Smith–Purcell radiation (SPR) is an important means of generating terahertz waves, and the enhancement of SPR is an attractive topic nowadays. Inspired by the phenomenon of special SPR, where the enhancement is achieved by using a high-duty-cycle grating, we describe a new, to the best of our knowledge, but more effective approach to this challenging problem. By deriving a simple analytical solution for the SPR from an annular electron beam passing through a cylindrical metallic grating, we show that the inverse structure, a low-duty-cycle grating can exhibit rather high SPR efficiencies in the presence of quasi-bound states in the continuum (quasi-BICs). The analytical prediction is supported by particle-in-cell simulations, which show that the quasi-BICs can enhance the superradiant SPR generated by a train of electron bunches by orders of magnitude. These results present an interesting mechanism for enhancing the SPR from metallic gratings, and may find applications in terahertz free-electron lasers.

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.

In this article, a method to enable efficient emission of coherent radiation by using an intense electron beam coupled with a quasi-bound state in the continuum (quasi-BIC) is investigated. We present an analytical solution providing an intuitive round-trip phase condition to explain the origin of quasi-BICs of dielectric gratings. Numerical study of the beam-wave interaction shows that the electrons can be bunched by the synchronous space harmonic enhanced by the quasi-BIC, resulting in self-excited coherent oscillation and consequently efficient Smith-Purcell radiation. This work presents an interesting solution for coherent radiation sources, and may find application in communications and physics.

We present a method utilizing the coupling between a pre-bunched electron beam and a silicon subwavelength grating to generate coherent terahertz waves. The grating that is connected to two opposite-traveling in-plane waveguides functions as a resonator. An example operating around 2 THz shows that, when the velocity and repetition frequency of the electron bunches respectively match the phase velocity and resonant frequency of the Bragg resonance in the grating, the strong electron-wave coupling leads to coherent radiation through the waveguide. The repetition frequency of the electron bunches can be halved by using its second harmonic to match the resonant frequency. This study might offer a potential approach for on-chip terahertz sources.

We systematically investigate the Smith–Purcell radiation emitted by electrons moving along a metal-plate array. Neglecting absorption loss, we show that Smith–Purcell radiation can be enhanced by leaky resonances near bound states in the continuum. When absorption loss is considered, by changing the period of metal plates so that the resonant frequency increases from microwave to terahertz, the amplitude of the resonance-enhanced Smith–Purcell radiation is found to decline in low frequencies ( < Privacy Policy and Terms of Service apply.