3D Dielectric Chiral Metamaterials Achieving Nondispersive Near-Zero Ellipticity

Chiral metamaterials based on mirror-symmetry broken resonators are engineered to achieve a strong interaction with circularly polarized (CP) waves. However, in contrast to the widely-observed enhanced circular dichroism (CD), moderate/narrow-band optical activity (OA) responses are usually seen in chiral metadevices. Here, by engineering the chirality parameter that determines the effective refractive indices associated with the two CP eigenstates, a three-dimensional (3D) metamaterial consisting of 3D-printed ceramic meta-atoms exhibiting broadband strong OA with nondispersive near-zero ellipticity in the millimeter-wave regime is demonstrated. With four-fold rotational symmetry (C-4), the 3D dielectric metamaterial shows near-identical co-polarized transmission magnitude but distinct phase retardations under left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) illumination over a broad frequency range. Field analysis indicates that the observed OA behavior originates from the handedness-dependent light-matter interaction between the chiral meta-atoms and CP waves. Furthermore, large transmission CD and CP polarization conversion are observed in a metamaterial with C-2-symmetry, demonstrating the proposed systems' versatility in CP wave control. As revealed by the present study, coherently exploring the complex parametric space offered by 3D meta-atoms based on state-of-the-art 3D fabrication techniques can become a promising paradigm for engineering metamaterials with sophisticated functionalities.