Circular dichroism of honeycomb-shaped elliptical hole absorber

Abstract

The circular dichroism of chiral structure has been widely used in analytical chemistry, industrial pharmacy, biological monitoring, etc. However, the light-matter interaction between natural chiral structures is extremely weak. Plasmonic nanostructures can significantly enhance light-matter interaction. During the fabrication of the visible-to-near-infrared chiral plasmonic metamaterial absorbers, there exists usually a trade-off between the absorption and the sample area, that is, the circular dichroism signal of the large-area structure is small. Besides, the preparation of chiral absorbers working in the visible and near-infrared region usually requires expensive etching or lithography equipment, such as reactive ion etching or electron beam lithography. Therefore, preparing cost-effective chiral absorbers with large circular dichroism is attractive for practical applications. In order to improve the circular dichroism of large-scale chiral absorbers, a honeycomb-shaped elliptical hole absorber is proposed in this paper, and its absorption, circular dichroism, and optical <i>g</i>-factor are studied. By reasonable design, the numerical calculation results show that the circular dichroism can reach about 0.8 under the excitation of chiral polarized light, and the corresponding optical <i>g</i>-factor can reach about 1.7 at 920 nm. Compared with the reported absorber, our chiral absorber has a maximum <i>g</i>-factor value. The giant circular dichroism originates from the symmetry breaking of the structure by tilting ellipse structures, and the tilt angle has a significant influence on circular dichroism. To further explain the absorption difference, the electric profile, surface current distribution, and absorption loss of the chiral absorption at resonant wavelength are analyzed. Finally, we point out that the structure can be prepared by existing technologies, such as nanosphere photolithography: first, a layer of polystyrene (PS) balls is formed by self-organization, which can control the period of the structure; then the size of the PS balls is reduced to a suitable size and spacing by the reactive ion etching; finally, a metallic layer is deposited by oblique angle evaporation. This work provides useful guidance for fabricating the large-scale chiral plasmonic absorbers.