Feasibility of generating surface plasmon polaritons with a given orbital momentum on cylindrical waveguides using diffractive optical elements

Three optical systems employing diffractive optical elements to generate surface plasmon po-laritons (SPP) with orbital angular momentum on axisymmetric conductors are considered. In all three systems, the incident radiation is first converted by binary spiral phase axicons into a set of plane waves converging to the optical axis. In the zone of intersection of these waves, a "twisted" Bessel beam is formed. By fitting the diameter of the first ring of the Bessel beam to the diameter of the cylindrical conductor, it is possible to generate a rotating SPP by the "end-fire coupling" method. The use of an additional lens makes it possible to convert the SPP-exciting Bessel beam into a vortex annular beam whose diameter is independent of the topological charge of the beam. In the third scheme, converging plane waves are “intercepted” by a cylindrical metal diffraction grating, which forms twisted SPPs on a cylindrical conductor connected to the grating. Examples of the possible use of the proposed systems in experiments on a terahertz free electron laser are presented.