  Transmission electron microscopy is an excellent experimental tool to study
the interaction of free electrons with nanoscale light fields. However, up to
now, applying electron microscopy to quantum optical investigations was
hampered by the lack of experimental platforms which allow a strong coupling
between fast electrons and high-quality resonators. Here, as a first step, we
demonstrate the broad-band excitation of optical whispering-gallery modes in
silica microresonators by fast electrons. In the emitted coherent
cathodoluminescence spectrum, a comb of equidistant peaks is observed,
resulting in cavity quality factors larger than 700. These results enable the
study of quantum optical phenomena in electron microscopy with potential
applications in quantum electron-light metrology.
