  Photoinduced carrier dynamics of nanostructures play a crucial role in
developing novel functionalities in advanced materials. Optical pump-probe
scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of
real-space imaging of such carrier dynamics with nanoscale spatial resolution.
However, combining the advanced technology of ultrafast pulsed lasers with STM
for stable time-resolved measurements has remained challenging. The recent
OPP-STM system, whose laser-pulse timing is electrically controlled by external
triggers, has significantly simplified this combination but limited its
application due to nanosecond temporal resolution. Here we report an
externally-triggerable OPP-STM system with a temporal resolution in the
tens-picosecond range. We also realize the stable laser illumination of the
tip-sample junction by placing a position-movable aspheric lens driven by piezo
actuators directly on the STM stage and by employing an optical beam
stabilization system. We demonstrate the OPP-STM measurements on GaAs(110)
surfaces, observing carrier dynamics with a decay time of $\sim170$ ps and
revealing local carrier dynamics at features including a step edge and a
nanoscale defect. The stable OPP-STM measurements with the tens-picosecond
resolution by the electrical control of laser pulses highlight the potential
capabilities of this system for investigating nanoscale carrier dynamics of a
wide range of functional materials.
