Abstract
A free-electron-driven terahertz (THz) source based on coherent Smith-Purcell radiation (SPR) assisted by bound state in the continuum (BIC) is proposed in this paper. A reflection-type quasi-BIC with ultrahigh quality factor is formed by continuously tuning the structural parameter of the compound grating, which results from the wavevectors mismatch between the incident plane wave and the guided mode. When a sheet electron beam flies over the surface of the grating, the SPR will be enhanced at the resonant frequency of the quasi-BIC and become coherent accordingly. The forming process of the BIC through parameter tuning is analyzed, and the frequency spectrum of the coherent SPR wave demonstrates the enhancement effect of the quasi-BIC. The proposed quasi-BIC-assisted coherent THz SPR scheme may bring possibilities to the design and applications based on compact THz sources.