Enhanced Terahertz Smith-Purcell Radiation with a Novel Grating

When a high-speed electron beam passes close to the surface of a mental grating, due to Smith-Purcell effect an electromagnetic radiation from the grating can be observed [1]. The Smith-Purcell radiation (SPR) arises from oscillating dipoles composed of the passing electrons and their image charges in the grating [2]. In general, if the grating period is designed in millimeter (mm) or sub-mm scale, the SPR generated will be in terahertz (THz) band. However, SPR is a type of incoherent radiation with weak radiation energy, which limits its application in developing THz sources. Herein, we propose a novel grating in Figure 1 to improve the coherence of THz SRP. Unlike the common rectangular grating where the electron beam can only move above it and induce image electrons along the plane, the novel annular grating enables the electron beam to move through its center and hence induce circularly distributed image electrons surrounding the beam. This special design can also enhance the interaction between the electron beam and the electromagnetic surface wave. Furthermore, we have conducted numerical simulations with the CST to compare the radiation energy distributions between the rectangular grating and the novel one. Apparently, in the rectangular grating case, all the fundamental, second and third harmonics in the THz band are observable over a wide range of observation angle, indicating an incoherent SRP. As to the novel grating case, the third harmonic is hardly recognized due to its lower energy, while both the fundamental and second harmonics are well bunched in a fairly small angle with higher energies, implying a coherent THz radiation. As a consequence, the novel annular grating is promising to enhance the THz SPR.