Optical Properties of Chiral Plasmonic Tetramers: Circular Dichroism and Multipole Effects

Chiral metal nanoparticle assemblies exhibit plasmonic circular dichroism (CD) in the visible spectral interval. It was found previously that the circular dichroism signals can be induced by dipolar interactions between nanoparticles in a chiral assembly. In order to enhance plasmonic circular dichroism response, one can take advantage of multipole effects and anisotropy of nanostructures. We calculate the plasmonic circular dichroism of several nanoparticle (NP) assemblies using the interacting point-dipole approach and the purely numerical method based on the discrete dipole approximation (DDA). We found that the multipole effects revealed by the DDA calculations are crucial to describe and understand CD responses of tightly packed assemblies. The chiral equilateral tetrahedral 4-NP complexes are especially interesting because they do not have the dipolar contribution to the CD signal. Therefore, CD signals of equilateral tetrahedral 4-NP complexes originate solely from the multipole interactions. The strength of CD signals rapidly decreases with the particle-particle distance as 1/R-9.7 for the helices and as 1/R-18.1 for the equilateral tetrahedral 4-NP complexes, where R is a particle-particle distance. We show that the CD spectra are much more sensitive to the geometry of a plasmonic complex compared to the extinction spectra. Small variations in geometry can result in large changes in CD responses. This study can be used to design nanostructures with strong CD for optical and sensor applications.