Compact Terahertz Vortex Beam Source Based on Smith–Purcell Radiation and a Double‐Layer Metasurface

ABSTRACT

Smith–Purcell (SP) radiation, a frequency‐tunable radiation mechanism excited by moving electrons, is a promising route toward compact light sources operating from the terahertz to optical regimes. Vortex beams carrying orbital angular momentum (OAM) have helical phase fronts and are useful for high‐speed optical communication, microscopic particle manipulation, and high‐capacity information encoding. Direct generation of tunable vortex beams on an SP radiation platform therefore provides an attractive pathway for multifunctional integrated photonic devices. However, existing approaches based on conventional gratings or single‐function metasurfaces often suffer from low radiation intensity and difficulty in simultaneously achieving radiation enhancement, polarization control, and high‐quality vortex beam generation. Here, we propose and numerically validate a double‐layer C‐ring metasurface for compact terahertz vortex beam generation. Radiation enhancement is achieved through electric‐field‐strength superposition between the radiation generated by Layer I and the field contribution excited in Layer II by the electron‐induced evanescent field. The second layer further combines dynamic and geometric phase modulation to generate vortex beams with controlled OAM. The generation of vortex beams under different polarization states is also verified, indicating that the design is robust against variations in polarization excitation. This work provides a feasible design strategy for high‐performance, controllable SP vortex beam radiation sources.