Harnessing modulated electrons to probe light-matter strong coupling

Due to recent advances in the control of the quantum properties of collimated free-electron beams, these appear to be one of the most promising probes for quantum matter at the nanoscale. In this work, we provide a model Hamiltonian describing the quantum interaction between a modulated electron wave-packet and a hybrid photonic-excitonic system comprising a quantum emitter and an optical nanocavity. This Hamiltonian is constructed using macroscopic QED ideas and fully parameterized in terms of the electromagnetic Dyadic Green's function. We explore the Jaynes-Cummings polariton ladder of the cavity-emitter system through both the free-electron and photon spectra and demonstrate the power of modulated electrons as near-field probes of light-matter interactions in the strong-coupling regime.