance the chirality [84, 85, 136, 137]. The number of layers is also carefully selected to balance the bandwidth and complexity, ensuring that the chiral metasurface demonstrates the required chiroptical response [70, 85]. Thus, by customizing the magnetoelectric couplings, inter-layer interactions, spacing within lay ers, the number of layers, and orientation angle, a broadband chiroptical response can be achieved [70; 134; 136; 137]. Some multi-layered chiral metasurfaces that use the orientation angle and the number of layers to enhance the chiroptical response are presented in Figure 3.  
Figure 3: Multi-layered chiral metasurfaces. (**a**,**b**) The frequency response changes as the number of layers increases; wavelength dispersion transmission of \(\mathrm{LCP}\,\mathrm{I}\,\mathrm{T}_{\mathrm{LL}}\,\mathrm{l}\) (red) and \(\mathrm{RCP}\,\mathrm{I}\,\mathrm{T}_{\mathrm{RR}}\,\mathrm{l}\) (black). (**c**) Bandwidth of a seven-layered twisted metasurface. ((**a**–**c**) reproduced from [84]). (**d**) Schematic of simulated transmission amplitude of co-polarization \(\,\mathrm{l}\,\mathrm{t}_{21\mathrm{RR}}\,\mathrm{l}\) and transmission coefficient of the cross-polarization \(\,\mathrm{l}\,\mathrm{t}_{21\mathrm{LR}}\,\mathrm{l}\) of a single-layered metasurface with a copper C-shaped SRR. (**e**) Schematic of C-shaped SRR metasurface arrays with four layers of similar spacing of D = 2.5 mm. (**f**) The simulated and experimentally measured transmission amplitude, in which the inset is an image of the experimental sample. ((**d**–**f**) reproduced from [84]). (**g**) Schematics of chiral films with opposing handedness. (**h**) CD response of a chiral two-layered film with different inter-layer angles. (**i**) CD response at interangle of 45\({}^{\circ}\) as the number of layers rises from 2 to 3. ((**g**–**i**) reproduced from [86]).