Understanding the spatio-temporal response of photoexcited nanostructures, low-dimensional systems and molecules at surfaces on their natural length and time scales is a key goal in surface dynamics. Many ultrafast techniques are available to probe the temporal evolution of the photoexcited state. Most approaches, however, lack high spatial resolution, though the dynamics of charge carriers will often be closely linked to their local environment, and can vary on nanometer down to atomic length scales. It is thus of key importance to study the local response of a given subsystem, from a fundamental point of view as well as for nanodevice applications.
In this talk, I will give an introduction to the concepts of femtosecond point-projection microscopy (fs-PPM) and photoexcited THz-STM and discuss first results obtained with both techniques. In particular, fs-PPM allows to probe the nanoscale electric field distribution at the surface of suspended nanostructures, providing insight into the nanoscale carrier dynamics in such systems. Aiming for higher spatial resolution, focusing single-cycle THz-pulses into an STM allows for femtosecond gating of the tunneling junction, and thus to probe the photoexcited state of the junction ultimately on atomic length scales. Conversely, I will show that using a simple metal-metal junction with short lifetimes of the photoexcited charge carriers allows for ultrafast sampling of the THz near-field in real time.