Understanding the Image Contrast of Material Boundaries in IR Nanoscopy Reaching 5 nm Spatial Resolution, ACS Photonics, 2018, 5 (8), pp 3372-3378, DOI: 10.1021/acsphotonics.8b00636

Stefan Mastel, Alexander A. Govyadinov, Curdin Maissen, Andrey Chuvilin, Andreas Berger, and Rainer Hillenbrand

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 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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• Corrsponding ORDP dataset: DOI: 10.13140/RG.2.2.19079.65441

Probes for Ultrasensitive THz Nanoscopy, ACS Photonics 2019, 6, 5, 1279-1288 DOI: 10.1021/acsphotonics.9b00324

Curdin Maissen, Shu Chen, Elizaveta Nikulina, Alexander Govyadinov and Rainer Hillenbrand

Abstract: Scattering-type scanning near-field microscopy (s-SNOM) at terahertz (THz) frequencies could become a highly valuable tool for studying a variety of phenomena of both fundamental and applied interest, including mobile carrier excitations or phase transitions in 2D materials or exotic conductors. Applications, however, are strongly challenged by the limited signal-to-noise ratio. One major reason is that standard atomic force microscope (AFM) tips, which have made s-SNOM a highly practical and rapidly emerging tool, provide weak scattering efficiencies at THz frequencies. Here, we report a combined experimental and theoretical study of commercial and custom-made AFM tips of different apex diameter and length, in order to understand signal formation in THz s-SNOM and to provide insights for tip optimization. Contrary to common beliefs, we find that AFM tips with large (micrometer-scale) apex diameter can enhance s-SNOM signals by more than 1 order of magnitude, while still offering a spatial resolution in the 100 nm range at a wavelength of λ = 119 μm. On the other hand, exploiting the increase of s-SNOM signals with tip length, we succeeded in sub-15 nm (<λ/8000) resolved THz imaging employing a tungsten tip with 6 nm apex radius. We explain our findings and provide novel insights into s-SNOM via rigorous numerical modeling of the near-field scattering process. Our findings will be of critical importance for pushing THz nanoscopy to its ultimate limits regarding sensitivity and spatial resolution.

Corresponding ORDP dataset: DOI: 10.13140/RG.2.2.28694.50246