Electron-beam induced terahertz radiation from graded metallic grating.

We have numerically analyzed, based on a simplified particle-in-cell finite-difference time-domain (PIC-FDTD) method, an electron-beam (e-beam) induced terahertz (THz) radiation from metallic grating structures with graded depths (graded grating). Upon exciting with e-beam, directional THz radiations with wide-band spectrum containing several sharp peaks are obtained only from the one of the edge of the grating, which cannot be expected from the conventional theory of Smith-Purcell radiation. It was clarified that each modes originate from different locations on the graded grating reflecting different dispersion characteristics of spoof surface plasmon polariton (spoof SPP) at each locations, and they can propagate toward only the shallower groove as a surface wave due to the cutoff at each locations, and all of these modes eventually emitted from the one of the edge of the graded grating. These directional radiations can be directed toward either backward or forward by making the groove depth deeper or shallower. The lowest and the highest frequency of the radiation can be chosen by appropriately designing the deepest and the shallowest groove depths, respectively. These unique radiations cannot be obtained from the uniformly grooved grating. Our findings may open the way for a development of novel THz radiation source based on the spoof SPP on the wide variety of metallic grating structures or metasurfaces.