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The dynamics of a tuning fork shear-force feedback system, used in a near-field scanning optical microscope, have been investigated. Experiments, measuring amplitude and phase of the tuning fork oscillation as a function of driving frequency and tip-sample distance, reveal that the resonance frequency of the tuning fork changes upon approaching the sample. Either amplitude or phase of the tuning fork can be used as distance control parameter in the feedback system. Using amplitude a second-order behavior is observed while with phase only a first-order behavior is observed, and confirmed by numerical calculations. This first-order behavior results in an improved stability of our feedback system. A sample consisting of DNA strands on mica was imaged which showed a height of the DNA of 1.4 nm.
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Ruiter et al. (1997) studied this question.
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