  We demonstrate spatially-resolved measurements of spontaneous and stimulated
electron-photon interactions in nanoscale optical near fields using electron
energy-loss spectroscopy (EELS), cathodoluminescence spectroscopy (CL), and
photon-induced near-field electron microscopy (PINEM). Specifically, we study
resonant surface plasmon modes that are tightly confined to the tip apexes of
an Au nanostar, enabling a direct correlation of EELS, CL, and PINEM on the
same physical structure at the nanometer length scale. Complemented by
numerical electromagnetic boundary-element method calculations, we discuss the
spontaneous and stimulated electron-photon interaction strength and spatial
dependence of our EELS, CL and PINEM distributions. We demonstrate that in the
limit of an isolated tip mode, spatial variations in the electron-near field
coupling are fully determined by the modal electric field profile, irrespective
of the spontaneous (in EELS and CL) or stimulated nature (in PINEM) of the
process. Yet we show that coupling to the tip modes crucially depends on the
incident electron energy with a maximum at a few keV, depending on the
proximity of the interaction to the tip apex. Our results provide elementary
insights into spontaneous and stimulated electron-light-matter interactions at
the nanoscale that have key implications for research on (quantum) coherent
optical phenomena in electron microscopy.
