# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 4 Conclusions and Outlook  
Chiral metasurfaces with CD, AT, OR, and other EM properties have been used to improve the chiroptical response. In this review, the fundamentals of chiral metasurfaces and their different types are summarized. First, we discuss single- and multi-layered chiral metasurfaces that can optimize the chiroptical response. Single-layer chiral metasurfaces are relatively easy to fabricate, whereas multi-layer chiral metasurfaces are the finest for providing a greater degree of freedom to manipulate waves and achieve a desirable chiral optical response. Furthermore, it is possible to control the chiroptical response of layered chiral metasurfaces using different geometrical parameters, including the orientation angle, the number of layers, and the distance between layers [69; 70]. Fabrication of multi-layered chiral metasurfaces poses challenges in attaining precise alignment, identical layer thickness, scalability, and material compatibility [84; 85; 86]. To address these issues, researchers can examine in detail advanced fabrication techniques and improved materials for compatibility. More efforts should be made to optimize procedures for scalability and to incorporate quality control measures to improve practical applications. Third, we review recently developed tunable chiral metasurfaces. These metasurfaces facilitate active control of EM waves. Electrically, optically, magnetically, and mechanically tunable chiral metasurfaces have also been developed. Although these metasurfaces are rapidly evolving, the majority of current tunable chiral metasurfaces have been achieved through numerical simulations and theoretical analyses, and only a few have been experimentally validated. Experimental techniques for evaluating the performance of tunable chiral metasurfaces consist of spectroscopic measurements of OR and CD augmentation under various conditions, such as changes in external stimuli. These measurements offer important information about the tunability of chiral metasurfaces. However, the existing validation limitations include evaluating tunable chiral metasurfaces over a wide range of wavelengths and the need for precise equipment to measure the chiroptical response. Furthermore, most of today's tunable chiral metasurfaces are composed of metal-dielectric metasurfaces, which limits their efficiency. Finally, DL-based approaches for chiral metasurface modeling approach have been examined, and it was found that it effectively reduced the numerical modeling computation time while maintaining accuracy. DL-based design of chiral metasurfaces enables optimization of optical parameters, including but not limited to amplitude distributions, phase profiles, and spectral responses. DL enables this optimization by effectively investigating complex design spaces and learning intricate correlations between desired optical outcomes and structural parameters. It speeds up the design process by iteratively refining metasurface topologies to attain specific optical capabilities, resulting in tailored responses and improved performances in applications such as holography, metalenses, and sensing [78; 79; 80; 81]. In chiral metasurface design, specific DL architectures such as CNNs and algorithms such as GANs have been successfully employed. They offer advantages over the traditional methods by learning complicated correlations in data effectively, leading to faster optimization and precise prediction of chiral metasurface features [79; 81]. In general, research on chiral metasurfaces focuses on geometrically chiral meta-atoms (i.e., intrinsic chirality) [74; 83; 107; 146; 147] as well as achiral and homogenous meta-atoms (i.e., extrinsic chirality) [148; 149; 150; 151; 152]. The chirality of an intrinsic chiral metasurface is mostly affected by the match between the meta-atom size and the wavelength spectra of the illumination light, whereas in an extrinsic chiral metasurface, it is primarily affected by the angle of the incident light.