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The experimental resolution that is obtained with a near-field microscope by optical tunneling detection is far beyond the Rayleigh criterion. We discuss the principal physical characteristics of this superresolution. Three different examples are presented. They show that the resolution increases as the collector width and collector-to-object distance decrease. It is interesting to note that, in the near-field microscope, as in all local probe microscopes, the resolution cannot be defined from the characteristics of the microscope only. In all tunnel devices the detector cannot be separated from the object. The superresolution that can be obtained results from this fact. This paper also points out the importance ofevanescent waves in near-field optics and makes the connection between resolving power and evanescent fields.

A simple model of the thermal coupling between a probe tip and a sample is given, assuming an exchange through a thermal conductive medium. Comparisons between one-dimensional and three-dimensional calculations show the near-field character of the ac exchange. The coupling is analysed according to the different important parameters: probe - surface distances, sample thermal conductivity and diffusivity.

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