# Enhancement of Optical Chirality Using Metasurfaces for Enantiomer-Selective Molecular Sensing
## 3 Results and Discussion  
To understand the working principle of the enhancement of optical chirality, we analyze the underlying physical mechanism of the resonances in two representative nanostructures. Figure 2a,g show the schematics of two classes of nanostructures, i.e., a nanorod with the length of 250 nm and a squared nanoblock with the length of 125 nm under the incident light of right-handed circular polarization. To elucidate the condition for effective C enhancement, the magnitude and phase of the electromagnetic field are evaluated by comparing them with the associated terms in Equation (1). Figure 2b-f shows the magnitude and relative phase of the electromagnetic field at the resonant nanorod. The fields are inspected in the cross-sectional plane located at the height of 30 nm as illustrated in Figure 1a. Despite the reasonable level the electric and magnetic fields overlap (Figure 2b,c), the relative phase in the overlapped area near the surface, i.e., two corners for x-polarization and vertical surfaces for y-polarization, renders the \(\cos(\beta_{i\mathbf{E},\ \mathbf{H}})\) close to zero that decreases C enhancement (Figure 2d,e). This undesirable condition of the phase occurs by the non-consistent phase delay between orthogonally polarized scattered fields and hinders the chirality's preservation (Figure 2f). As the second class of geometry, the squared nanoblock is evaluated, which features the relative phase value reaching close to unity at the substantially overlapped area of the electromagnetic field (Figure 2h-k). As such, the nature of CPL is effectively preserved and the chirality is enhanced near the surface of the nanoblock compared to the example of the nanorod (Figure 2l). Next, we construct a dimer structure to multiply the C enhancement when the molecules are placed in the gap region. To investigate the effect of optical interaction between each element of the dimer on the chirality, the C enhancement for various conditions of separation is analyzed, which is shown in Figure 3. Figure 3a shows the four representative examples of dimer structures consisting of squared nanoblocks. As the separation decreases from 220 nm to 135 nm, the electric field is more concentrated at the center region of the hotspot, which is similar to the effect of the plasmonic dimer. This also facilitates the enhancement of optical chirality, which reaches approximately 55 times higher than the free space chirality as the separation decreases to 135 nm, i.e., gap size of 10 nm. Figure 3b exhibits the values of C enhancement at the center of the gap for the continuous adjustment of separation. The chirality monotonically increases as the separation decreases to 180 nm and significantly increases as the separation decreases further. Figure 3c shows the C enhancement at the wavelength of 475 nm, which is indicated by the black dotted line in Figure 3b. The consistent increase of C enhancement can be explained by the near-field interaction combined with the continuity of electric flux at the interface of the gap, which results in the enhancement of both the electromagnetic fields and the optical chirality. When the molecules are adsorbed on the overall surface area of the nanostructure, the average chirality near the surface region is evaluated as the separation of the dimer is tuned (Figure 4). There is an oscillatory variation of chirality as the separation is tuned from 355 nm to 835 nm (Figure 4a). At 355 nm, the chirality shows the maximum value of 25 at the overall surface area. However, as the separation increases up to 610 nm, the chirality decreases to less than 20 and increases again as the separation increases further to 835 nm. This trend of variation in the optical chirality is visualized in Figure 4b, which exhibits the average chirality calculated within 10 nm from the nanostructure surface for continuous tuning of separation. The modulated enhancement is observed with the approximated  
Figure 2: The enhanced optical chirality in two types of Si structures at the wavelength of 475 nm.