We investigate the transient thermal expansion of a scanning tunneling microscope tip after excitation with intense femtosecond laser pulses. The expansion dynamics are measured electrically by monitoring the time-resolved tunneling current and mechanically by use of an atomic force microscope. The tip expansion reaches values as high as 15 nm and exceeds the typical working distance of a scanning tunneling microscope by far. This results in a mechanical contact between tunneling tip and surface leading to surface modifications on a nanometer scale. Our findings clarify the mechanism of the recently proposed focusing of laser radiation in the near field of a tip technique [J. Jersch and K. Dickmann, Appl. Phys. Lett. 68, 868 (1996)] for nanostructuring.

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