  Scattering-type scanning near-field microscopy (s-SNOM) at terahertz (THz)
frequencies could become a highly valuable tool for studying a variety of
phenomena of both fundamental and applied interest, including mobile carrier
excitations or phase transitions in 2D materials or exotic conductors.
Applications, however, are strongly challenged by the limited signal to noise
ratio. One major reason is that standard atomic force microscope (AFM) tips,
which have made s-SNOM a highly practical and rapidly emerging tool, provide
weak scattering efficiencies at THz frequencies. Here we report a combined
experimental and theoretical study of commercial and custom-made AFM tips of
different apex diameter and length, in order to understand signal formation in
THz s-SNOM and to provide insights for tip optimization. Contrary to common
beliefs, we find that AFM tips with large (micrometer-scale) apex diameter can
enhance s-SNOM signals by more than one order of magnitude, while still
offering a spatial resolution of about 100 nm at a wavelength of 119 micron. On
the other hand, exploiting the increase of s-SNOM signals with tip length, we
succeeded in sub-15 nm resolved THz imaging employing a tungsten tip with 6 nm
apex radius. We explain our findings and provide novel insights into s-SNOM via
rigorous numerical modeling of the near-field scattering process. Our findings
will be of critical importance for pushing THz nanoscopy to its ultimate limits
regarding sensitivity and spatial resolution.
