STM-induced enhancement of dynamic dipole moments on crystal surfaces: Theory of the lateral resolution

Abstract

A scanning tunnel microscope (STM) tip placed over an atom or molecule on a crystal surface can enhance the dynamic dipole moment very substantially, with the consequence that fluorescence intensities, Raman cross sections, and diverse optical interaction cross sections can be enhanced greatly. In this paper, we extend our earlier theory on this phenomenon to the case where the atom or molecule is not directly under the tip, but rather is displaced from this location. We are then able to explore the lateral resolution associated with the enhancement effects, through studies of scans of isolated dipoles, and dipole pairs. The numerical calculations suggest that the lateral resolution is on the scale of ${(\mathrm{RD})}^{1/2},$ with R the radius of curvature of the STM tip, and D its distance from the substrate.