Coupling free-electrons and whispering gallery modes

Free-electron beams in dedicated electron microscopes are an extremely functional probe for microstructure and composition [1], reaching a spatial resolution below an Angstrom. Optical control of the electrons in such microscopes facilitates ultrafast and ultrasensitive imaging and spectroscopy modalities. However, the weak coupling of electrons with photons is a limiting factor for emerging applications [2,3] of light-based electron control.

This talk demonstrates the merits of combining two scientific paradigms: electron microscopy and optical cavities. In particular I focus on whispering gallery mode (WGM) cavities for an efficient coupling to relativistic free-electrons [4,5]. I show how basic features of WGMs, such as light storage, modal population, and light coupling are expressed in the interaction with electrons. Importantly, an optimized arrangement of microresonators drives an unprecedented modulation of the electron beam, expressed as a broad and coherent electron-energy spectrum, including hundreds of sidebands [5]. In the future, the strong-coupling of electrons to resonant traveling modes can be used for fundamental electron-photon research, such as entangled electron-photon pairs, optical electron-phase manipulation, and generally, the merging of electrons into the realm of quantum optics. Furthermore, the combination of resonators with electron microscopy allows for dynamical imaging and spectroscopy with nanometer resolution and a temporal resolution down to the attosecond-scale.

1. Krivanek et al., Nature 464, 571–574 (2010)

2. Priebe, et al., Nature Photonics 11, 793–797 (2017).

3. Schwartz et al., Nature Methods 16, 1016–1020 (2019)

4. Kfir, Phys. Rev. Lett. 123, 103602 (2019)

5. Kfir et al., Nature 582, 46–49 (2020)