Tip-sample interaction in a "shear-force" near-field scanning optical microscope

The interaction between the tip of a near-field scanning optical microscope (NSOM) and the sample it scans is analyzed and compared to a simple tappin g model. The approach curves acquired with the NSOM are in excellent agreem ent with the model, and additional experiments strongly point against a non contact interaction (such as shear force). Based on this model we are also able to explain the oscillations pattern of the feedback loop. We conclude that our straight-fiber tip, operating under "shear-force" control, intermi ttently contacts the surface it is scanning, in a way similar to the tappin g mode in atomic force microscope. (C) 1999 American Institute of Physics. [S0034-6748(99)04509-8].