Plasmon Hybridization In Nanorhombus Assemblies

Understanding resonant coupling in plasmonic nano- assemblies is a challenging scientific endeavor, especially for particles with complex nano-architectures. Our ability to both model and measure this optical behavior, however, has rapidly developed in the last 20 years via a confluence of fabrication, spectroscopy, and theoretical analysis. Here, we precisely nanofabricate, characterize, and model the coupling and infrared optical responses of different plasmonic nanorhombus assemblies. Ranging from a monomer to a pentamer ensemble, experimental and simulated point spectra, spectrum images, and near-field maps agree well with the results of an analytical coupled normal mode model developed here. The analytical model reveals that the infrared optical responses of the nanorhombus systems can be explained by the coupling of the major and minor axis dipole and a quadrupole localized surface plasmon modes arising from the individual nanorhombus monomers. This model can be used to predict the plasmonic behavior of more complicated systems, and it elucidates the role that short-, intermediate-, and far-field coupling effects play in extended plasmonic assemblies more generally. This work also highlights how localized electron probes in the new generation of monochromated aberration-corrected scanning transmission electron microscopes can be used to study the optical responses of nanofabricated assemblies in the infrared spectral regime.