  Scanning near-field field optical microscopy (SNOM) is a technique, which
allows sub-wavelength optical imaging of photonic structures. While the
electric field components of light can be routinely obtained, imaging of the
magnetic components has only recently become of interest. This is so due to the
development of artificial materials, which enhance and exploit the typically
weak magnetic light-matter interactions to offer extraordinary optical
properties. Consequently, both sources and detectors of the magnetic field of
light are now required.
  In this paper, assisted by finite-difference time-domain simulations, we
suggest that the circular aperture at the apex of a metal coated hollow-pyramid
SNOM probe can be approximated by a lateral magnetic dipole source. This
validates its use as a detector for the lateral magnetic near-field, as
illustrated here for a plasmonic nanobar sample. Verification for a dielectric
sample is currently in progress. We experimentally demonstrate the equivalence
of the reciprocal configurations when the probe is used as a source
(illumination mode) and as a detector (collection mode). The simplification of
the probe to a simple magnetic dipole facilitates the simulations and the
understanding of the near-field images.
