Upper bound of efficiency for Smith-Purcell emission and evanescent-to-propagating wave conversion in metal-groove metasurfaces

Abstract

When a charged particle moves parallel and close to the surface of a metasurface, intense Smith-Purcell radiation can be observed at resonant frequencies. Here, we present a systematic investigation on the Smith-Purcell radiation and evanescent-to-propagating wave conversion in metal-groove metasurfaces. Based on a coupled mode theory, analytic formulas are derived for the resonant frequency, Q-factor, and wave conversion efficiency at resonant frequency. The accuracy of the formulas is verified by numerical simulations. It is found that the resonant frequency and Q-factor depend on the depth and filling ratio of the grooves, respectively. A high Q-factor can be obtained by decreasing the filling ratio of the grooves. As the Q-factor increases, the wave conversion efficiency at resonant frequency increase but exhibits an upper limit. Such an upper bound of efficiency (Cr,max = 4) can be approached at a moderate Q-factor (Q = 16) or an optimal filling ratio of the grooves (fs = 0.05). Our results may benefit the construction of compact high-power free-electron light sources.

Enhanced Smith–Purcell radiation from bound states in the continuum of metallic gratings

“…It is noticeable that the dispersion relation can also be easily derived using the above formulation. In the absence of incident field, equations ( 27) and (28) become…”

“…To date, most studies of SPR used metallic structures. Previous studies have shown that for a metallic grating, the SPR can be enhanced by the resonance effect of the TEM waveguide mode in the presence of a small groove width [26][27][28]. The enhanced SPR, which is called special SPR (S-SPR), has a narrow spectrum and radiates at specific directions.…”

Ultra-monochromatic far-infrared Cherenkov diffraction radiation in a super-radiant regime

“…Tamm and Frank have developed a theoretical approach to describe the classical ChR characteristics 14 . Recently more advanced ChR mechanisms have been considered for intense radiation generation including excitation of structural materials such as metamaterials 15 , gratings 8 , 9 , 16 , or photonic crystals 17 , 18 .…”

“…Those currents give rise to an entire family of polarization radiation mechanisms that have been widely applied for both charged particle beam diagnostics and the generation of intense EM radiation beams, e.g. transition radiation 2,3 , diffraction radiation 4,5 , Smith-Purcell radiation [6][7][8][9] , and parametric X-ray radiation 10 . More recently it has been proposed to use plasmons generated by the field of passing particle in a thin conducting material to reinforce polarization radiation photon yield at well-defined frequencies 11,12 .…”