Due to the short de Broglie wavelength and strong interaction with matter, free electrons are widely used to probe minute physical structures and local excitations. In particular, electron microscopes enable imaging, diffraction, and spectroscopy at an ultra-high spatial resolution, having revolutionized material science and structural biology. The interaction of free electrons with light, a fundamental and special type of light-matter interaction, has been exploited in exquisite control and measurement schemes for electron beams. The ultrafast transmission electron microscope (UTEM) provides time-resolved electron microscopy with femtosecond temporal resolution by using an electron-pump-optical-probe technique in a transmission electron microscope (TEM). In particular, photon-induced near-field electron microscopy in UTEM has been used to investigate electron-light interactions with a high spatiotemporal resolution, leading to the demonstration of quantum-coherent control of free electrons, attosecond electron pulse trains, and hyperspectral imaging of nanophotonic structures.
Recently, there has been growing interest in investigating electron-light interactions mediated by dielectric photonic cavities. The project “Strong electron-photon interactions with high-Q microresonators” aimed at exploiting strong electron-photon interactions with photonic chip-based high-Q optical microresonators. In this project, we have demonstrated electron-photon interactions based on the combination of electron microscopy and integrated photonics. The cavity enhancement and electron-photon phase matching lead to an enhanced interaction strength, while photonic integrated circuits provide flexibility and compactness to tailor the interaction. This breakthrough has enabled continuous-beam electron phase modulation with continuous-wave optical fields, the generation of correlated electron-photon pairs, and free electron interaction with nonlinear optical states.
