Universal metric for <i>plasmonicity</i> of excitations at the nanoscale

A promising trend in plasmonics involves shrinking the size of plasmon-supporting structures down to a few nanometers, thus enabling control over light−matter interaction at extreme-subwavelength scales. In this limit, quantum mechanical effects, such as nonlocal screening and size quantization, strongly affect the plasmonic response, rendering it substantially different from classical predictions. For very small clusters and molecules, collective plasmonic modes are hard to distinguish from other excitations, such as single-electrons ones. Using rigorous quantum mechanical computational techniques for a wide variety of physical systems, we describe how the plasmonic character of a nanostructure’s optical resonance can be quantified. We define a universal metric, the generalized plasmonicity index (GPI), which can be straightforwardly implemented in any computational electronic-structure or classical electromagnetic approach to discriminate plasmons from single-particle excitations and photonic modes [ACS Nano, 11, 7321 (2017); PNAS, 115, 9134 (2018)]. The GPI metric deepens our fundamental understanding of what is a plasmon down to the molecular limit of plasmon-supporting nanostructures and provides a rigorous foundation for further development in molecular plasmonics.