PROBES FOR SCANNING TUNNELING OPTICAL MICROSCOPY - A THEORETICAL COMPARISON

In near-field optical microscopy two kinds of probe are used: a dielec tric tip and a small-aperture tip. The purpose of this paper is to com pare theoretical images of the same sample obtained with these two pro bes. We describe the use of a scanning tunneling optical microscope wh en the sample is a transparent dielectric rough surface illuminated by total internal reflection. The dielectric tip is modeled as a small s cattering 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 diel ectric tip picks up the square modulus of the electric near field. The small-aperture probe is sensitive to both the electric and the magnet ic fields. The models are used for calculating and comparing images of a periodic grating and of a two-dimensional object (a letter) that ar e smaller than the wavelength. The images are quite different, and pol arization of the incident light is an important parameter for scanning tunneling optical microscope images, with different behavior for the two tips.