Thermal effects in pulsed laser assisted scanning tunneling microscopy

The thermal response of a tunneling tip after illumination of the apex with an ultrashort laser pulse of 1 ps duration is investigated. The finite element method is applied to calculate the resulting time-dependent temperature distribution and the thermal expansion taking into account the elastic properties of the tip material. The calculation reveals the three-dimensional movement of the tip apex. The expansion of the tip occurs within a few nanoseconds and after 10 μs the tip has almost reached its original length again. The bending of the tip due to the asymmetric illumination of the tip occurs on the same time scale and is of the same order of magnitude as the axial expansion. Under tunneling conditions the absolute magnitude of the expansion can lead to the formation of nanocontacts. This accounts for the laser induced nanostructuring of surfaces that has been reported in literature. The application of the thermal expansion as a fast switch for the tunneling current is proposed.