UDC 535.15: 621.391: 535.016 Interference Refractometry of Terahertz Surface Plasmon-Polaritons Launched by a Free-Electron Laser V. V. Gerasimov∗, B. A. Knyazev†, A. K. Nikitin‡, V. V. Nikitin‡, T. A. Rijova§ ∗ Budker Institute of Nuclear Physics SB RAS Lavrentiev av. 11, Novosibirsk, Russia † Novosibirsk State University Pirogova str. 2, 630090 Novosibirsk, Russia ‡ Scientific and Technological Center for Unique Instrumentation of RAS Butlerova str., 15, 117342 Moscow, Russia § General Physics Department People’s Friendship University of Russia Miklukho-Maklaya str., 6, 117198 Moscow, Russia The problem of terahertz (THz) surface plasmon-polaritons (SPP) refractometry, i.e. determination of their complex refractive index κ = κ′ +i · κ′′ employing interferometric measurements, is considered in the paper. <...> It is stated that one can determine both parts of κ provided the interference pattern formed by a reference bulk wave and the wave produced by the SPP is recorded. <...> The idea was tested for SPP generated by monochromatic radiation (wavelength 140 µm) of Novosibirsk THz free-electron laser on gold samples covered with different thickness ZnS layers. <...> Besides, intensity distribution of the SPP field in air over the track has been registered instantly with an uncooled vanadium oxide microbolometer focal plane array consisting of 320Ч240 sensitive elements. <...> The results obtained are in good agreement with the theory provided the covering layer thickness is equal or exceeds 2 µm. <...> Key words and phrases: terahertz radiation, surface plasmon-polaritons, refractometry, surface electromagnetic waves, free-electron laser. 1. <...> Introduction Surface plasmon-polaritons (SPP) corresponding a complex of p-polarized electromagnetic wave and a wave of free charges induced by the former onto a conducting surface and propagating along the conductor-dielectric interface are widely used in optics for surface control [1]. <...> Having determined the complex refractive index κ = κ′ +i · κ′′ of the SPP, one can calculate two unknown parameters of the layer or optical constants of the surface. <...> By analogy with optics of bulk materials the division of SPP science dealing with determination of κ may be called SPP refractometry. <...> This concept is well developed for the visible and middle infrared (IR) spectral ranges [2], but not for the far IR range <...>