Smith-Purcell radiation (SPR) is the electromagnetic wave generated by free electrons passing above a diffraction grating, which has played an important role in free electron light sources and particle accelerators. Since SPR is derived from the scattering of the evanescent wave around the free electrons by grating, the characteristics of SPR, including radiation wavelength, intensity, and polarization, could be controlled by manipulating the free electrons and the structure interacting with electrons. In recent years, orbital angular momentum (OAM) has been known as a new degree of freedom of light, which shows promising application in increasing the bandwidth of optical communication, manipulating micro-particles, fabrication of nano-structures, and so on.

In order to generate SPR carrying OAM, Nano OptoElectronics Laboratory led by Prof. Yidong Huang, Department of Electronic Engineering, Tsinghua University, proposes an integratable method to flexibly generate "vortex Smith-Purcell radiation" by having periodically bunched free electrons pass on a holographic grating. The simulation results illustrate that coherent OAM wave with different topological charges and in different frequency regions, including ultraviolet, visible light, infrared, terahertz, and microwave region, could be derived by designing different holographic gratings. It is also found that the radiation angle of OAM wave could be tuned easily and the topological charge would be multiplied when high-order SPR is generated. Related research results are published in Photonics Research, Volume 8, No. 8, 2020 (M. Wang, et al., Vortex Smith-Purcell Radiation Generation with Holographic Grating).

The corresponding author of this paper, associate Prof. Fang Liu, believes that this work introduces a new way to manipulate the OAM of free-electron-induced radiation and to develop an integratable and tunable vortex wave source in different frequency regions.

Further work will focus on observing the vortex SPR experimentally and realizing the on-chip integrated vortex SPR emitter.

Generating "vortex Smith-Purcell radiation" by having periodically bunched free electrons pass on a holographic grating