  Energy carrier evolution is crucial for material performance. Ultrafast
microscopy has been widely applied to visualize the spatiotemporal evolution of
energy carriers. However, direct imaging of small amounts of energy carriers on
nanoscale remains difficult due to extremely weak transient signals. Here we
present a method for ultrasensitive and high-throughput imaging of energy
carrier evolution in space and time. This method combines femtosecond
pump-probe techniques with interferometric scattering microscopy (iSCAT), named
Femto-iSCAT. The interferometric principle and unique spatially-modulated
contrast enhancement increase the transient image contrast by >2 orders of
magnitude and enable the exploration of new science. We address three important
and challenging problems: transport of different energy carriers at various
interfaces, heterogeneous hot electron distribution and relaxation in single
plasmonic resonators, and distinct structure-dependent edge state dynamics of
carriers and excitons in optoelectronic semiconductors. Femto-iSCAT holds great
potential as a universal tool for ultrasensitive imaging of energy carrier
evolution in space and time.
