# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 3 Types of Chiral Metasurfaces
### Single-Layered Chiral Metasurfaces
Because of the negligible absorption loss beyond 1.5 \(\upmu\)m of germanium (Ge), the illumination light would either be reflected or transmitted, resulting in the CDs for reflection and transmission being roughly opposite (Figure 2c). The massive chirality (approximately 88%) of these and similar Ge metasurfaces far exceeds the limit of prior ultrathin metallic chiral metasurfaces. The dielectric nature of these metasurfaces allows them to reduce the absorption loss and mitigate the fabrication problems associated with metal-based chiral metasurfaces. In addition, these designs offer a platform for controlling wavefronts by modifying them in line with the handedness of circularly polarized light (CPL) to create multipurpose chiral metasurfaces [102]. The spiral chiral metasurfaces (Figure 2e) have fewer sharp corners, allowing for a smooth distribution of fields with enhanced optical chirality [82]. The spiral meta-atom has a rotation of one and a half, whereas the gammadion has only one 90-degree kink within each arm. Thereby, the two polarizations of incidents exhibited distinct behaviors. Compared to C4 gammadion meta-atoms, nanosprial meta-atoms offer stronger optical chirality and more uniform enhancement with a stronger twist. However, because of their discontinuous responses and fabrication challenges, these spiral metasurfaces are impractical for some applications [82]. Despite the rapid advancements in planar and dielectric chiral metasurfaces, there are some limitations, such as absorption and scattering losses, discontinuous responses, and limited chiroptical responses. These limitations can be overcome using chiral metasurfaces with bound states in the continuum (BICs) [112; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124]. BICs are EM modes that exist within the radiation continuum but do not radiate energy away [125]. In chiral metasurface, BICs can be harnessed to improve chiroptical response by concentrating and confining EM fields at specific frequencies, leading to stronger interactions with chiral materials [112; 118; 119; 120; 121; 122]. Tailoring chiral metasurfaces to host BICs at specific wavelengths enhances the sensitivity and efficiency of chiroptical devices such as modulators and sensors [125]. Moreover, BIC non-coplanar symmetry breaking has a direct chirality manipulation potential [112]. Thus, the choice between plasmonic and dielectric materials for single-layer chiral metasurfaces rests on factors such as desired operation wavelength, fabrication feasibility, and material losses [82; 113; 114; 82]. Dielectric materials, with broader spectral ranges, and lower losses are preferred for applications requiring broadband chiroptical responses. Plasmonic materials provide strong field confinement and sensitivity but are limited to specific narrowband applications because of their inherent losses, emphasizing the importance of material selection for optimizing the chiral metasurface's performance for various scenarios [113; 114; 126]. A chiral, substantially transparent metasurface was achieved by transforming BICs into quasi-BICs with the highest chiroptical response, which resulted in the highest peak unit height in the CD spectrum [112]. To perturbate the antiparallel dipole BICs in the symmetric dimers of dielectric bars, the design features a slight vertical shift distance d and a corresponding rotation angle \(\theta\). The lattice in Figure 2f has a reflection symmetry regarding three forms of mirror planes: \(\sigma_{1}\) and \(\sigma_{2}\) and the plane of the drawing. When the vertical C2 axes (the point where \(\sigma_{1}\) and \(\sigma_{2}\) planes intersect) pass exactly through their centers, all the dimers are symmetric (Figure 2f). The chirality of different kinds of resonant chiral metasurfaces that work in the visible and NIR spectral regions can be enhanced using this design. However, this structure is not as simple to fabricate as pure planar chiral metasurfaces.