# Chiral Metasurfaces: A Review of the Fundamentals and Research Advances
## 1 Introduction  
The word \"chirality\" comes from the Greek word name \(\chi\)\(\iota\)\(\iota\) (kheir), which translates to \"hand\". Based on the description of chirality, a device is chiral if it is non-superimposable with its mirror image [1; 2; 3]. Chiral devices have right- or left-handedness and lack any inversion symmetry or mirror symmetry plane [1; 4]. Chiroptical response is the optical response of an object with chirality. Chiral media exhibit unique optical responses to the left circularly polarized (LCP) and right circularly polarized (RCP) light, resulting in a chiroptical response. Specifically, optical activity (OA) is the rotation in polarization of a linearly polarized light when it propagates through a chiral media, and circular dichroism (CD) is the absorption difference between RCP and LCP light. Chiroptical response is important for distinguishing enantiomers in chemistry, physics, and biology [5; 6]. Artificially engineered chiral metamaterials have been presented to improve the weak chiroptical signals in chiral media [5]. Among these, metasurfaces, which are 2D artificial composite meta-atoms, stand out because of their advantageous fabrication process, cost, and capabilities. They feature characteristic electromagnetic (EM) wave-manipulating abilities similar to those of three-dimensional (3D) metamaterials and support a wide spectrumof designed optical responses owing to their comparative flexibility in structural fabrication [6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. Thus, metasurfaces that respond differently to LCP and RCP have emerged as recent academic and industrial hotspots [14; 15; 16; 17; 18; 19; 20; 21]. Chiral metasurfaces play a crucial role in advancing science and technology by enabling precise control of light-matter interactions, paving the way for innovations in material science, optics, and sensing applications. Chiral metasurfaces have demonstrated significant improvements in chiroptical responses in practical applications such as improved CD spectroscopy for biomolecular analysis, enhanced chiral molecule detection in chemical sensing, and advanced chiral photonics components for telecommunications and quantum information processing [22; 23; 24]. Various geometries of metasurface have been investigated to optimize the chiroptical response, including nanorods [25; 26; 27; 28; 29], Slavic symbols [30; 31; 32], V-shaped nanoantennas [33; 34; 35; 36; 37; 38], Y-shaped meta-atoms [39; 40; 41], split-ring resonators [42; 43; 44; 45; 46], helix meta-atoms [47; 48; 49; 50; 51], twisted cross meta-atoms [52; 53], multi-layered arc meta-atoms [54; 55; 56; 57], and others that support optical resonances in subwavelength meta-atoms. Active chiral metasurfaces using optical, electrical, and mechanical methods have also been introduced to actively modify their chiroptical responses [9; 10; 11; 58; 59; 60; 61; 62; 63; 64; 65; 66]. Metasurfaces designed to improve the chiral optical response can be broadly categorized into single- [21; 67; 68] and multi-layered [15; 20; 69; 70; 71; 72; 73] chiral metasurfaces. Single-layer chiral metasurfaces are typically favored for their ease of fabrication, simplicity, and lower optical loss, making them suitable for efficient and compact applications like light structuring and metaholography. However, a multi-layered metasurface provides a greater degree of freedom to manipulate waves and attain the desirable chiroptical response, enabling advanced functions such as enhanced sensing sensitivity and polarization manipulation [20; 69; 70]. In a layered metasurface, geometrical parameters, including the orientation angle, number of layers, and distance between layers, affect the chiroptical response [69; 70]. Thus, the choice between single- and multi-layered chiral metasurface depends on the specific application needs and involves balancing design complexity, fabrication constraints, and the target application's specific requirements.