Intrinsic chiral optical vortex emission from the interactions between free electrons with bound states in the continuum

We present an approach for free-electron-driven optical vortex emission through interaction with the quasi-bound states in the continuum (quasi-BICs). Based on a unified framework for free electron-nanophotonic interaction and time domain simulations, we show that the free electron beam can efficiently excite the quasi-BIC under the phase-matching condition. The intrinsic k-space polarization anisotropy associated with the quasi-BIC imprints a geometric phase onto the emission, and thus generates an intrinsic optical vortex. We then reveal a link between the topological charge of the quasi-BIC and the emitted optical vortex by analyzing the far-field polarization structure and phase distribution. This mechanism offers a promising approach to creating nanoscale electron-beam-driven structured light sources without the need for complex chiral nanostructures, thereby advancing the interdisciplinary study of free-electron optics and nanophotonics.