Smith-Purcell radiation from helical grating to generate wideband vortex beams

A broadband vortex beam generator provides a promising solution for various applications. Due to its azimuthal phase perturbation, the helical grating is an ideal structure to support the orbital angular momentum (OAM) mode. Since the space-charge wave of the free electron bunch inherently covers a wide frequency range, the free-electron-driven devices can be utilized to generate broadband radiation. This work presents a wideband tunable multi-mode vortex beam generatorbased on the Smith-Purcell radiation (SPR) from a helical grating. The generated broadband vortex radiation presents prominent flexibility in the manipulation of frequency, topological charge, and radiation direction. The frequency range and the topological charge depend on the operating harmonic order, operating voltage, and structural parameters. The radiation direction varies with the operating frequency and covers a wide range in each band. Moreover, the proposed principle can be scaled to the terahertz frequency band. This work advances the application of vortex beam in the millimeter wave-terahertz communication system.