Abstract
Recent advances in vortex Smith-Purcell radiation (V-SPR) have spurred many breakthroughs in the generation of vortex beams for their potential applications in chiral detection, particle acceleration, communication, and imaging. However, advanced multi-mode applications of the V-SPR suffer from the incoherent spectrum and poor purity in producing high-order topological charges. Here, by exploiting the rotational symmetry and Brillouin-folding phenomenon, it is proved that the coherent V-SPR with independently steerable high-order topological charge is directly emitted from chiral spoof surface plasmons. Moreover, both topological charges and operation frequencies can be readily tuned, thus enabling flexible adaptability over a broad bandwidth. A proof-of-concept prototype is designed and fabricated in the microwave regime, in which the method is demonstrated from the perspective of symmetry and tunability. The proposed methodology can be exploited as a platform to investigate the interaction between chiral plasmons and swift electrons, promising a feasible scheme for generating coherent and tunable vortex beams from the microwave to the terahertz band.