Exciting Bright and Dark Eigenmodes in Strongly Coupled Asymmetric Metallic Nanoparticle Arrays

The strong coupling between planar arrays of gold and silver nanoparticles mediated by a near-field interaction is investigated both theoretically and experimentally to provide an in-depth study of symmetry breaking in complex nanoparticle structures. The asymmetric composition allows to probe for bright and dark eigenmodes, in accordance with plasmon hybridization theory. The strong coupling could only be observed by separating the layers by a nanometric distance with monolayers of suitably chosen polymers. The bottom-up assembly of the nanoparticles as well as the stratified structures themselves gives rise to an extremely flexible system that, moreover, allows the facile variation of a number of important material parameters as well as the preparation of samples on large scales. This flexibility was used to modify the coupling distance between arrays, showing that both the positions and relative intensities of the resonances observed can be tuned with a high degree of precision. Our work renders research in the field of “plasmonic molecules” mature to the extent that it could be incorporated into functional optical devices

Plasmon Coupling in Self-Assembled Gold Nanoparticle-Based Honeycomb Islands

Metallic nanostructures that sustain plasmonic resonances are indispensable ingredients for many functional devices. Whereas structures fabricated with top-down methods entail the advantage of a nearly unlimited control over all plasmonic properties, they are in most cases unsuitable for a low cost fabrication on large surfaces; and eventually a truly nanometric size domain is difficult to reach due to limitations in the fabrication resolution. Although ordinary bottom-up techniques based on colloidal nanolithography promise to lift these limitations, they often suffer from their incapability to self-assemble nanoparticles at large surfaces and at a density necessary to observe effects that strongly deviate from those of isolated nanoparticles. Here, we rely on the application of sequential bottom-up fabrication steps to realize honeycomb structures from gold nanoparticles that show strong extinction bands in the near-infrared. The extraordinary properties are only facilitated by densely packing the nanoparticles into clusters with a finite size; causing the clusters to act as plasmonic macromolecules. These strongly interacting bottom-up materials with a deterministic geometry but fabricated by self-assembly might be of use in future sensing applications and in material platforms to mediate strong light–matter-interactions