Surface plasmon polaritons launched using a terahertz free-electron laser: propagation along a gold–ZnS–air interface and decoupling to free waves at the surface edge

In this paper, we experimentally study the propagation of surface plasmon polaritons (SPPs) along gold–ZnS–air interfaces and their diffraction at the surface edge. The SPPs were launched by the waveguide method using monochromatic radiation of the Novosibirsk Free-Electron Laser, operated at the wavelength 140 μm. SPP characteristics were studied via examination of the electromagnetic field of diffracted waves employing two terahertz sensors: a movable Golay cell and an optical system, consisting of a TPX lens and a 320×240 microbolometer focal plane array (MBFPA) recording images with a rate of 17  frames/s. The experimentally recorded intensity distribution of the diffracted wave in the direction normal to the surface differed from the evanescence wave distribution in the SPP, but their characteristic widths were practically the same and coincided with the theoretical calculations made within the Drude model [Phys. Rev. A87, 023828 (2013)]. Diffracted wave characteristics drastically changed when ZnS-layer thickness increased from 0 to 0.75 μm. The angular distributions grew from 0.16 to 3.6 deg, but the characteristic beam width decreased from 8 to 0.35 mm, which is promising for a number of applications. The propagation length of SPPs therewith decreased from 31 to 11 mm, which is 3 orders of magnitude less than the Drude theory predicts. Prospects for further studies are discussed.