Polarization Control by Using Anisotropic 3-D Chiral Structures

Due to the mirror symmetry breaking, chiral structures show fantastic electromagnetic (EM) properties involving negative refraction, giant optical activity, circular dichroism and asymmetric transmission. Aligned electric and magnetic dipoles excited in chiral structures contribute to the extraordinary properties. However, the chiral structures that exhibit n-fold rotational symmetry show a limited tunability of the chirality. In this paper, we propose a compact, light, and highly tunable anisotropic chiral structure to overcome this limitation and realize a linear-to-circular polarization conversion. The anisotropy is due to simultaneous excitations of two different pairs of aligned electric and magnetic dipoles. The 3-D omega-shaped structure, etched on two sides of one printed circuit board (PCB) with connecting metallic vias, achieves 60% of linear-to- circular conversion (transmission) efficiency at the operating frequency of 9.2 GHz. The desired 90 degrees phase shift between the two orthogonal linear polarizations is not only from the chirality but also from the anisotropic chiral response slightly off the resonance. The work enables elegant and practical polarization control of EM waves.