## 2 Materials and Methods  
Figure 1b-d presents the schematic drawings of the Archimedes spiral disk designs under study. Using the investigation results for similar structures in the microwave region [22] and taking into account our technological limitations, we rescaled the spiral disk parameters to the range of 0.2-0.5 THz, which was compatible with our instrumentation capabilities: the inner radius \(r=18\)\(\upmu\)m, the outer radius \(R=192\)\(\upmu\)m, the spiral arm line width \(w=8\)\(\upmu\)m, the number of arms \(n=12\). We considered two types of arrays differed in the lateral periodicity \(p\) of the single spirals in the \(X\) and \(Y\) directions: (1) \(p=768\)\(\upmu\)m, distantly spaced disks; and (2) \(p=408\)\(\upmu\)m, closely spaced disks with a \"hybridization effect\" (excitation of hybridized plasmon modes). The transmittance spectra of a periodic array of spiral disks in the range of 0.2-0.5 THz were simulated using the commercial CST Microwave Studio Suite software, wherein the regime of Floquet ports, periodic boundary conditions applied to the structure's unit cell were employed. In this regime,  
Figure 1: Four different designs of plasmonic metasurfaces (PMSs) with the metal disk elements (the unit cells are shown; \(p\) is the lattice constant): (**a**) the azimuthally patterned disk; (**b**) the spiral disk; (**c**) the spiral disk with a C-shaped resonator (\(\alpha\) is the resonator’s arch angle); (**d**) the spiral disk with a C-shaped resonator and a shifted center (\(\delta r\) is center shift). the oblique illumination of the structure was modeled through specifying the orientation angles (\(\theta\), \(\varphi\)) of the incident wave vector \(\mathbf{k}\) in the \(XYZ\) coordinate system: the polar angle \(\theta\) describes the angle between \(\mathbf{k}\) and the \(Z\) axis, while the azimuthal angle \(\varphi\) defines the angle between the orthogonal projection of \(\mathbf{k}\) on the \(XY\) plane measured from the \(X\) direction. Since a THz beam was horizontally polarized in our experiments and a rotary stage afforded the rotation of the sample only around a vertical axis, our investigations were limited by TM-polarized excitation. Two marginal cases were considered: \(\varphi=0\) and \(\varphi=90^{\circ}\). The former was of primary importance since it enabled the proper excitation of the C-shaped resonator (\(\mathbf{E}_{in-plane}\parallel\mathrm{X}\)), as elucidated in Section 3.1. The spirals were modeled as 0.35 \(\upmu\)m-thick aluminum layers with dispersionless conductivity \(\sigma=1.5\times 10^{7}\) S/m [25; 26] placed on top of a 15 \(\upmu\)m-thick polypropylene (PP) film with the dielectric permittivity \(\epsilon_{\mathrm{PP}}=2.25\)-(1 - j\(10^{-3}\)) [25; 26]. PP has low absorption in the THz range and was intentionally chosen as a substrate material to minimize the dielectric losses in the structure; we also used the thinnest PP substrate available to diminish interference effects in the PP layer. The designed structures were fabricated with a standard contact photolithography technique, which was specifically adapted for operations on flexible polymeric substrates [26; 27; 28]. Each 15 \(\upmu\)m PP film sample was carefully tightened onto a bearing \(Al\) ring with a clear aperture diameter of 3 inches which specified the optical diameter of the output PMS. \(Al\) metallization was sputtered onto the PP films by means of a vacuum thermal deposition method. Prior to sputtering, the PP substrates were treated with a glow discharge in O\({}_{2}\) atmosphere to improve the adhesion of \(Al\) to PP. The sensing performance of the structure was tested by depositing a photoresistive material (AR-P 3250 produced by ALLRESIST (GmbH, Strausberg, Germany) [29]) of variable thickness (from 0.3 to 10 \(\upmu\)m) on the PP film using a standard spin coating deposition technique.