Dynamics of the tip–sample interaction in near-field scanning optical microscopy and the implications for shear force as an accurate distance measure

Near-field scanning optical microscopy uses shear-force feedback as the primary method to control the probe–sample distance. We describe the nonlinear interaction between the tip and sample with a simple truncated driven harmonic oscillator model. The model accurately describes the measured dynamics of this system. Insights are gained into the mechanism behind this interaction, and we give strong evidence that the probe taps on sample surface adlayers in normal operation, but will tap the underlying sample surface when the oscillation is nearly quenched.