Multi-frequency terahertz Smith–Purcell radiation via momentum-mismatch-driven quasi-bound states in the continuum

Bound states in the continuum (BICs) have gained considerable attention for their ability to strengthen light–matter interactions, enabling applications in lasing, sensing, and imaging. These properties also show great promise for intensifying free-electron radiation. Recently, researchers realized momentum-mismatch-driven quasi-BICs in compound grating waveguides. This category of quasi-BICs exhibits high Q factors over a broad frequency spectrum. In this paper, we explore the possibility of achieving multi-frequency terahertz Smith–Purcell radiation empowered by momentum-mismatch-driven quasi-BICs in silicon compound grating waveguides. By leveraging the low-loss properties of silicon in the terahertz range, quasi-BICs are achieved through guided-mode resonance, delivering exceptionally high Q factors over a broad frequency spectrum. The broadband nature of these quasi-BICs enables efficient energy extraction from electron beams across varying voltages, while their multimode characteristics support simultaneous interactions with multiple modes, further boosting radiation intensity. The findings demonstrate significant enhancement of free-electron radiation at multiple frequencies, addressing the limitations of narrowband methods and high-loss metallic systems. By integrating broadband performance with the advantages of low-loss dielectric platforms, this work advances the development of compact, tunable terahertz free-electron radiation sources and provides valuable insights into optimizing quasi-BIC systems for practical applications.