THz Near-Field Imaging of Extreme Subwavelength Metal Structures

TL;DR

This study systematically investigates how the size and connectivity of subwavelength metallic nanostructures influence near-field optical contrast in THz s-SNOM imaging, revealing size independence of resolution and the importance of structure configuration.

Contribution

It provides a comprehensive analysis of the effects of nanostructure size and connectivity on near-field contrast in the THz regime, clarifying previously puzzling experimental results.

Findings

Near-field contrast is greatly affected by nanostructure size and connectivity.

Spatial resolution remains practically independent of nanostructure size.

Size and configuration critically influence near-field optical contrast in the THz regime.

Abstract

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.