In near-field optical microscopy two kinds of probe are used: a dielectric tip and a small-aperture tip. The purpose of this paper is to compare theoretical images of the same sample obtained with these two probes. We describe the use of a scanning tunneling optical microscope when the sample is a transparent dielectric rough surface illuminated by total internal reflection. The dielectric tip is modeled as a small scattering dipolar center. The intensity detected by the small-aperture probe is calculated with use of the diffraction theory of Bethe [ Phys. Rev. 66, 163 ( 1944)] and Bouwkamp [ Rep. Phys. 27, 35 ( 1954)]. It is shown that the two probes do not detect the same information: The dielectric tip picks up the square modulus of the electric near field. The small-aperture probe is sensitive to both the electric and the magnetic fields. The models are used for calculating and comparing images of a periodic grating and of a two-dimensional object (a letter) that are smaller than the wavelength. The images are quite different, and polarization of the incident light is an important parameter for scanning tunneling optical microscope images, with different behavior for the two tips.
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