Non-Invasive Near-field Spectroscopy of Single Sub-Wavelength Complementary Resonators

TL;DR

This paper compares far-field and near-field THz spectroscopy methods for studying subwavelength metallic resonators, demonstrating that near-field microscopy effectively probes single resonators and provides detailed spatial and spectral information.

Contribution

It introduces a near-field THz microscopy technique that overcomes limitations of far-field methods for analyzing individual subwavelength resonators, enabling detailed single-resonator studies.

Findings

Near-field spectroscopy yields high signal-to-noise spectral data for single resonators.

Far-field spectroscopy is ineffective for single resonator analysis due to weak interaction.

Near-field technique maps confined fields and surface waves, revealing resonator spectral responses.

Abstract

Subwavelength metallic resonators provide a route to achieving strong light-matter coupling by means of tight confinement of resonant electromagnetic fields. Investigation of such resonators however often presents experimental difficulties, particularly at terahertz (THz) frequencies. A single subwavelength resonator weakly interacts with THz beams, making it difficult to probe it using far-field methods; whereas arrays of resonators exhibit inter-resonator coupling, which affects the resonator spectral signature and field confinement. Here, traditional far-field THz spectroscopy is systematically compared with aperture-type THz near-field microscopy for investigating complementary THz resonators. Whilst the far-field method proves impractical for measuring single resonators, the near-field technique gives high signal-to-noise spectral information, only achievable in the far-field with resonator arrays. At the same time, the near-field technique allows us to analyze single resonators - free from inter-resonator coupling present in arrays - without significant interaction with the near-filed probe. Furthermore, the near-field technique allows highly confined fields and surface waves to be mapped in space and time. This information gives invaluable insight into resonator spectral response in arrays. This near-field microscopy and spectroscopy method enables investigations of strong light-matter coupling at THz frequencies in the single-resonator regime.