  Understanding and actively controlling the spatio-temporal dynamics of
non-equilibrium electron clouds is fundamental for the design of light and
electron sources, novel high-power electronic devices, and plasma-based
applications. However, electron clouds evolve in a complex collective fashion
on nanometer and femtosecond scales, producing electromagnetic screening that
renders them inaccessible to existing optical probes. Here, we solve the
long-standing challenge of characterizing the evolution of electron clouds
generated upon irradiation of metallic structures using an ultrafast
transmission electron microscope to record the charged plasma dynamics. Our
approach to charge dynamics electron microscopy (CDEM) is based on the
simultaneous detection of electron-beam acceleration and broadening with
nanometer/femtosecond resolution. By combining experimental results with
comprehensive microscopic theory, we provide deep understanding of this highly
out-of-equilibrium regime, including previously inaccessible intricate
microscopic mechanisms of electron emission, screening by the metal, and
collective cloud dynamics. Beyond the present specific demonstration, the here
introduced CDEM technique grants us access to a wide range of non-equilibrium
electrodynamic phenomena involving the ultrafast evolution of bound and free
charges on the nanoscale.
