## 1 Introduction  
Localized plasmon resonance (LPR) is the resonant oscillation of the charge density of free electrons in conducting particles of sub-wavelength size in an oscillating external field, accompanied by the enhancement of the near field at the surface of the particles [1]. LPRs exhibit strong spatial localization and amplification of the electromagnetic field near the particle surface with the resonance frequency depending on the shape, size and conductivity of the particle and its environment. These properties have wide applications in the visible frequency range: in chemical sensors and biosensors [2], surface-enhanced Raman spectroscopy [3], fluorescence amplification [4], second harmonic generation [5], solar cells [6] and photocatalysis [7]. The terahertz (THz) spectral range (covering the frequency domain from 0.1 to 10 THz) is of great importance for biology due to the nonionizing nature of THz waves and the fact that the rotational and vibrational modes of complex intra- and extra-molecule bonds of many important chemical and biological substances lie in this region [8; 9; 10]. This explains the high prospects of using plasmon resonances in the THz range for sensing and spectroscopy. It is worth noting, however, that the THz frequencies (\(\omega_{\rm THz}\sim 10^{12}\) Hz) are several orders of magnitude lower than the plasma frequency of free electrons in metal (\(\omega_{\rm p}\sim 10^{16}\) Hz) and their collision frequency (\(\omega_{\rm c}\sim 10^{14}\) Hz [11]) and therefore, plasmon resonances cannot formally be excited in this spectral region. To overcome these limitations, [PERSON] and his colleagues suggested making a periodic corrugation of the flat surface [12] that leads to forming bound surface states with a high confinement of the electromagnetic field, so called \"spoof surface plasmon resonances\". It has been shown that sub-wavelength metallic structures [13] are able to support spoof localized surface plasmon resonances (LSPRs) in the long-wave region. This idea was elaborated in detail in the metamaterial community through the concept of plasmonic metasurfaces (PMSs) [14; 15], which are understood as thin metallic screens with periodically patterned subwavelength apertures or substrate-backed arrays of subwavelength metallic elements. It is essential that LSPRs excited on a PMS are directly related to the geometry of the PMS unit cells that provides high flexibility in engineering the electromagnetic (EM) properties of the structures both in far- and near-field. Alongside with the feasibility of versatile frequency-selective and polarization-transforming devices, this approach enables creating high-performance PMS-based sensors of thin-film analytes. Exploiting the LSPR-induced effect of near-field confinement, which enhances light-matter interaction, such sensors exhibit high sensitivity to the dielectric environment and allow the determining of analytes with thicknesses several orders of magnitudes smaller than the LSPR wavelength via tracking a frequency shift of the LSPR relative to that of a bare PMS (without a deposited analyte). Applicable to THz, a comprehensive review of thin film sensing with metasurfaces is given in [16]. From a technological viewpoint, the THz range is attractive due to its relatively large (compared to VIS and IR domains) wavelengths as it allows one to use well mastered and inexpensive micromachining technologies, such as photolithography, in the PMS fabrication. Furthermore, the Ohmic losses in metal and dielectric losses in substrates, which serve as an obstacle at shorter wavelengths, do not play such a crucial role at THz frequencies and can be neglected in many cases. Plasmon resonances with lower losses have higher \(Q\)-factors which facilitate high-sensitivity sample detection via the precise measurement of small resonant shifts. In this work, we proposed using PMSs with spiral-disk-shaped metallic elements patterned on low-loss polypropylene substrates for the THz sensing of thin dielectric films.