Broadband infinite-Q plasmons enable intense Smith–Purcell radiation

With the rapid development of nanophotonics for enhancing free-electron radiation,bound states in the continuum(BICs)have emerged as a promising approach for emitting intense Smith–Purcell radiation(SPR)with enhanced intensity.However,current BIC-based methods are limited to single-frequency operation,thereby restricting their applications requiring spectral and angular tunability,such as particle detectors and light sources.To overcome this limitation,this work proposes an approach for constructing plasmonic BICs over a broad spectral range in symmetry-broken systems.By leveraging the high-Q resonances near the BICs,we achieve intense SPR with broadband tunability,potentially improving the radiation intensity by six orders compared to traditional methods.Experimentally,we validate the construction of BIC using plasmonic antennas and achieve broadband demonstration.Our proposed concept can be extended to other plasmonic or guided-wave systems,paving the way toward compact and efficient free-electron sources in hard-to-reach frequency regimes.