Understanding the Image Contrast of Material Boundaries in IR Nanoscopy Reaching 5 nm Spatial Resolution

• Published in: ACS photonics 2018-08, Vol.5 (8), p.3372-3378
• Main Authors: Mastel, Stefan, Govyadinov, Alexander A, Maissen, Curdin, Chuvilin, Andrey, Berger, Andreas, Hillenbrand, Rainer
• Format: Article
• Language: English
• ISSN: 2330-4022; 2330-4022
• DOI: 10.1021/acsphotonics.8b00636

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 read/write head of a commercial hard disk drive (HDD) as a most suitable test sample for fundamental studies, given its well-defined sharp material boundaries perpendicular to its ultrasmooth surface. We obtain unprecedented and unexpected insights into the s-SNOM image formation process, free of topography-induced contrasts 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 crucial importance for an accurate understanding and proper interpretation of high-resolution s-SNOM images of nanocomposite materials. We also demonstrate that with ultrasharp tungsten tips the apparent width (resolution) of sharp material boundaries can be reduced to about 5 nm.