Location of objects beyond the horizon line by terahertz surface plasmons

We report on the first experiments on the over-the-horizon location of objects on the conducting surface by means of terahertz (THz) surface plasmon polaritons (SPPs) generated by free-electron laser radiation with the wavelength $\lambda =130 {\mu } \mathrm {m}$. The gold surface, guiding the SPPs, consisted of two flat faces 35 mm long each joined by a rounded edge with a radius of 10 mm. The intensity of SPPs reflected by the metal object placed on the surface at 3.5 cm beyond the horizon line depended on the height of the object in accordance with the distribution of the SPP field over the surface. The distance, orientation, coordinates and height of an object beyond the horizon line can be determined using the THz SPP locator.