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

Recently experiments have been performed with a new kind of Scanning N ear-field Microscope. The apparatus is derived from a Scanning Tunneli ng Optical Microscope by simply inverting the direction of light propa gation: 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 theoreti cal calculation of the detected intensity measured in ISTOM experiment s. In this model, the sample is a relief on the hemisphere surface and the source is an aperture tip described within the Bethe-Bouwkamp app roximation. 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 v arious parameters on the detected intensity is thus quite easy. We ill ustrate the formulas by some examples showing the variations of the de tected signal versus tip-surface distance, tip radius and detection an gle.