## 2 Materials and Methods  
### Experimental Setup  
To measure the characteristics of THz surface plasmon polaritons (SPPs), we developed and created a plasmon Michelson interferometer, the schematic diagram of which is shown in [37; 38], the results of the first testing are presented in [39], and a detailed description of the SPP interferometer, its technical characteristics and measurement technique are presented in the paper [40]. The source of radiation was a unique installation of the Budker Institute of Nuclear Physics SB RAS, the Novosibirsk free electron laser (NovoFEL) [41], which generates a periodic sequence of 100-ps pulses with a frequency of 5.6 MHz. The NovoFEL radiation was linearly polarized and fully coherent in the beam cross-section; the temporal coherence was 30 \(\div\) 100 ps (depending on the operating mode of the laser). The radiation wavelength was \(\lambda_{0}\) = 141 \(\upmu\)m, with a line width of less than 1%. The average radiation power at the input to the interferometer was 30 \(\div\) 40 W, and the diameter of the Gaussian beam was 12 mm. The use of the interferometer, similarly to the classical Michelson interferometer, relies on an analysis of the interferogram formed not by bulk waves but by collinear SPP beams directed by the surface under study. The real part of the SPP refractive index \(n_{\rm s}\) was determined from a comparison of the NovoFEL emission spectrum with the SPP spectrum resulting from the Fourier analysis of the interferogram. In the normal mode of NovoFEL operation, the emission spectrum is not stable enough: the wavelength shift can be of up to 0.2 \(\upmu\)m during measurements. For the variations in the laser radiation spectrum to be taken into account, the interferogram formed by the NovoFEL radiation beams was recorded simultaneously with the plasmon interferogram. For this purpose, a Michelson interferometer for bulk waves (BWs) [39] was added to the scheme of the plasmon interferometer (Figure 1). The \(p\)-component corresponding to the SPP polarization was extracted from the NovoFEL radiation beam entering the input of the installation (the lithographic polarizer \(P_{1}\) was used). The polypropylene film beam splitter \(BS_{1}\) split the linearly polarized radiation into two beams. The cylindrical mirror \(CL\) focused the transmitted beam on the edge of the flat substrate of the plasmon interferometer, where, due to diffraction, the radiation was converted into SPPs. The mirror \(M_{5}\) directed the reflected beam, the intensity of which was controlled by the polarizer \(P_{2}\), to the BW interferometer. In the plasmon interferometer, fixed (\(M_{1}\)) and movable (\(M_{2}\)) mirrors were used (40 \(\times\) 20 \(\times\) 5 mm\({}^{3}\) glass plates with gold coating on the reflecting faces), and a 40 \(\times\) 25 \(\times\) 1 mm\({}^{3}\) plane-parallel plate \(BS_{2}\) of Zeonex polyimide, at an angle of 45\({}^{\circ}\) to the incident SPP beam, served as an SPP beam splitter [42]. The lower faces of the mirrors and the beam splitter were optically polished and closely adhered to the substrate, providing optical contact with its surface. At the exit from the plasmon interferometer, the combined collinear SPP beams from both arms passed onto the convex surface of the cylindrical out-coupling element adjacent to the end of the substrate for the conversion of the SPPs into BWs. The element was a 1/8 of a cylinder with a curvature radius of 60 mm; its convex polished surface contained a 300 nm layer of gold coated with a ZnS layer 1 mm thick. Having reached the opposite edge of the convex face of the element, the resulting SPP beam diffracted on it and was converted into BWs, which were registered by the radiation detector.