title: Electromagnetic coupling on an atomic scale.
abstract: Light emission from a scanning tunneling microscope is used to investigate the electromagnetic coupling (EMC) of a metal tip and a metal sample. Subatomic scale modifications of the tip-sample region cause spectral shifts of the fluorescence as demonstrated for a monatomic step and by variation of the tip-sample distance. For sharp tips the EMC is confined to a lateral range of a few nm. The results are consistent with model calculations of the electromagnetic response of an appropriate tip-sample geometry.