# Enhancement of Optical Chirality Using Metasurfaces for Enantiomer-Selective Molecular Sensing
## 3 Results and Discussion
The first type is designed as a nanorod with a length of 250 nm (**a–f**), and the second type is designed as a squared nanoblock with a length of 125 nm (**g–l**). (**a–c**) The schematic of the nanorod (**a**), and the images of the electric field (**b**) and the magnetic field (**c**) monitored from the cross-sectional x-y plane in (**a**), i.e., top view. (**d–f**) The spatial distributions of the relative phase between the electric and magnetic fields for x-directed (**d**) and y-directed polarizations (**e**) and the C enhancement (**f**). (**g–i**) The schematic of the squared nanoblock (**g**) and the images of the electric and magnetic fields in the squared nanoblock for the same conditions of (**b,c**). (**j–l**) The spatial distributions of the relative phase of the electric and magnetic fields (**j,k**) and C enhancement (**l**) in the squared nanoblock for the same conditions of (**d–f**). oscillation period of a wavelength, including the effective length of Si and free space. This is explained by the effect of the radiated fields generated in the resonant nanostructure interacting with each other, which influences the resonance of nearby nanostructures and induces super- or sub-radiant conditions [36; 37]. Under the super-radiant condition with the optimal separation, high chirality appears by the constructive interference. Figure 4c clearly shows such modulation at the wavelength of 470 nm, which shows the maximum C enhancement in Figure 4b. The modulation that occurs in the C enhancement spectrum is determined by a simple theoretical calculation as below. \[d= \frac{\lambda}{2n}\times(2m),\ m=0,\ 1,\ 2,\ \cdots \tag{2}\] \[d= \frac{\lambda}{2n}\times(2m+1),\ m=0,\ 1,\ 2,\ \cdots \tag{3}\]  
Equations (2) and (3) indicate the conditions for constructive interference and destructive interference of two waves, respectively, and \(d\), \(n\), and \(\lambda\) are the distance between two sources of waves, the effective refractive index, and the wavelength of a wave. Using the two equations, the radiation fields theoretically induce the super  
Figure 4: The average enhancement of optical chirality with changes in the separation of the Si nanoblock dimer. (**a**) Images of the enhanced optical chirality for four representative separations of the dimer at the wavelength of 470 nm. (**b**) Image of the spectrums of the optical C enhancement as the separation is tuned from 135 nm to 2025 nm. White circle marks in the map indicate the theoretical maximum point of the radiation field, and black cross marks in the map indicate the theoretical minimum point of the radiation field. (**c**) The C enhancement at the resonance wavelength of 470 nm. The inset in (**c**) shows the spectrums for four representative separations in (**a**). Figure 3: The enhanced optical chirality at the gap point of the Si nanoblock dimer. (**a**) Images of enhanced optical chirality at the four representative separations (\(d\) = 220 nm, 175 nm, 150 nm, and 135 nm, from top to bottom) at the resonant wavelength of 475 nm. (**b**) Two-dimensional map of the optical C enhancement at the center of the gap in the nanoblock dimer for the various separations of the dimer. (**c**) The C enhancement for the separation of the dimer at the wavelength of 475 nm. The inset in (**c**) shows the spectrums in four representative dimers in (**a**). These points are displayed in Figure 4b, which is almost identical to the maximum and minimum points of the C enhancement. The slight difference between the tendency of C enhancement and theoretically calculated radiation fields is the result of balancing between the electric field, the magnetic field, and the relative phase changes' intensities [38; 39]. We further analyze the effect of super- and sub-radiant conditions on the C enhancement by evaluating the amplitude and relative phase of two examples (Figure 5). Figure 5a shows the scattering efficiency of the dimer for various separations (blue line) and the relative phase difference between the electric and magnetic fields (red line).