  This study proposes an ultrathin chiral metamaterial slab stacked with
twisted complementary split-ring resonators (CSRRs) for highly efficient
broadband polarization transformation. The polarization of linearly polarized
electromagnetic waves can be rotated in a specific direction by passing it
through such a slab having a thickness of about one-tenth the operational
wavelength. Microwave experiments verified the theoretically predicted
conversion efficiency of up to 96% covering a bandwidth of 24% of the central
wavelength. CSRRs with circular symmetry provide increased interlayer coupling
strength, which produces a high-efficiency broadband response and strong
isolation of the original polarization.
