Giant circular dichroism of chiral L-shaped nanostructure coupled with achiral nanorod: anomalous behavior of multipolar and dipolar resonant modes

Chirality, which has long been known as an intrinsic property of living organisms, has caught the interest of researchers due to the rapid emergence of chiral metamaterials. The chiroptical response of noble metal nanostructures in visible and near-infrared regions has been widely investigated. Herein, we propose a bilayer Ag metastructure, in which a chiral L-shaped nanostructure at the bottom is coupled with an achiral nanorod acquiring different positions in the top layer with respect to the long and/or short arm of the chiral L-shaped nanostructure at the bottom layer. The metastructure generates a giant circular dichroism (CD) signal resulting from the strong coupling of the multipolar and dipolar resonant modes on the two layers, in the visible and near-infrared regions. With changing the position of the achiral nanorod, an unusual reversal of the CD spectra is observed, along with a fourfold increase in CD intensity in the short wavelength range due to the multipolar resonant modes. The position of the achiral nanorod is tailored by the azimuthal angle of the substrate during the fabrication of the metastructure using the oblique angle deposition method. This study provides insights into the variation of the coupling strength between a chiral L-shaped nanostructure and an achiral nanorod. The results can be useful in designing chiral-achiral composite nanoantennas for sensing devices.