Abstract
The electromagnetic coupling of a metal tip and a metal sample which varies upon sub-A variations of their distance, leading to measurable spectral shifts of the lowest localized plasmon resonance was experimentally demonstrated. This effect explains spectral shifts occurring near atomic steps on the sample surface and allows a direct measurement of the lateral etension of the localized surface plasmon on the surface. Moreover, lateral extensions of the coupling in the range of a few nanometers, consistent with model calculations of the electromagnetic response of a tip-sample geometry was found.