Abstract
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 write/read head of a commercial hard disk drive (HDD) as a most suitable test sample for fundamental studies, given its well20 defined sharp material boundaries perpendicular to its ultra-smooth surface. We obtainunprecedented 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 numericalsimulations. 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 ultra-sharp tungsten tips the apparent width (and thus resolution) of sharp material boundaries can be reduced to about 5 nm.
Highlights
The spatial resolution in microscopy is often evaluated by measuring the width of a typically pointsymmetric line profile across the sharp boundary between two different materials.[9-11]
The characteristic width w of the Edge Response Function (ERF) can be determined via its derivative, which is known as the Line Spread Function (LSF)
In Scattering-type scanning near-field optical microscopy (s-SNOM) experiments, w, is typically measured directly in line profile recorded across the boundary[13-16] or via its derivative[17]
Summary
We cross-correlated the line profiles for the second demodulation order n=2 in order to obtain the lateral offset between them. We used the second demodulation order for finding the offsets because it provides a better SN than higher orders, which enables a higher accuracy of the cross-correlation. The cone was attached by FIB induced deposition of silicon oxide. Details of this procedure can be found in reference 25. With Pt/Ir we achieved apex radii of about 10 nm and with Au not better than 12 nm. We assign this finding to diffusion of metal atoms under ion bombardment, which is higher for Au than for Pt/Ir and W. Further studies are needed to clarify the mechanisms involved in the tip sharpening process
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