Tunable vortex radiation source driven by free electrons

In this Letter, we propose a scheme to generate tunable vortex radiation through the interaction of free electrons with a dual-layer metasurface. The metasurface combines a field-enhancement layer with a phase-modulation layer to produce enhanced Smith–Purcell radiation (SPR) while enabling gradient phase control. This configuration efficiently converts the free-electron evanescent field into a coherent vortex beam. Simulation results indicate that the proposed structure generates coherent radiation with approximately 4.5 times the intensity of conventional vortex radiation under identical electron-beam conditions. The frequency and direction of the vortex radiation can be tuned by varying the electron velocity. Moreover, the structure enables coherent vortex radiation generation with a small number of electrons, including low-velocity electrons, without requiring strong focusing magnetic fields or periodic electron bunching. This work establishes an innovative approach for developing compact, efficient, and tunable vortex radiation sources based on free electrons.