Enhanced Smith-Purcell radiation based on quasibound states in the continuum in dimers aligned in a chain

In this work we theoretically investigate the radiation from electrons passing along a periodic chain of dimers-the particles consisting of two coupled subwavelength parts. High-quality resonances can be excited in dimers due to interaction between the constituent particles, making quasibound states in the continuum. We show that at the resonant frequencies, the intensity of Smith-Purcell radiation increases sharply. Although numerical analysis is performed for the dimers consisting of two equal-coupled particles, the theory constructed is valid for more general binary objects consisting of two different subwavelength-coupled particles of arbitrary form and dielectric properties, and we have formulated the analytical conditions describing this effect. We prove that there is a wide range of the parameters for which Smith-Purcell radiation can be boosted by up to two orders of magnitude. Apparently, the effect can be even stronger with an appropriate choice of parameters. A resonant mechanism of enhancement of Smith-Purcell radiation paves the way to the novel and very effective radiation source based on metasurfaces made of resonant elements.