Plasmonic Circular Dichroism of the Helical Nanosphere Assemblies and the Helical Nanoellipsoid Assemblies

We investigate the circular dichroism (CD) of both the helical gold nanosphere assemblies and the helical gold nanoellipsoid assemblies by means of the discrete dipole approximation (DDA). For the helical gold nanosphere assemblies, we find that, with increasing the number of the nanospheres within a helical pitch, the CD spectra of the assemblies are red-shifted and the intensity of the CD signals is strengthened. We also investigate the E-field distributions of the helical nanosphere assemblies. The results show that the strong CD response of the nanosphere assemblies can be ascribed to the collective dipole and multipole interactions between nanoparticles. The CD signals of the solid helical structure exhibit periodical behaviors in half pitch, which is quite different from that of the nanosphere assemblies. The helical nanoellipsoid assemblies exhibit complicated plasmon-induced CD signals at the localized surface plasmon resonance (LSPR) frequency. The strength and wavelength of the CD signals can be manipulated in the range of 525 to 688 nm via tuning the aspect ratio of the nanoellipsoids. These results are important for designing the chiral plasmonic nanostructures with strong and tailorable optical properties.