Free-electron driven light-matter interactions

Impact statement

The interaction of free electrons with light and matter has historically given rise to fundamental radiation mechanisms—such as Cherenkov radiation, transition radiation, Smith–Purcell radiation, and bremsstrahlung—as well as powerful sources including free-electron lasers, with far-reaching applications across physics, materials science, and biology. A defining feature of free-electron–based radiation is its exceptional spectral tunability, spanning from microwaves to hard X-rays, combined with extremely high brightness.

A central theme of this research is the development of new compact light sources and accelerator concepts based on structured light–electron interactions. By treating the free electron as a genuine quantum-optical object, on equal footing with photons and other quantum emitters, we seek to access new regimes of electron–photon coupling. Our goal is to translate concepts traditionally associated with large-scale accelerators to the nanoscale, by exploiting the evanescent electromagnetic near field carried by electrons even at comparatively low energies. When electrons interact with tailored nanophotonic structures—such as periodic media supporting well-defined optical modes—this near field can be efficiently converted into propagating radiation, enabling highly controllable, on-chip free-electron light sources. More broadly, this approach opens the door to new forms of ultrafast, nanoscale spectroscopy and imaging in which electron beams act as versatile probes of matter.

Research questions

• How efficient can free-electron radiation processes such as Smith–Purcell emission be in dielectric, semiconductor, or hybrid nanophotonic structures, and what design principles enable optimal performance?
• Can low-energy electrons be used to realize compact, tunable light sources that are directly integrable with photonic circuits?
• How can the polarization, directionality, spectral content, and coherence properties of free-electron radiation be precisely engineered through nanophotonic mode design?
• Is it possible to reach collective or superradiant emission regimes using electron beams generated by tabletop instruments such as scanning electron microscopes?
• What are the ultimate frequency limits accessible in nanoscale free-electron radiation, and what experimental challenges arise in terms of in-vacuum integration, detection, and spectroscopy?
• Under what conditions do quantum effects and strong electron–photon coupling become significant, and can signatures such as vacuum Rabi splitting or coherent oscillations be observed?
• How can these concepts be leveraged to develop novel electron microscopy modalities that integrate laser excitation and readout, microwave probes, and quantum sensing techniques for enhanced contrast, functionality, and information content?