  Carrier distribution and dynamics in semiconductor materials often govern
their physical properties that are critical to functionalities and performance
in industrial applications. The continued miniaturization of electronic and
photonic devices calls for tools to probe carrier behavior in semiconductors
simultaneously at the picosecond time and nanometer length scales. Here, we
report pump-probe optical nanoscopy in the visible-near-infrared spectral
region to characterize the carrier dynamics in silicon nanostructures. By
coupling experiments with the point-dipole model, we resolve the size-dependent
photoexcited carrier lifetime in individual silicon nanowires. We further
demonstrate local carrier decay time mapping in silicon nanostructures with a
sub-50 nm spatial resolution. Our study enables the nanoimaging of ultrafast
carrier kinetics, which will find promising applications in the future design
of a broad range of electronic, photonic, and optoelectronic devices.
