A THEORETICAL-MODEL FOR THE INVERSE SCANNING TUNNELING OPTICAL MICROSCOPE (ISTOM)

Recently experiments have been performed with a new kind of Scanning Near-field Microscope. The apparatus is derived from a Scanning Tunneling Optical Microscope by simply inverting the direction of light propagation: it is thus an Inverted Scanning Tunneling Optical Microscope (ISTOM) where the tip is used in emission mode and where detection can be mediated via homogeneous or evanescent waves. We propose a theoretical calculation of the detected intensity measured in ISTOM experiments. In this model, the sample is a relief on the hemisphere surface and the source is an aperture tip described within the Bethe-Bouwkamp approximation. The use of plane wave expansions of the various fields and of a perturbation method for matching the boundary conditions lead to concise analytical formulas. The discussion of the influence of the various parameters on the detected intensity is thus quite easy. We illustrate the formulas by some examples showing the variations of the detected signal versus tip-surface distance, tip radius and detection angle.