Tip size dependence of passive near-field microscopy.

We improve the spatial resolution and investigate the tip-sample coupling in a passive scattering-type scanning near-field optical microscope (s-SNOM), which probes thermally excited surface waves without any external light source. We study the spatial resolution, the intensity, and the decay behavior of the thermally excited near-field signals with different radii of curvatures of tungsten-tip apexes. We also study the tip size dependence of the interference pattern in the far-field region. The spatial resolution is closely related to the tip size, but the decay behavior of the near field is unrelated. These results suggest that the strength of the tip-sample coupling is unrelated to the tip size in the passive s-SNOM. We propose a theoretical model able to interpret the experimental data for the passive s-SNOM.