## 1 Introduction
However, test measurements have demonstrated the fundamental impossibility of integrating the capabilities of the SPP and TDS methods in real conditions. The main reason is parasitic radiation, both from the conversion (coupling) elements and from the SPP track (because of inhomogeneities on the real surface). As a result, the photodetector receives a \"mixture\" of bulk waves (BWs) from the conversion output element and the BWs generated at the coupling input element and on surface inhomogeneities on the SPP track. As a result, the signal-to-noise ratio and measurement accuracy become very low [32]. It is important to note that in all the works we know on the refracometry of metal surfaces, the metal dielectric permittivity value \(\varepsilon_{\text{m}}\) in the THz range, as determined from an analysis of the spectrum of radiation reflected from or transmitted through the studied metal layer, is two to three orders of magnitude greater than the \(\varepsilon\)m values found from the characteristics of the SPPs for the same surface [2]. The main reason for this difference is apparently the different length of interaction of the radiation with the sample under study. Indeed, in the transmission/reflection geometry, the interaction length is equal to the sample thickness (in the case of transmission, the thickness of metal films is up to 120 nm), which is several times greater than the skin layer thickness \(\delta_{\text{skin}}\) of metal (for gold, \(\delta_{\text{skin}}\approx 40\) nm). The grainy surface structure, inherent in an evaporated metal [33; 34], as well as the roughness and small defects on optically polished substrates, form an optically-inhomogeneous surface layer of the metal with a thickness comparable to \(\delta_{\text{s}}\)[35]. The optical constants of such a layer can differ significantly from the constants of the metal below it, composing the majority (in thickness) of the metal layer. Therefore, it is the metal under the transition layer that makes the main contribution to the transmission/reflection coefficient, and consequently, the optical constants obtained from the analysis of the transmission/reflection spectra are closer to the bulk metal constants, which are well described by the Drude model [27; 28]. As for the THz SPP refracometry method, the field of the surface wave generated by the probing radiation penetrates into the metal only to the depth \(\delta_{\rm m}\approx\delta_{\rm s}\)[36]. Therefore, the SPP characteristics are determined mainly by the optical properties of the transition layer of the metal. As a result, the effect of inhomogeneities and surface defects on the SPP characteristics exceeds that on the characteristics of the reflected probing radiation. In addition, taking into account the resonant enhancement of the field in the near-surface region during SPP-enhanced photon generation and the difference in the distribution of the fields of the SPPs and the reflected plane wave (the \(\delta_{\rm m}\) value is slightly less than \(\delta_{\rm s}\)) [11], we can assume that the collisional frequency \(\omega_{\rm\tau}\) of the conduction electrons for the SPPs exceeds this value for a plane wave of probing radiation incident on the metal. Therefore, in our opinion, it can be stated that the value of the effective permittivity \(\varepsilon_{\rm m}\) of the metal surface depends not only on the manufacturing and processing of the metal surface but also on the method of its determination. More precisely, it depends on the nature of the interaction of the probing radiation with the object of study. Due to the intensive mastering of the THz range and the creation of THz plasmonic devices in recent years, the development of the THz SPP refracometry method is of particular relevance.