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 polaritons (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.