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We use a modulation technique to detect a short-range interaction between a probe and a sample of a tunneling near-field optical microscope. This interaction might explain the resolution enhancement of the apertureless near-field techniques compared with conventional aperture near-field ones. To detect this interaction we measure approach curves of the modulating probe towards the sample and fit the experimental data with a simulation of the intensity, where a coupled dipoles interaction is added to the conventional near-field interaction. While the effect of the coupled dipoles interaction is negligible in conventional near-field microscopes, it dominates the modulated signal at scanning distances. Thus better resolution can be achieved, even in an aperture near-field microscope, by vibrating the tip vertically over the sample.