  Modern scattering-type scanning near-field optical microscopy (s-SNOM) has
become an indispensable tool in material research. However, as the s-SNOM
technique marches into the far-infrared (IR) and terahertz (THz) regimes,
emerging experiments sometimes produce puzzling results. For example, anomalies
in the near-field optical contrast have been widely reported. In this Letter,
we systematically investigate a series of extreme subwavelength metallic
nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range.
We find that the near-field material contrast is greatly impacted by the
lateral size of the nanostructure, while the spatial resolution is practically
independent of it. The contrast is also strongly affected by the connectivity
of the metallic structures to a larger metallic ground plane. The observed
effect can be largely explained by a quasi-electrostatic analysis. We also
compare the THz s-SNOM results to those of the mid-IR regime, where the
size-dependence becomes significant only for smaller structures. Our results
reveal that the quantitative analysis of the near-field optical material
contrasts in the long-wavelength regime requires a careful assessment of the
size and configuration of metallic (optically conductive) structures.
