Abstract
We experimentally investigate the chiral optical response of an individual nanostructure consisting of three equally sized spherical nanoparticles made of different materials and arranged in \ang{90} bent geometry. Placing the nanostructure on a substrate converts its morphology from achiral to chiral. Chirality leads to pronounced differential extinction, i.e., circular dichroism and optical rotation, or equivalently, circular birefringence, which would be strictly forbidden in the absence of a substrate or heterogeneity. This first experimental observation of the substrate-induced break of symmetry in an individual heterogeneous nanostructure sheds new light on chiral light-matter interactions at substrate-nanostructure interfaces.
Highlights
L ord Kelvin defines chirality as the property of a geometrical figure whose “image in a plane mirror, ideally realized, cannot be brought to coincide with itself”.1 Chiral molecules and nanostructures exhibit circular anisotropies;[2,3] left- and right-hand circular polarizations (LCP and RCP) experience different real and imaginary parts of the refractive index
In the coupled-dipole model (CDM), each of the nanoparticles is modeled as a point-dipole, whose electric- and magnetic-dipole polarizability is obtained from the Mie theory in free space.[49]
The CDM cannot account for strong near-field enhancement in the gaps between the actual nanoparticles, which significantly contributes to the scattering, absorption, and extinction spectra
Summary
L ord Kelvin defines chirality as the property of a geometrical figure whose “image in a plane mirror, ideally realized, cannot be brought to coincide with itself”.1 Chiral molecules and nanostructures exhibit circular anisotropies;[2,3] left- and right-hand circular polarizations (LCP and RCP) experience different real and imaginary parts of the refractive index. QPNs may show a strong chiroptical response in differential transmission (ΔT), differential absorption (ΔA), differential scattering, and asymmetric polarization conversion of CPL.[10,11,25−36] All aforementioned differential measures must invert their sign with the reversal of the illumination direction if the QPNs are embedded in a homogeneous background.
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