Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators

Metamaterial resonators have become an efficient andversatileplatform in the terahertz frequency range, finding applications inintegrated optical devices, such as active modulators and detectors,and in fundamental research, e.g., ultrastrong light-matterinvestigations. Despite their growing use, characterization of modessupported by these subwavelength elements has proven to be challengingand it still relies on indirect observation of the collective far-fieldtransmission/reflection properties of resonator arrays. Here, we presenta broadband time-domain spectroscopic investigation of individualmetamaterial resonators via a THz aperture scanning near-field microscope(a-SNOM). The time-domain a-SNOM allows the mapping and quantitativeanalysis of strongly confined modes supported by the resonators. Inparticular, a cross-polarized configuration presented here allowsan investigation of weakly radiative modes. These results hold greatpotential to advance future metamaterial-based optoelectronic platformsfor fundamental research in THz photonics.