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Controlling free electrons with optical whispering-gallery modes


Free-electron beams are versatile probes of microscopic structure and composition1,2, and have revolutionized atomic-scale imaging in several fields, from solid-state physics to structural biology3. Over the past decade, the manipulation and interaction of electrons with optical fields have enabled considerable progress in imaging methods4, near-field electron acceleration5,6, and four-dimensional microscopy techniques with high temporal and spatial resolution7. However, electron beams typically couple only weakly to optical excitations, and emerging applications in electron control and sensing8,9,10,11 require large enhancements using tailored fields and interactions. Here we couple a free-electron beam to a travelling-wave resonant cavity mode. The enhanced interaction with the optical whispering-gallery modes of dielectric microresonators induces a strong phase modulation on co-propagating electrons, which leads to a spectral broadening of 700 electronvolts, corresponding to the absorption and emission of hundreds of photons. By mapping the near-field interaction with ultrashort electron pulses in space and time, we trace the lifetime of the the microresonator following a femtosecond excitation and observe the spectral response of the cavity. The natural matching of free electrons to these quintessential optical modes could enable the application of integrated photonics technology in electron microscopy, with broad implications for attosecond structuring, probing quantum emitters and possible electron–light entanglement.

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Fig. 1: Experimental scheme.
Fig. 2: Electron spectral broadening induced by WGMs.
Fig. 3: Spectral and temporal properties of the interaction between free electrons and WGMs.

Data availability

The data supporting the findings of this study are available within the paper and at the Open Science Framework repository at Source data for Figs. 13 are provided with the paper.


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We acknowledge the joint effort of the UTEM team in Göttingen and especially the support of M. Möller, J. H. Gaida, T. Danz and T. Domröse. We thank J. Liu for productive discussions. O.K. gratefully acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 752533. T.R.H. acknowledges the support of a postdoctoral fellowship from the Alexander von Humboldt Foundation and its sponsor, the German Federal Ministry for Education and Research. This work was funded by the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center ‘Atomic Scale Control of Energy Conversion’ (DFG-SFB 1073, project A05) and the Priority Program ‘Quantum Dynamics in Tailored Intense Fields’ (DFG-SPP 1840).

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Authors and Affiliations



O.K. conceived the experiment. C.R. directed the study. O.K. and H.L.-M. conducted the experiment with contributions from A.F. and T.R.H.; O.K., H.L.-M., G.S. and M.S. prepared the samples. O.K. analysed the data with contributions from A.F.; O.K. and C.R. wrote the manuscript with contributions from T.J.K., H.L.-M., A.F. and M.S. and based on discussions with all authors.

Corresponding authors

Correspondence to Ofer Kfir or Claus Ropers.

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The authors declare no competing interests.

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Peer review information Nature thanks Nahid Talebi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Kfir, O., Lourenço-Martins, H., Storeck, G. et al. Controlling free electrons with optical whispering-gallery modes. Nature 582, 46–49 (2020).

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