Coupling Free Electrons and Whispering Gallery Modes

Free-electron beams in dedicated electron microscopes are an extremely functional probe for microstructure and composition [1]. Technological improvements in electron-beams control have repeatedly revolutionized the scientific reach of nanoscopic phenomena [2,3]. Light – the newest insertion into electron microscopes – creates novel ultrafast imaging modalities, facilitating direct observations of dynamics in phase transitions [4], phonons [5,6], and more. However, the weak coupling of electrons with photons is a limiting factor for emerging applications [7] of light-based electron control.

This talk presents a roadmap towards a strong coupling of electrons and light, using whispering gallery mode (WGM) microresonators [8,9]. I will start by discussing the important properties of these rotating modes for electron-light coupling. I describe the expected entanglement of electrons and photons, the statistical properties of the electron-photon states, and show that in the weak coupling regime they disentangle and reproduce known phenomena.  Experimentally, 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 a dramatic modulation of the electron beam, stretching it coherently over hundreds of electron volts within a sub-micron interaction length. In the future, the strong-coupling of electrons to resonant optical modes can give birth to research of entangled electron-photon pairs on one hand, and to phase-sensitive imaging capabilities of light-induced phenomena on the other hand.

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

2. Haider et al., Nature 392, 768–769 (1998).

3. Nobel Prize in Chemistry (2017)

4. van der Veen et al.," Nature Chem 5, 395–402 (2013).

5. Cremons, Plemmons, and Flannigan, Nat Commun 7, 1–8 (2016).

6. Feist et al., Structural Dynamics 5, 014302 (2018).

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

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

9. Kfir et al., arXiv:1910.09540 (2019).