Time-resolved thermal expansion of an STM tip after ultrafast optical excitation

摘要

The FOLANT (focusing of laser radiation in the near-field of a tip) technique developed by Dickmann et al. (1997) for nanostructuring of surfaces has attracted considerable attention. In the FOLANT configuration, intense laser pulses illuminate the gap region between the tip of a scanning probe microscope (SXM) and a sample surface. Above a certain threshold, each laser pulse causes modifications of the surface enabling the "writing" of complex patterns as the tip is moved across the sample surface. The physical mechanism for generating these nanostructures is still under debate. While Dickmann et al. attribute the surface modifications to material ablation resulting from a tip-induced focusing of laser radiation onto the surface, it was often argued that the observed structures are caused by a thermal expansion of the tip leading to a crash of the tip into the sample surface. Here we present time-resolved SXM measurements after short laser pulse excitation that clearly demonstrate that thermal tip expansion is the dominating mechanism. Our results show that an STM combined with intense ultrafast laser pulses should be considered a microscopic light-driven "air hammer" rather than a nanometric laser drill.