Interference Refractometry of Terahertz Surface Plasmon-Polaritons Launched by a Free-Electron Laser

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 320×240 sensitive elements. The results obtained are in good agreement with the theory provided the covering layer thickness is equal or exceeds 2 μm.

terahertz radiation, surface plasmon-polaritons, refractometry, surface electromagnetic waves, free-electron laser