Low-loss and high-Qplanar metamaterial with toroidal moment

We experimentally observe toroidal dipolar response in a planar metamaterial comprised of asymmetric split-ring resonators (ASRRs) at microwave frequency. It is shown that a toroidal molecule can be constructed through rational arrangement of planar ASRRs as meta-atoms via manipulating structural symmetry and thus coupling of the meta-atoms. We find that the toroidal resonance provides a subwavelength-scale electromagnetic localization style, and that confining the electromagnetic field inside a dielectric medium with toroidal geometry is beneficial for low-loss metamaterials. The planar scheme of manipulating the coupling among the ASRRs may stimulate research in optical regions involving toroidal multipoles. The toroidal geometry together with the Fano resonance of ASRR-induced high-Q response will have enormous potential applications in enhancing light-matter interactions, e.g., for low-threshold lasing, low-power nonlinear processing, and sensitive biosensing.