Quantifying the Plasmonic Character of Optical Excitations in Nanostructures

The microscopic definition of plasmons in nanosystems is a tremendous challenge. Any sharp distinction of the excitation nature (nonplasmonic vs plasmonic) becomes blurred at the nanoscale, where quantum effects become important. Here we introduce the concept of plasmonicity index, i.e., a direct measure of the plasmonic character of the optical excitations in nanosystems. Its definition is based on a rigorous theoretical derivation, which leads to the physically sound result that the plasmonicity index is related to the capability of enhancing locally an applied electromagnetic radiation. The proposed expression is general and can be applied to any finite system. We show its usefulness in modeling metallic nanoparticles, prototypical C-based molecules, and paradigmatic hybrid systems, starting from first-principles calculations, based on (TD)DFT. Our results represent a step forward in the fundamental understanding of what a plasmon is in nanometer-sized particles and molecular systems.