The heterodyne phase-controlled oscillator method to monitor the resonance frequency and quality
factor of the tip oscillations was used to control the scanning near-field optical microscope ~SNOM!
and to study the nature of the shear-force interaction routinely used in SNOM. Both optical and
nonoptical ~tuning fork-based! detection schemes of the shear force have been investigated using the
same electronic unit, which enables a direct comparison of the results. It is shown that the possibility
to record simultaneously the topography and dissipative interaction ~Q-factor! channels gives
additional information about the sample and helps to interpret the data in a manner analogous to that
of a usual dynamic force microscope. The peculiarities of the recorded approach curves ~increase of
the resonance frequency and Q factor when the tip approaches the sample! are consistent with the
”repetitive bumping” mechanism of tip’sample interaction for the shear force. Evidence for the
transition from the bumping to the permanent sliding mechanism has been obtained for the case of
larger vibration amplitudes of the tip.