Electron-beam induced terahertz radiation from graded metallic grating

Abstract

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.

Electron Beam-Induced Directional Terahertz Radiation from Metamaterials

“…The PIC-FDTD method has been widely used to study underlying physical mechanism of the Smith-Purcell superradiance [8][9][10]. To save computational time and memory, we have used simplified version of PIC-FDTD method [17,18]. In our simplified model, the electron-bunch is treated as one negatively charged particle, the movement of the particle is restricted only in two-dimensional (2D) (x-y) plane, and only transverse electric (TE) mode, with E x , E y , and H z fields, has been analyzed.…”

“…A 20-μm-wide (w) bunched electron-bunch with Gaussian charge distribution (Eqs. (16) and (17)) was sent 20 μm( w) above the grating at the relativistic speed. The maximum current density at the center of the electron-bunch was assumed to be 1.0 Â 10 6 A/m 2 , and the acceleration energy of the electron-bunch is 30 keV, which is comparable to the recent experimental condition by Urata et al [5].…”

“…Here, we show an e-beam-induced THz radiation from such graded grating based on PIC-FDTD method [3,17]. We have obtained THz radiation with unique characteristics such as arbitrarily chosen bandwidth and unique directionality, which cannot be expected from the conventional theory developed for the SPR.…”

Enhancing spoof surface-plasmons with gradient metasurfaces

“…Moreover, based on this effect, a Cherenkov radiation source operating in the visible or ultraviolet frequency range was proposed 10 . Recently, with the structured metal surface, the generation of spoof surface-plasmons (SSP) with free electron bunches has also been studied 11 12 , yielding an effective surface mode mimicking the SPP waves.…”

“…Secondly, in the tangent or propagation direction, the electric field of SPP mode is very weak, especially in the low frequency such as the THz or microwave regime. Thirdly and most importantly, the electric-field force will change the velocity of electrons, especially in the strong coupling regime (this effect may be neglected in the weak coupling 10 11 12 ). Thus, a significant velocity mismatch between the electrons and SPP waves (or relative motion of electrons with respect to the SPP field) can be resulted.…”