Bimodal Vortex Smith-Purcell Radiation

A vortex Smith-Purcell radiation (VSPR) with twin operation points, namely bimodal VSPR, is proposed based on a depth-modulated triple-slots helical grating. Due to the Brillouin-zone folding phenomenon, the dispersion of the helical grating is folded twice in the momentum space. Thus, two spoof surface plasmon (SSP) modes carrying orbital angular momentum can be directly diffracted into the vortex beam. When a ring-shaped e beam flies over the surface of the helical grating, the two SSP modes can be effectively excited owing to the momentum matching. Accordingly, a bimodal vortex Smith-Purcell radiation is achieved when the operating frequencies are in the radiation region, which is validated by the probed intensities and field distributions over the grating. The proposed bimodal vortex Smith-Purcell radiation may facilitate the generation and applications of vortex beams in microwave and terahertz band.