# Enhancement of Optical Chirality Using Metasurfaces for Enantiomer-Selective Molecular Sensing
## 1 Introduction  
Chirality refers to the property that an object cannot be superimposed with its mirror image [1; 2]. Chirality is an observable property in diverse molecules and plays a crucial role in determining the molecules' chemical and biological properties, which are important in the fields of life science, analytical chemistry, biochemistry, and medicine [3; 4; 5; 6]. One of the most representative examples of chiral structures is the enantiomers that are mirror images of each other. Although they exhibit the same molecular formula, the response of each molecule has either a positive or a severe side effect in biology. The need for developing technology to distinguish such chiral molecules has been noticed since the discovery that the enantiomer of thalidomide, which was sold as an anti-morning drug in the 1950s, caused birth defects and malformed babies. Circular dichroism (CD) spectroscopy utilizes different reactivity to circularly polarized light (CPL) with opposite handedness in chiral molecules, and it is used to distinguish two optical isomers [7; 8]. However, it still remains challenging to effectively distinguish chirality using CD signals due to most enantiomers' weak chiroptical reactivity to the light. To overcome such limitations, significant research is conducted to develop the platform of CD spectroscopy for the effective enhancement of chiroptical signal. Researchers have explored and designed various nanostructures to study the novel light-matter interactions in the form of chirality into two distinct classes. In one class, nanostructures with intrinsic chiral geometry are designed and studied to enhance CD signals induced by the nanostructure itself. In the other class, achiral nanostructures exhibiting mirror symmetry are exploited, which amplify the optical chirality of orthogonally polarized incidentlight equally. The former approach helps the researchers to elucidate the mechanism and how the CPL interact differently in chiral nanostructures [9; 10; 11; 12; 13]; however, the intrinsic chiral geometry in such nanostructure severely distorts and eludes the detection of the target molecules' chirality, which is not appropriate for the CD spectroscopy. In the latter approach, on the other hand, diverse methods of achiral nanostructures are developed as a platform to amplify the optical chirality [14; 15; 16; 17; 18; 19; 20; 21], which affords effective detection of enantiomers with enhanced CD signals and provides tremendous opportunities for developing novel CD spectroscopy by optimizing the nanostructures. It typically requires two crucial conditions to enhance chirality in achiral nanostructures; optimal phase difference between the electric and magnetic fields of CPL and significant amplification of the field. To achieve these, various designs of metasurfaces are proposed to secure a high-field value and to induce an appropriate phase difference by driving strong light-matter interaction, such as plasmonics [9; 10; 11; 13; 22; 23; 24] or dielectric resonances [14; 16; 17; 18; 19; 20; 21; 25; 26]. For example, a nanodimer structure that invokes strong field concentration facilitates the strong enhancement of optical chirality in the gap regions [27; 28; 29] and even the dynamic tuning of the chiral signal is possible using an electrically tunable metasurface [23]. Despite such progress, the underlying physics of the relation between chiral enhancement (C enhancement) and geometric design of resonant nanostructures is still elusive, and thus the in-depth understanding of chiroptical response in a nanostructured surface is essential to pave a pathway of realizing efficient CD spectroscopy. In this paper, we explore the design of nanostructured surfaces, i.e., metasurfaces, by investigating the interaction between electric and magnetic resonances in diverse unit structures.