  We investigate the radiation patterns of sharp conical gold tapers, designed
as adiabatic nanofocusing probes for scanning near-field optical microscopy
(SNOM). Field calculations show that only the lowest order eigenmode of such a
taper can reach the very apex and thus induce the generation of strongly
enhanced near-field signals. Higher order modes are coupled into the far field
at finite distances from the apex. Here, we demonstrate experimentally how to
distinguish and separate between the lowest and higher order eigenmodes of such
a metallic taper by filtering in the spatial frequency domain. Our approach has
the potential to considerably improve the signal-to-background ratio in
spectroscopic experiments on the nanoscale.
