Figure 2 shows the simulated transmission (green, solid-curve in the left-scale) and reflection (red, dashed-curve in the right-scale) amplitude results for the three structures depicted in

Figure 1. The supercell of each structure is depicted as an inset in each part of

Figure 2 for clarity.

Figure 1a presents the results of the symmetric structure. Due to the fact that the electric field is horizontally polarized, we expect the excitation of the dipole mode to only be shown at higher frequencies (not shown here). Indeed, no resonance feature is noticed in this configuration as expected. The results of the nonmirrored design are shown in

Figure 1b, where the gap has been moved by 20.5 μm.

In this case, we notice the excitation of a weak

LC resonance. The sharpness of the resonance can be assessed by evaluating its

Q-factor. It is defined as the ratio between full-width half-maximum (FWHM) bandwidth and the resonance frequency (

f_{r}). As a result, we observe the excitation of a broad spectral response with FWHM of 231 GHz at the resonance frequency (

f_{r}) of 0.78 THz. Hence, the

Q-factor (

f_{r}/FWHM) is 3.4. Remarkably, a much sharper spectral response is revealed when the mirrored design depicted as an inset in

Figure 2c is used. It features FWHM of 90 GHz at a resonance frequency of 0.76 THz and hence a

Q-factor of 8.44. Therefore, comparing the

Q-factor of the mirrored supercell to the nonmirrored supercell, the improvement factor is two and a half times larger. This is a direct result of enhanced out-of-phase dipole moments that resulted from mirroring the resonators [

31]. The resonance of the mirrored structure in part (c) of the figure is slightly red-shifted and can be interpreted to the increased coupling in the mirrored resonators. In order to get an insight into the electric field distribution of both nonmirrored supercell shown in

Figure 1b and mirrored supercell shown in

Figure 1c, the electric field spatial distribution at the surface of both metasurfaces at the resonance frequency is demonstrated in

Figure 3. It is evident that once the resonators are mirrored as depicted in

Figure 3b of the figure, a much larger electric field can be confined within the complementary resonators, i.e., in the area where there is no metal. This not only helps in understanding why the

Q-factor has been improved but also visualizes the areas where the analyte can be placed. In this case, it is not needed or useful to cover the whole device, except in areas with high field confinement. It is worth mentioning that this issue was a topic of a recent paper [

14]. So, if we coat two sides of the resonators where the E-field is highly confined, we will need to coat only {[(

w = 3) × (

l = 50) × (2 sides of each resonator) × (4 resonators)]/[total area =

p^{2} = 120 × 120] = 1/12} of the whole area. This means only 8% of the whole device is required to be covered with the analyte. Therefore, the required analyte volume is reduced by 92%. It is also worth mentioning that the same results would be achieved if the gaps were moved to the opposite sides or placed on the outer corner of the structure. The final overall structure would feature the exact same frequency response.