## 1 Introduction  
The value of the dielectric permittivity of the surface of metal and the metallized elements of optical circuits may differ significantly from that calculated by the Drude model, especially in the infrared (IR) and terahertz (THz) ranges, where the reflectivity of noble metals tends to 100% and the anomalous skin effect manifests itself in a weak form [1, 2]. In addition, in the THz range, the reflectivity of a metal surface (directly related to its dielectric constant) strongly depends on the quality and preparation of the mirror surface [3, 4, 5]. Therefore, in order to model correctly the operation of optical systems, it is necessary to use a measured value of the dielectric constant of the given reflective surface of metal (metallized) elements, not reference data. For frequencies in the visible and near-IR ranges, a number of methods are known (reflectometry, polarimetry, conductometry, ellipsometry, Fourier spectroscopy, and calorimetry) for the determination of the dielectric permittivity of thin-layer and bulk conducting samples [6; 7]. However, since a weakly pronounced anomalous skin effect occurs in metals at THz frequencies [8], their dielectric permittivity is no longer adequately described by the Drude model, and the results of its calculation via the above methods and Kramers-Kronig relations are of low reliability and in poor agreement [2; 9]. For example, the results of measurements of reflectivity coincide with calculations based on conductometric measurements and Kramers-Kronig transformations only for very pure metals with minimal roughness and without oxide layers [3]. The waveguide method, in which a thin layer of the metal under study is deposited onto a plate dielectric waveguide, has proved to be more reliable in the far IR range [10]. However, with this method, the radiation interacts with the metal surface (within the skin layer, whose depth at THz frequencies does not exceed 100 nm [4]) adjacent to the plate, not bordering with the environment. New possibilities for the refracometry of metals have opened up with the proposal to use surface plasmon polaritons (SPPs) for this purpose. SPPs are a kind of surface electromagnetic waves, which exist at the interface between media, one of which (metal) contains conduction electrons and has a negative dielectric permittivity [11]. The SPP characteristics (field distribution, attenuation coefficient, and phase velocity) are determined mainly by the optical constants of the metal and the ratio of the SPP frequency \(\omega\) to the plasma frequency \(\omega_{\rm p}\) of the metal: the greater their difference (in the THz range, the \(\omega_{\rm p}\) is about three orders of magnitude greater than the \(\omega\)), the lower the SPP decay and the closer the SPP phase velocity to the velocity of the corresponding plane wave in the medium adjoining the metal. This makes THz SPPs promising for use in high-speed lines for data transmission and processing [12; 13; 14]. With the discovery of the ability of a structured (by holes, relief, or other ordered inhomogeneities) metal surface to direct THz SPPs with high attenuation and high field concentration in the near-surface region, it became possible to create both active and passive microdevices in which information is carried by SPPs [15; 16; 17]. First, due to the attenuated total reflection (ATR) phenomenon, SPPs were applied to the determination of the optical constants of metals with high conductivity in the visible range, both transparent metal layers [18; 19; 20] and bulk samples [21]. Later on, a similar technique was tested in the mid-IR range for a transparent layer of palladium (a transition metal with a reflectivity of about 90%) [22].