Journal
ACS PHOTONICS
Volume 5, Issue 8, Pages 3372-3378Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsphotonics.8b00636
Keywords
scattering-type scanning near field optical microscopy (s-SNOM); IR and THz nanoscopy; focused ion beam (FIB) machining; ultrasharp near field probes
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Funding
- Spanish Ministry of Economy, Industry, and Competitiveness of the Marie de Maeztu Units of Excellence Program [MAT2015-65525, MDM-2016-0618]
- H2020 FET OPEN project PETER (GA) [767227]
- Swiss National Science Foundation [172218]
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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.
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