Probes for Ultrasensitive THz Nanoscopy

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 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

Functional Mid‐Infrared Polaritonics in van der Waals Crystals

“…Near‐field imaging of phonon polaritons in α‐MoO 3 nanodisks allows direct measurement of their dispersion, demonstrating the elliptical and hyperbolic types at corresponding frequencies, as shown in Figure c,d . Here, the near‐field imaging was performed with a scattering‐type scanning near‐field optical microscope (s‐SNOM), where a nanotip effectively couples the far‐field with the near‐fields via inelastic scattering, overcoming the momentum mismatch and thus enabling the excitation and detection of polaritonic modes . The confinement factor of α‐MoO 3 phonon polaritons is measured to be as high as n eff ≈ 60, and, surprisingly, the lifetime reaches up to 8 ps (γ −1 ≈ 200) which is about four times higher than that of the phonon polaritons in isotopically enriched h‐BN .…”

“…[11] Here, the near-field imaging was performed with a scattering-type scanning near-field optical microscope (s-SNOM), [31] where a nanotip effectively couples the far-field with the near-fields via inelastic scattering, overcoming the momentum mismatch and thus enabling the excitation and detection of polaritonic modes. [32] The confinement factor of α-MoO 3 phonon polaritons is measured to be as high as n eff ≈ 60, and, surprisingly, the lifetime reaches up to 8 ps (γ −1 ≈ 200) which is about four times higher than that of the phonon polaritons in isotopically enriched h-BN. [11] The strong field confinement and ultralow loss of hyperbolic phonon Adv.…”

Towards broadband THz spectroscopy and analysis of sub-wavelength-size biological samples

“…Scanning THz near-field microscopy set-ups can be classified in two different kinds. The scattering Scanning Near-field Microscopy (s-SNOM) [10,11], uses the oscillating tip of an Atomic Force µscope (AFM) to concentrate the incident light on its nm-sized apex enabling to probe the dielectric constant of a material at a resolution similar to the apex radius (today's record ~ 15 nm [12]). It can be used as a nano-spectroscopy system [13], for instance in the recent report of the characterization of lactose [14].…”

Antenna-coupled field-effect transistors as detectors for terahertz near-field microscopy

“…In this work, we employ homodyne interferometric detection as yet another alternative for sub-terahertz s-SNOM sensing. 12 It offers the interferometric signal enhancement of the pseudo-heterodyne technique (in our case about 50Â higher signals in comparison to non-interferometric detection), phase sensitivity by two subsequent measurements with different mirror settings, and it has the option for samples, which do not introduce an optical phase shi, to record the near-eld signal in a single measurement by setting the interferometer to the maximum-signal position.…”