Broadband chiral metamaterials with large optical activity

We study theoretically and experimentally a type of metamaterial with hybrid elements composed of twisted pairs of cross-shaped meta-atoms and their complements. We reveal that such two-layer metasurfaces demonstrate large, dispersionless optical activity at the transmission resonance accompanied by very low ellipticity. We develop a retrieval procedure to determine the effective material parameters for this structure, which has symmetry (${\mathrm{C}}_{4}$) of a lower order than other commonly studied chiral structures. We verify our theoretical approach by reproducing numerical and experimental scattering parameters.