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

Abstract

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.

Molecular Chirality Detection with Periodic Arrays of Three-Dimensional Twisted Metamaterials

“…The varying gap would reduce the CD strength. However, the observable CD response is considerably larger compared to those of many chiral nanostructures reported in the literature. ,,,− Table S1 in the Supporting Information lists the experimentally obtained CD from various chiral metamaterials reported in the literature. It should be noted that the twisted-arc metamaterials we used in this study do not exhibit rotational symmetry.…”

Ultrathin Suspended Chiral Metasurfaces for Enantiodiscrimination

“…[39][40][41] Electron beam lithography (EBL) and focused ion beam (FIB) have also been widely used to fabricate chiral metamaterials at the nanoscale. For example, a few kinds of chiral metamaterials made by EBL or FIB, such as multilayer nanoarrays, [42][43][44][45] kirigami structures, [46] and chiral nanoholes arrays, [29,47] have demonstrated intriguing chiroptical features of giant CD, [48][49][50] excellent rotation of linearly polarized lights, [51] and helical dichroism. [52] Nevertheless, the processing of fabrication for multilayer chiral metamaterials is sophisticated and requires multiple steps of precise alignment in EBL.…”

“…Moreover, the asymmetric transmission properties may find potential applications in information processing, polarimetric imaging and encryption. [49,[53][54][55][56][57][58]…”

Wideband High-Reflection Chiral Dielectric Metasurface

“…This would lead to an increase in the volume of the system. With the development of metasurface technology, ultra-thin metal chiral structures have been proposed [5][6][7][8], and some special designs can enhance the chiroptical response by multiple orders of magnitude, such as three-dimensional gold helix [9][10][11], two-dimensional fish-scale structure [12][13][14], and L-shaped gold nanoantennas [15][16][17][18]. However, due to metal absorption loss, the efficiency is limited.…”