Probing Strong Coupling in Core–Shell Nanoparticles With Fast Electron Beams

Collective optical excitations, such as localized surface plasmons in metallic nanoparticles and Mie resonances in high‐index dielectrics, play a central role in nanoscale light–matter interactions. When interacting with electronic transitions in matter, they can couple strongly, like coupled harmonic oscillators, forming hybrid light–matter states. Here, this coupling is probed with fast electrons in cathodoluminescence (CL) and electron energy‐loss (EEL) spectroscopy. Owing to their highly localized fields, fast electrons can excite modes inaccessible to light‐based spectroscopies, including higher‐order nonradiative modes, which offer greater field confinement and potentially stronger coupling. We develop an analytical framework to calculate EEL and CL probabilities for core–shell nanospheres under aloof and penetrating electron trajectories, and apply it to two representative systems: an excitonic core with metallic shell, and a silicon core with excitonic shell. We examine how the electron beam position and velocity affect the ability to probe this coupling. Depending on electron beam parameters, we find that the spectral signature of strong coupling remains robust in plasmonic nanospheres, but can be significantly suppressed or even completely obscured in dielectric nanospheres. Our formalism enables a deeper understanding of the coupling mechanisms in electron–light–matter interactions, thereby accelerating progress in single‐nanoparticle‐based polaritonic studies.