We study theoretically and experimentally a novel 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 optic
al 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 lower-order symmetry ($mathrm {C}_4$) than other commonly studied chiral structures. We verify our new theoretical approach by reproducing numerical and experimental scattering parameters.
We propose and verify experimentally a new concept for achieving strong nonlinear coupling between the electromagnetic and elastic properties in metamaterials. This coupling is provided through a novel degree of freedom in metamaterial design: intern
al rotation within structural elements. Our meta-atoms have high sensitivity to electromagnetic wave power, and the elastic and electromagnetic properties can be independently designed to optimise the response. We demonstrate a rich range of nonlinear phenomena including self-tuning and bistability, and provide a comprehensive experimental demonstration of the predicted effects.
We introduce a chiral metamaterial with strong, non-resonant optical activity, and very low polarization ellipticity. We achieve this by combining a meta-atom and its complementary structure into a meta-molecule, resulting in the coupling of magnetic
and electric dipole responses. In contrast to either a pair of crosses, or complementary crosses, this structure has low dispersion in the optical activity at the transmission resonance. We also study the excitation mechanism in this structure, and optimize the optical activity through changing the twist angle.
We introduce the concept of controlling the nonlinear response of the metamaterial by altering its internal structure. We experimentally demonstrate tuning of the nonlinear response of two coupled split-ring resonators by changing their mutual positi
on. This effect is achieved through modification of the structure of the coupled resonant modes, and their interaction with the incident field. By offsetting the resonators we control the maximum currents through the nonlinear driving elements, which affects the nonlinear response of the system.
We experimentally observe the tuning of metamaterials through the relative rotation of the elements about their common axis. In contrast to previous results we observe a crossing of resonances, where the symmetric and anti-symmetric modes become dege
nerate. We associate this effect with an interplay between the magnetic and electric near-field interactions and verify this by calculations based on the interaction energy between resonators.
