  Scattering-type scanning near-field optical microscopy (s-SNOM) allows for
nanoscale resolved Infrared (IR) and Terahertz (THz) imaging, and thus has
manifold applications ranging from materials to biosciences. However, a
quantitatively accurate understanding of image contrast formation at materials
boundaries, and thus spatial resolution is a surprisingly unexplored terrain.
Here we introduce the write/read head of a commercial hard disk drive (HDD) as
a most suitable test sample for fundamental studies, given its well20 defined
sharp material boundaries perpendicular to its ultra-smooth surface. We
obtainunprecedented and unexpected insights into the s-SNOM image formation
process, free of topography-induced artifacts that often mask and artificially
modify the pure near-field optical contrast. Across metal-dielectric
boundaries, we observe non-point-symmetric line profiles for both IR and THz
illumination, which are fully corroborated by numericalsimulations. We explain
our findings by a sample-dependent confinement and screening of the near fields
at the tip apex, which will be of crucial importance for an accurate
understanding and proper interpretation of high-resolution s-SNOM images of
nanocomposite materials. We also demonstrate that with ultra-sharp tungsten
tips the apparent width (and thus resolution) of sharp material boundaries can
be reduced to about 5 nm.
