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

Abstract

The enhancement of Smith–Purcell radiation (SPR) produced by electrons moving closely to a grating is a longstanding topic of interest. Here, we systematically investigate the resonant enhancement of SPR for planar metallic gratings. Using an analytic solution for the amplitude of SPR, we show that metallic gratings with a small dutycycle support two type of bound states in the continuum (BICs), i.e. symmetry-protected BICs and accidental BICs, both of which enable the SPR to be enhanced by orders of magnitude at the resonant frequency. The required electron energy for the excitation of BICs can be reduced by employing a higher-order diffraction wave for SPR. Our results present a mechanism for enhancing the SPR produced by metallic gratings, and may find applications in free-electron lasers.

Free-electron terahertz radiation based on silicon gratings with in-plane waveguide emission

“…Such phenomena can be explained by considering the effect of incident fields from electrons. For a 2D model, the current density of a single charge q moving with velocity v 0 can be given by J z (x, z, t) = qv 0 δ(x − g)δ(z − v 0 t) [45]. Using Fourier transform, we can obtain the current density in the frequency domain, i.e., J z (x, z, ω) = qδ(x − g)exp(−ik z0 z), with k z0 = ω/v 0 .…”

Improved Smith–Purcell free-electron laser based on quasi-bound states in the continuum

“…Other platforms supporting quasi-BICs, e.g. metal gratings [15,16], can also be applied as the electromagnetic structure. Additionally, recent advances in manipulating the characteristics of quasi-BICs can also be utilized here to tailor the beam-wave interaction (e.g.…”

“…1/2 are the reflection coefficient, z wavenumber and x wavenumber of the nth-order space harmonic, respectively [15,16]. Importantly, the absolute value of reflection coefficient, denoted by |r n |, is essentially the amplitude of the nth-order space harmonic at the grating surface.…”

“…The quasi-BIC can be found in the Smith-Purcell effect with a grating. The excitation of such resonances by electrons enables strong SPR at the resonant frequency in the spontaneous regime [13][14][15][16].…”