Abstract
Fluorescence induced by the tunnelling current of a scanning tunnelling microscope is used to investigate the electromagnetic coupling of a metal tip and a metal sample. Sub-atomic scale modifications of the tunnelling junction geometry cause spectral shifts of the fluorescence. Such shifts are observed when the tip is vertically displaced relative to a flat sample surface. Similarly, monatomic sample steps close to the tip position shift the fluorescence. These experimental results are consistent with model calculations of the electromagnetic response of an appropriate tip-sample geometry. We find that for sharp tips the electromagnetic coupling of the tip and the sample is confined to a lateral range of a few nm.