Ultrafast scanning-probe microscopy: toward ultrafast movies of moving atoms

By combining ultrashort laser pulse techniques with scanning tunneling microscopy (STM), we have developed an instrument that obtains simultaneous 2-ps time resolution and 50-angstrom spatial resolution. This is a 9-orders-of-magnitude improvement over the time resolution previously attainable with STM. We have used this instrument to measure the response of the tunneling gap to excitation by a subpicosecond electrical pulse. Our technique is not limited to STM; it can be implemented in a variety of scanning-probe microscopies, allowing the observation of ultrafast dynamics on the atomic scale. The nonlinear tip-sample interaction in scanning-probe microscopy (SPM) enables one to measure laser-induced dynamical events on surfaces. The technique will be a powerful new tool for the observation of processes and excitations that propagate at velocities of a few angstroms per femtoseconds. We believe that it will be possible to spatially and temporally resolve many dynamic phenomena on an atomic scale. Future investigations will focus on vibronic motion of atoms on surfaces; carrier transport in semiconductors, molecules, and semiconductor devices; and hot-carrier effects.