  The local optical field of a semiconductor micrograting (GaAs, 10x10 micro m)
is recorded in the middle field region using an optical scanning probe in
collection mode at constant height. The recorded image shows the micro-grating
with high contrast and a displaced diffraction image. The finite penetration
depth of the light leads to a reduced edge resolution in the direction to the
illuminating beam direction while the edge contrast in perpendicular direction
remains high (~100nm). We use the discrete dipole model to calculate the local
optical field to show how the displacement of the diffraction image increases
with increasing distance from the surface.
