Cherenkov Radiation from Photonic Bound States in the Continuum: Towards Compact Free-Electron Lasers

In conventional materials, Cherenkov radiation (CR) due to a moving charged particle is associated with a broad frequency range and velocity threshold. Here, we show that using a periodic grating structure, which possesses photonic bound states in the continuum (BICs) and quasi-BICs, unusual CR can be generated in a very narrow frequency band at a particle velocity below the common threshold. This effect arises from an interesting light-amplification process in the BIC structure, which can dramatically improve the efficiencies of evanescent-to-propagating wave conversion and CR generation. Our results present a mechanism for realizing Cherenkov lasing at low electron velocity and may find applications in compact free-electron lasers.