  This paper addresses recent experimental findings about luminescence of a
gold tip in near-field interaction with a gold surface. Our electrochemically
etched gold tips show a typical, intrinsic luminescence that we exploit to
track the plasmon resonance modeled by a Lorentzian oscillator. Our
experimental device is based on a spectrometer optically coupled to an atomic
force microscope used in tuning fork mode. Our measurements provide evidence of
a strong optical coupling between the tip and the surface. We demonstrate that
this coupling strongly affects the luminescence (Intensity, wavelength and
FHWM) as a function of the tip position in 2D maps. The fluctuation of these
parameters is directly related to the plasmonic properties of the gold surface
and is used to qualify the optical near field enhancement (which subsequently
plays the predominant role in surface enhanced spectroscopies) with a very high
spatial resolution (typically around 20 nm). We compare these findings to the
independently recorded near-field scattered elastic Rayleigh signal.
