## 1 Introduction
However, for noble metals, because of their high conductivity, this technique is not applicable, since the propagation length \(L_{\rm s}\) (the distance at which the wave intensity decreases by a factor of \(e\approx 2.718\)) of infrared SPPs reaches several centimeters, which leads to the distortion of the reflected beam due to the re-emission of SPPs into the ATR prism. A fundamentally new method for the SPP refracometry of metal surfaces in the IR range was proposed in [23]. The method includes the measurement of the propagation length of monochromatic SPPs over a bare metal surface (\(L_{0}\)) and the same surface containing a thin non-absorbing film (\(L\)) of known thickness \(d\) and refractive index \(n_{\rm layer}\) and then the calculation of the optical constants of the metal using the \(L_{0}\), \(L\), \(d\), and \(n_{\rm layer}\) values and the radiation wavelength \(\lambda_{0}\) via simultaneous solution of the SPP dispersion equations for two- and three-layer structures [11]. However, the reliability of the results obtained via such a technique is low, since it implicitly assumes (in the derivation of the dispersion equations) that the observed attenuation of the SPPs is due solely to the Joule losses in the metal, while in reality it also involves other significant mechanisms of dissipation of the SPP energy: scattering on the inhomogeneities and surface roughness, scattering on the granules of the near-surface layer of the metal, and heat losses due to the skin effect. In the THz range, it was proposed to determine the dielectric permittivity of the surface of non-transparent metal objects from measured SPP characteristics such as the propagation length and depth of the field penetration into the environment adjacent to the metal [24]. However, this technique also does not take into account the influence of the SPP energy dissipation on the inhomogeneities of a real surface on both measured quantities, which reduces the reliability of the results. New prospects for the refracometry of metals seemed to appear in connection with the development in the early 1990s of the THz time-domain spectroscopy (THz TDS) [25; 26]. A significant advantage of the TDS method in comparison with those mentioned above lies in the measurement of not the power but rather the amplitude and phase of the probing THz radiation in the form of a picosecond pulse. After interaction with the sample, the broadband THz radiation is strobed by femtosecond light pulses (for the triggering of the THz radiation photodetector), which yields the time dependence of the photocurrent. The full inverse Fourier transform of this dependence makes it possible to obtain both the amplitude and phase spectrum of the reflected (transmitted) radiation in the entire THz range. The method has been tested for the THz refracometry in a transmission/reflection geometry of transparent metal layers (up to 120 nm thick) on terahertz spectrometers with a wide dynamic range and with the use of reference dielectric plates [27; 28]. Along with the advantages, which are impressive at first glance, the TDS method has a number of disadvantages: (1) the impossibility of studying metal samples with roughness, at which the reflected radiation is scattered; (2) the strict requirements for the parallelism of the faces of the substrates; and (3) the long measurement time because of the need to average the measurement results over a large number of pulses because of the stringent requirements for the synchronism and identity of THz and light pulses [29]. There were attempts to combine the possibilities of the SPP refracometry and TDS spectroscopy for studying the surfaces of metals and semiconductors in the THz range [30; 31; 32]. A THz radiation pulse was converted into a pulse of broadband SPPs, and after they traveled a macroscopic distance over the sample, the SPPs were inversely converted into bulk radiation, which was detected, and the resulting photocurrent dependence on time was processed by the standard TDS method.