Understanding the image contrast of material boundaries in IR nanoscopy reaching 5 nm spatial resolution

S. Mastel, A. A Govyadinov, C. Maissen, A. Chuvilin, A. Berger and R. Hillenbrand

ACS Photonics, Just Accepted (2018)
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, the details on image contrast formation at materials boundaries - and subsequently the spatial resolution – is a widely unexplored terrain. Here we introduce the write/read head of a commercial hard disk drive (HDD) as an ideal test sample for fundamental studies, owing to its well-defined sharp material boundaries perpendicular to its ultra-smooth surface. We obtain unprecedented 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 numerical simulations. We explain our findings by a sample-dependent confinement and screening of the near fields at the tip apex, which will be of critical importance for the 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.