Seeing double: coupling between substrate image charges and collective plasmon modes in self-assembled nanoparticle superstructures.

The interaction between adjacent metal nanoparticles within an assembly induces interesting collective plasmonic properties. Using dark-field imaging of plasmon scattering, we investigated rings of gold nanoparticles and observed that the images were dependent on the substrate. In particular, for nanoparticles assembled on carbon and gold substrates, intensity line sections perpendicular to the ring revealed a significant broadening beyond the optical resolution accompanied by an intensity dip in the middle of the line profile. Overall, this appeared in the image as a "splitting" into two offset circles along the direction of the scattered light polarization. This effect was not observed for a substrate with a low permittivity, such as glass. By varying the substrate as well as selecting different detected wavelengths and polarization components of the excitation light, we were able to confirm that the observed effect was due to coupling of collective plasmon modes with their induced image charges in the supporting substrates. These results suggest that plasmon scattering in extended nanostructures can be spatially modulated by tuning the permittivity of the substrate.