Hybrid Helix Metamaterials for Giant and Ultrawide Circular Dichroism

Abstract

Circular polarization-resolved imaging and spectroscopy have versatile applications ranging from astronomy and biological and biomedical studies to remote sensing. To achieve real-time imagery with high contrast and rich information, integratable circular polarizers with high extinction ratios, broad operation bandwidths, and low insertion loss are urgently desired, while to our knowledge no structures proposed previously can balance these three vital parameters. In this paper, a concept of a hybrid helix metamaterial has been proposed to achieve ultrawide and giant circular dichroism with low insertion loss simultaneously. By coupling the localized and delocalized resonant modes, a broad and strong blocking band for unselected polarization can be realized with a pencil-like configuration. A giant extinction ratio of 5.9 × 10 5 can be obtained with a six-pitch hybrid single-helix array, which is almost 7 times larger than the best result reported by Behera et al. in 2015. A very high average extinction ratio of 8.5 × 10 3 is achieved in the operation band of 3.5–9.5 μm, which is due to an average transmittance as low as 0.2% for the unselected polarizations. More importantly, the top tapered part well-matches the impedance and contributes to a high average transmittance of 72% in the whole range. On the basis of the general scalablility, the validity of such a concept is demonstrated experimentally in the microwave range. The experimental results agree well with theoretical ones. The proposed concept can also be broadly extended to other chiral metamaterial systems for the design of novel high-performance circular polarizers.