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Abstract

The interaction between free electrons and optical fields constitutes a unique platform to investigate ultrafast processes in matter and explore fundamental quantum phenomena. Specifically, optically modulated electrons in ultrafast electron microscopy act as noninvasive probes that push space–time–energy resolution to the picometer–attosecond–microelectronvolt range. Electron energies well above the involved photon energies are commonly used, rendering a low electron–light coupling and, thus, only providing limited access to the wealth of quantum nonlinear phenomena underlying the dynamical response of nanostructures. Here, we theoretically investigate electron–light interactions between photons and electrons of comparable energies, revealing quantum and recoil effects that include a nonvanishing coupling of surface-scattered electrons to light plane waves, inelastic electron backscattering from confined optical fields, and strong electron–light coupling under grazing electron diffraction by an illuminated crystal surface. Our exploration of electron–light–matter interactions holds potential for applications in ultrafast electron microscopy.