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Description
Localized surface plasmons (LSPs) can confine light to sub-wavelength dimensions, which is a key concept in the fast-growing field of nanophotonics. Thanks to their sub-nanometer spatial resolution, transmission electron microscopes (TEMs) are powerful and well-established tools for studying nanophotonic samples, e.g. by probing the energy of LSPs using electron energy-loss spectroscopy (EELS). It has been experimentally shown that the change of the transversal momentum of the electron can be measured as well and provides us with information about the vector structure of the induced near-fields.[1] The radiative decay of LSPs leads to coherent emission of cathodoluminescence (CL) photons.[2] Recent rapid technological improvement of TEMs, especially the development of the Timepix detector, has opened the way for electron–photon coincidence experiments, allowing us to reach into the quantum properties of the system through correlations on the single-particle level. This has been already used to demonstrate the entanglement between free-space electrons and a different type of coherent CL emission, transition radiation.[3,4] However, to the best of our knowledge, electron–photon coincidence measurement has never been done for an electron probing LSPs. This type of measurement is necessary to understand the behavior of the system from the perspective of conservation of energy, momentum, and angular momentum, and could pave the way towards new quantum methods for probing optical excitations in general.
Herein, we present simulations of momentum-resolved EELS and momentum-resolved CL for a swift electron interacting with LSPs, as well as preliminary results from experiments carried out at a custom-modified FEI Tecnai G20.[4] For the numerical simulations, we used a modified version of Matlab toolbox MNPBEM [5] to calculate surface charges and surface currents excited in a metallic nanoantenna by an electron beam. By processing the induced electromagnetic response using quantum and classical formalisms within the quasistatic and retarded frameworks, we obtained maps of the energy-loss probability and the transversal momentum-transfer probability of the electron and a map of the probability of the photon being emitted into a particular direction. We discuss the suitability of the used approaches for specific experimental scenarios and confirm that the phenomena should be experimentally observable, which moves us towards an understanding of electron–photon momentum transfer at the fundamental level.
References
- KREHL, J. et al. Spectral field mapping in plasmonic nanostructures with nanometer resolution. Nature Communications. 2018, vol. 9, no. 1, p. 4207. Available from DOI: 10.1038/s41467-018-06572-9.
- GARCÍA DE ABAJO, F. J. Optical excitations in electron microscopy. Reviews of Modern Physics. 2010, vol. 82, no. 1, pp. 209–275. Available from DOI: 10.1103/RevModPhys.82.209.
- HENKE, J.-W. et al. Observation of quantum entanglement between free electrons and photons. arXiv preprint. 2025. Available from DOI: 10.48550/arXiv.2504.13047.
- PREIMESBERGER, A. et al. Experimental verification of electron-photon entanglement. arXiv preprint. 2025. Available from DOI: 10.48550/arXiv.2504.13163.
- HOHENESTER, U. et al. MNPBEM–A Matlab toolbox for the simulation of plasmonic nanoparticles. Computer Physics Communications. 2012, vol. 183, no. 2, pp. 370–381. Available from DOI: 10.1016/j.cpc.2011.09.009.