Probing quantum optical excitations with fast electrons

Probing optical excitations with nanometer resolution is important for understanding their dynamics and interactions down to the atomic scale. Electron microscopes currently offer the unparalleled ability of rendering spatially resolved electron spectra with combined meV and sub-nm resolution, while the use of ultrafast optical pulses enables fs temporal resolution and exposure of the electrons to ultraintense confined optical fields. Here, we theoretically investigate fundamental aspects of the interaction of fast electrons with localized optical modes that are made possible by these advances. We use a quantum optics description of the optical field to predict that the resulting electron spectra strongly depend on the statistics of the sample excitations (bosonic or fermionic) and their population (Fock, coherent, or thermal), whose autocorrelation functions are directly retrieved from the ratios of electron gain intensities. We further explore feasible experimental scenarios to probe the quantum characteristics of the sampled excitations and their populations. In particular, we present realistic simulations for electron beams interacting with optical cavities infiltrated with optically pumped quantum emitters, which we show to undergo a varied temporal evolution in the cavity mode statistics that causes radical modifications in the transmitted electron spectra depending on pump-electron delay. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement