Monochromatic Unidirectional Electron-Driven Photon Source

High-energy electron beams generate coherent radiation when approaching nanostructures. This phenomenon gives rise to electron-driven photon sources (EDPHS), which exhibit significant potential for nanophotonic applications, including on-chip integrated light sources and ultrafast spectral interferometry in electron microscopy. These applications necessitate meticulous manipulations of directionality and wavelength at the nanoscale. In this work, we achieve a monochromatic unidirectional EDPHS through selective excitations of toroidal dipole modes. The unique electromagnetic mode possesses distinctive symmetry properties and low radiative losses, enabling the realization of highly directional, linearly polarized emission with a quality factor (Q-factor) of 35 and degree of linear polarization (DOLP) reaching 0.999. The strategy also allows tunability in both wavelength and emission direction. Besides serving as nanoscale light sources, their exceptional monochromaticity and polarization purity make them particularly suitable as coherent reference sources in electron microscopy, facilitating ultrafast spectral interferometry and phase imaging to reconstruct nanoscale dynamics with temporal resolution.