## 3 Results and Discussion
### Asymmetric Spiral Disks with Shifted Center  
Another way to produce high-order resonances is to break the symmetry of the disk by shifting its center at some value _sr_ (see Figure 1d). The _sr_ magnitude affects the number and frequencies of the dark modes. For the new design we took spiral disks with _sr_ = 35 \(\upmu\)m, at which multiple resonances were excited in the spectral region of interest 0.2-0.5 THz, and arranged them in the array with higher density packing (_p_ = 408 \(\upmu\)m). The advantage of densely packed structures lies in red shifting the LSPRs relative to the diffraction lobes' onset point that improves the angular stability of the resonances [35]. Furthermore, in the case of closely spaced disks, new strong plasmon modes can be excited due to hybridizing the EM fields of nearby disks [19; 36]. The results of simulating the transmittance spectra for the closely spaced spiral disks with a 135\({}^{\circ}\) C-resonator and _sr_ = 35 \(\upmu\)m covered with photoresist films of different thicknesses are shown in Figure 6a. For clarity, the graphs in Figure 6 are plotted in a reduced spectral region of 0.36-0.48 THz. There are many narrow resonances, which are red shifted when increasing the photoresist thickness (the shifts are indicated by arrows). The experimental results of the fabricated samples with the same parameters are presented in Figure 6b. As compared with the simulations, there is only one distinct wideband resonance at 0.438 THz in the spectral range considered; other narrow resonances are broadened and disappeared due overlapping with adjacent resonances. The _E\({}_{Z}\)_-field distribution of two nearby disks at 0.438 THz (Figure 6c) presumably corresponds to an electric sextupole hybrid mode. The _FWHM(d)_ values plotted in Figure 7c are approximately the same as for the distantly spaced disks without shifted centers (see Figure 5c), while _sr(d)_ and _sr_ (Figure 7a,b) are significantly  
Figure 6: Transmittance spectra of the closely spaced (_p_ = 408 \(\upmu\)m) spiral disks with centers shifted at _sr_ = 35 \(\upmu\)m, covered by photoresist films of different thicknesses: (**a**) the numerical simulations; (**b**) the experiments; (**c**) the _E\({}_{Z}\)_-field distribution at resonance (0.438 THz). Normal incidence (_σ_ = 0\({}^{\circ}\)), _E_\({}\parallel X\). larger. The greater sensitivity can be explained by the high-density arrangement of the disks, which enhances EM field localization near the PMS. The larger sensitivity yields the greater _FOM_. The highest _FOM_\(\approx\) 2950 (RIU-mm)\({}^{-1}\) is reached at the smallest analyte thickness (\(d\) = 0.3 \(\upmu\)m), which is \(\approx\)77% bigger than in the case of the distantly spaced disks. This result is comparable with the best _FOM_ value (3450 (RIU-mm)\({}^{-1}\)) obtained at the same analyte thickness for a labyrinth metasurface absorber implemented with a similar fabrication techniques [16; 35]. As seen from numerical simulations in Figure 6a, the illumination of the array of asymmetrical disks excites multiple narrow resonances as compared with only several peaks excited in the symmetrical disks (Figure 4a). The sharp resonances arising in asymmetrical structures originate from the physics of bound states in the continuum (BIC) [37]. The number of BIC resonances and their \(Q\)-factors depend on the level of asymmetry. In our case, this level defined by the \(\delta r\) value (see Figure 1d) effect on the number, as well as the width and depth of the resonances. Comparing the results obtained for symmetrical and asymmetrical disks (Figures 4 and 6), we can assume that the number of resonances, their widths and depths increase with increasing \(\delta r\). Besides, the augmentation of a packing density of the disks produces new hybrid modes. However, in our experiments (Figure 6b), the adjacent resonances overlap with each other which leads to the formation of broad resonances with lower \(Q\)-factors.