Vortex Surface Plasmon Polaritons on a Cylindrical Waveguide: Generation, Propagation, and Diffraction

In this paper, we experimentally demonstrated excitation of terahertz vortex surface plasmon polaritons by end-fire coupling of radially-polarized annular beams with orbital angular momentum (the wavelength was 141 µm, and the topological charges were ±3 and ±9) to a 70 mm long, simply connected axis-symmetric transmission line and their propagation to the end of the line and diffraction into a free wave possessing the same topological charge as the input beam. The diameter of the line exceeded greatly the radiation wavelength, and, in contrast to experiments with nanowires, no azimuthal electromagnetic modes existed. We observed that 18 plasmons, locally excited on the input face perimeter by a wave with topological charge of ±9, traveled rotating over the tapering cylindrical line and transformed into 18 lobes of the decoupled free wave. The evidence of the possibility of the transformation of a vortex beam into vortex plasmons and back into vortex beam with the conservation of the topological charge opens a way for the development of plasmonic communication lines with coding and decoding of information at the free-wave stage. Application of this technique to complex beams consisting of a combination of different vortex modes can be a technological base for the development of multiplex plasmonic communication lines in the terahertz range. Since wave characteristics are easier to record than plasmon characteristics, sorting of free waves by the topological charges can be used for demultiplexing combined plasmons in multiplex systems.