## 3 Results and Discussion
### Symmetric Spiral Disks
The transmittance spectra of the samples measured with the BWO spectrometer are shown in Figure 4b. The simulated spectra (see Figure 4a) are in good accordance with the experiments. With increasing \(d\), all the resonances were gradually shifted towards lower frequencies. We evaluated the resonance shifts \(\Delta f\) (GHz) from the spectra at 0.297, 0.339 and 0.373 THz as a function of the photoresist thickness \(d\) (see Figure 5a). Figure 2: (a) Transmittance spectra of the distantly spaced (\(p=768\)\(\upmu\)m) spiral disks with different C-resonator angles \(\alpha\). Normal incidence (\(\theta=0^{\circ}\)), \(\mathbf{E}\parallel X_{i}\); (b) The resonance frequencies vs. \(\alpha\) (—number of resonances), the red circles—the resonances produced by the bright mode of the C-resonator. The errors for \(\Delta f\) in the experiments and simulations were determined with the inaccuracy (\(\approx\)0.5-1 GHz) of identifying the resonance frequency for the minima of the transmittance spectra. The experimental errors depended on the step of the frequency scanning (0.5 GHz). The experimental and simulated results were in qualitative agreement. Additional simulations for the larger thicknesses showed that \(\Delta f\) tended to saturation at \(d>50\) um, at which the sensor was insensitive to the thickening of the photoresist layer. Figure 4: Transmittance spectra of the distantly spaced (\(p\) = 768 μm) spiral disks with a 90\({}^{\circ}\) C-resonator and photoresist coatings of different thicknesses: (**a**) the numerical simulations; (**b**) the experiments; (**c**) the \(E_{Z}\)-field distributions at the resonances (0.297 THz-natural electric dipole mode (ED), 0.339 THz and 0.373 THz-magnetic dipoles (MDs)). Normal incidence (\(\theta=0^{\circ}\)), \(E\parallel X\). Figure 3: TM-transmittance spectra of the distantly spaced spiral disks with a 90\({}^{\circ}\) C-resonator under oblique illumination: \(\theta\) = 0, 20 and 40\({}^{\circ}\): (**a**) the simulations and (**b**) the experiments. Azimuthal angles \(\varphi\) = 0 and \(\varphi\) = 90\({}^{\circ}\) correspond to the zero projections of the incident **E**-field on the \(Y\) and \(X\) axes, respectively. According to [16], the frequency sensitivity of the sensor was defined as the ratio of the resonance frequency variation to the product of the analyte thickness and the refractive index \(n_{\text{a}}\): \(S=\Delta f/(d\cdot n_{\text{a}})\), with the units of GHz:(the refractive index unit (RIU)\(\cdot\)mm)\({}^{-1}\). The dependencies \(S(d)\) plotted in Figure 5b are almost consistent with each other, except for \(d=0.3\)\(\upmu\)m at which the experimental value \(S\) is larger than the simulated one. The reason for this discrepancy presumably lies in a deviation of the real thickness (averaged over a THz-illuminated area) of the photoresist layer deposited on a flexible 15 \(\upmu\)m PP substrate of our PMS sensor. In sensing applications, the full width at the half minimum (_FWHM_) in the units of frequency is another important characteristic related with the \(Q\)-factor of the resonance. The _FWHM_ vs. \(d\) is plotted in Figure 5c. The _FWHM_ values were determined with the accuracy of 0.5-1 GHz depending on the resonance bandwidth. It is worth noticing that the resonance at 0.297 THz has about two times larger _FWHM_ compared to the other two resonances. This effect is explained by their different nature: the mode excited at 0.297 THz is electric dipole, while the modes at 0.339 THz and 0.373 THz are magnetic dipole, as illustrated in Figure 4c by the \(E_{Z}\)-field distributions simulated at the resonant frequencies at a distance of 2 \(\upmu\)m from the metallized layer. According to [22], the magnetic modes potentially have higher \(Q\)-factors than the electric ones. In many cases, the sensitivity alone is not enough to determine the sensor quality. It is clear that to minimize the overlapping between the detection thresholds upon shifting the resonance, resonances with a narrower bandwidth can be distinguished better in the experiment.