Probes for Ultrasensitive THz Nanoscopy

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

doi:10.1021/acsphotonics.9b00324

Curdin Maissen, Elizaveta Nikulina + Show 3 more

Open Access

https://doi.org/10.1021/acsphotonics.9b00324

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Journal: ACS photonics	Publication Date: Apr 1, 2019
Citations: 76	License type: cc-by

Affiliation: CIC nanoGUNE, Ikerbasque

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 119 micron. On the other hand, exploiting the increase of s-SNOM signals with tip length, we succeeded in sub-15 nm 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.

Highlights

Scattering-type scanning near-field optical microscopy (s-SNOM)[1] is an emerging scanning probe technique, which extends the power of optical techniques deep into the subwavelength regime for nanoscale imaging and spectroscopy from visible to terahertz frequencies
In s-SNOM, a metallic, cantilevered atomic force microscopy (AFM) tip is illuminated with focused laser radiation and the backscattering from the tip is detected by a far-field detector
We note that our findings show a clear similarity to findings made in scanning electrostatic force microscopy (EFM), where an increase of the tip radius leads to stronger EFM signals owing to an increased capacitive coupling between tip and sample.[47-50]

Summary

Introduction

Scattering-type scanning near-field optical microscopy (s-SNOM)[1] is an emerging scanning probe technique, which extends the power of optical techniques deep into the subwavelength regime for nanoscale imaging and spectroscopy from visible to terahertz frequencies. When the tip is brought in close proximity to a sample surface, the near-field interaction between tip and sample modifies the tip-scattered field, depending on the local dielectric sample properties. By collecting the tip-scattered field with a distant (far-field) detector, information about the local sample properties close to the tip apex are obtained. Improved background suppression is obtained in combination with interferometric detection, which yields amplitude and phase of the scattered field that are related to the reflection and absorption properties of the sample, respectively[19-21]

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