Validating the First-Order Topological Charge of Vortex Smith-Purcell Radiation

The vortex Smith-Purcell radiation with topological charge (TC) of 0 and −2 is proposed based on spoof surface plasmons (SSPs) carrying orbital angular momentums (OAMs) in this work. Due to the chirality of the helical grating, their eigenmodes are a series of SSPs carrying OAMs concerning the harmonic order. By customizing the slot depths over two propagation periods, these SSPs are exposed to the light cone. When the swift electrons skim over the grating and the wavenumber-matching conditions are met between the electron and SSPs carry ±1ℏ, the vortex beams with TCs = 0 and −2 are emitted into the free space with broadband tunability. The proposed method and validation may prompt the development of mode-division multiplexed communications and chiral detections based on vortex beams.