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 one order of magnitude, while still offering a spatial resolution of about 100 nm 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. TOC graphics

Modern scattering-type scanning near-field optical microscopy (s-SNOM) has become an indispensable tool in material research. However, as the s-SNOM technique marches into the far-infrared (IR) and terahertz (THz) regimes, emerging experiments sometimes produce puzzling results. For example, "anomalies" in the near-field optical contrast have been widely reported. In this letter, we systematically investigate a series of extreme subwavelength metallic nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range. We find that the near-field material contrast is greatly impacted by the lateral size of the nanostructure, while the spatial resolution is practically independent of it. The contrast is also strongly affected by the connectivity of the metallic structures to a larger metallic "ground plane". The observed effect can be largely explained by a quasi-electrostatic analysis. We also compare the THz s-SNOM results to those of the mid-IR regime, where the size-dependence becomes significant only for smaller structures. Our results reveal that the quantitative analysis of the near-field optical material contrasts in the long-wavelength regime requires a careful assessment of the size and configuration of metallic (optically conductive) structures.

Epitaxial BiFeO3 films pulsed laser deposited on SrTiO3, Nb:doped SrTiO3, and DyScO3 were studied using variable angle spectroscopic ellipsometry, vacuum ultraviolet ellipsometry, micro-Raman spectroscopy, and x-ray diffraction. The energy band gap of the film deposited on DyScO3 is 2.75 eV, while the one for the film deposited on Nb:doped SrTiO3 is larger by 50 meV. The blueshift in the dielectric function of the BiFeO3 films deposited on Nb:doped SrTiO3 compared to the films deposited on DyScO3, indicates a larger compressive strain in the films deposited on Nb:doped SrTiO3. This is confirmed by Raman spectroscopy and by high resolution x-ray diffraction investigations.

We show that magnetometry measurements based upon the magneto-optical Kerr effect and high resolution optical microscopy can be used as a noninvasive probe of magnetization reversal for individual nano-structures. Our measurements demonstrate single pass hysteresis loop measurements for sample sizes down to 30 nm width. A quantitative signal-to-noise ratio evaluation shows that our approach achieves an at least 3-fold improvement in sensitivity if compared to focused laser based nano-magnetometry. An analysis of the physical limits of our detection scheme enables us to estimate that measurements for structures with single digit nm widths and magnetic moments of 10−16 Am2 are feasible.

Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered structure of vdW materials, the polaritons can exhibit a hyperbolic dispersion and propagate as nanoscale‐confined volume modes in thin flakes. On the other hand, surface‐confined modes can be found at the flake edges. Surprisingly, the guiding of these modes in ribbons—representing typical linear waveguide structures—is widely unexplored. Here, a detailed study of hyperbolic phonon polaritons propagating in hexagonal boron nitride ribbons is reported. Employing infrared nanoimaging, a variety of modes are observed. Particularly, the fundamental volume waveguide mode that exhibits a cutoff width is identified, which, interestingly, can be lowered by reducing the waveguide thickness. Further, hybridization of the surface modes and their evolution with varying frequency and waveguide width are observed. Most importantly, it is demonstrated that the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus enables linear waveguiding of the polaritons in arbitrarily narrow ribbons. The experimental data, supported by simulations, establish a solid basis for the understanding of hyperbolic polaritons in linear waveguides, which is of critical importance for their application in future photonic devices.