Vortex Smith-Purcell Radiation based on Archimedean Spiral Grating

A free-electron-driven terahertz (THz) vortex generator based on Smith-Purcell radiation and Archimedean spiral grating (ASG) is proposed in this paper. The structural parameters are optimized to operate in the THz band. When the electron beam rotates around the singularity of the ASG constrained by the magnetic field, the vortex beam with the value of the topological charge of 1 can be generated above the grating. The vorticity of the vortex beam is determined by the rotation direction of the structure. For instance, if the vorticity of the ASG is anticlockwise, the topological charge of the vortex beam should be +1. The proposed scheme indicates its promising applications in developing compact on-chip THz vortex generators and the next-generation communication technology based on orbital angular momentum multiplexing.