Thermal-expansion effects in near-field optical microscopy fiber probes induced by laser light absorption

We present a study on thermal-expansion effects induced by laser light coup ling into metal-coated probes for near-field scanning optical microscopy. A n expansion of the probe edge of 170 nm per mW of coupled power has been ob served as an effect of the temperature raise due to light absorption in the metal coating. The phenomenon has been studied in both time and frequency domains by modulating the coupled laser power, in order to measure the typi cal time constants related to heat exchange processes, that turned out to b e of the order of a few milliseconds. An analytical model, taking into acco unt both the heat conduction through the coating and the convection losses, provides scales for the system parameters and fits the experimental data r emarkably well. (C) 1999 American Institute of Physics. [S0003-6951(99)0444 7-2].