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

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 interacts weakly with THz beams, making it difficult to probe it using far-field methods, whereas arrays of resonators exhibit inter-resonator coupling, which affect 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. While 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 to analyze single resonators without significant interaction with the near-field 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 spectral response in resonator arrays. This near-field microscopy and spectroscopy method enables investigations of strong light-matter coupling at THz frequencies in the single-resonator regime.