Scanning near-field optical microscopy based on the heterodyne phase-controlled oscillator method

The heterodyne phase-controlled oscillator method to monitor the resonance frequency and quality factor of the tip oscillations was used to control th e scanning near-field optical microscope (SNOM) and to study the nature of the shear-force interaction routinely used in SNOM. Both optical and nonopt ical (tuning fork-based) detection schemes of the shear force have been inv estigated 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 additi onal information about the sample and helps to interpret the data in a mann er 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 "repeti tive bumping" mechanism of tip-sample interaction for the shear force. Evid ence for the transition from the bumping to the permanent sliding mechanism has been obtained for the case of larger vibration amplitudes of the tip. (C) 2000 American Institute of Physics. [S0021-8979(00)04017-2].