  The ultrafast dynamics of charge carriers in solids plays a pivotal role in
emerging optoelectronics, photonics, energy harvesting, and quantum technology
applications. However, the investigation and direct visualization of such
non-equilibrium transport phenomena remains as a long-standing challenge, owing
to the nanometer-femtosecond spatio-temporal scales at which the charge
carriers evolve. Here, we propose and demonstrate a novel interaction mechanism
enabling nanoscale imaging of the femtosecond dynamics of charge carriers in
solids. This imaging modality, which we name charge dynamics electron
microscopy (CDEM), exploits the strong interaction between terahertz (THz)
electromagnetic near fields produced by the moving charges and synchronized
free-electron pulses in an ultrafast scanning transmission electron microscope.
The measured free-electron energy at different spatio-temporal coordinates
allows us to directly retrieve the THz near-field amplitude and phase, from
which we reconstruct movies of the generated charges by comparison with
microscopic theory. The introduced CDEM technique thus allows us to investigate
previously inaccessible spatio-temporal regimes of charge dynamics in solids,
for example revealing new insight into the photo-Dember effect, showing
oscillations of photo-generated electron-hole distributions inside a
semiconductor. Our work lays the foundation for exploring a wide range of
previously inaccessible charge-transport phenomena in condensed matter using
ultrafast electron microscopy.
