Split-ring resonators imaged by THz s-SNOM

Split-ring resonators (SRRs) are one of the most common building blocks to design metamaterials operating over a large range of frequencies in the electromagnetic spectrum. Here we acquire s-SNOM images with high signal-to-noise ratio on SRRs by scattering-type scanning near-field optical microscopy (s-SNOM) in the terahertz (THz) domain. By mapping the electric field distribution with subwavelength spatial resolution, we observe clear spatial asymmetries within a single SRR. These effects are more pronounced for the SRRs designed to resonate at the frequency of the probing laser (2.52 THz), compared to the SRRs that do not support a resonant mode at this frequency. This asymmetrical distribution is attributed to combined far-field and near-field multiplication effects, which depend on the position of the probing tip on the micro-resonator.