Femtosecond laser assisted scanning tunneling microscopy

The excitation of the tunneling junction of a scanning tunneling microscope using ultrashort laser pulses combined with detection of a tunneling current component which depends nonlinearly on the laser intensity allows, in principle, to simultaneously obtain ultimate spatial and temporal resolution. To achieve this goal, a laser system that produces ultrashort laser pulses is combined with an ultrahigh vacuum scanning tunneling microscope. The basic technical considerations are discussed and it is shown that atomic resolution can be achieved under pulsed laser excitation of the tunneling junction. The pulsed illumination gives rise to several contributions to the measured total current. Experimental evidence for signal contributions due to thermal expansion, transient surface potentials and multiphoton photoemission are presented.