Which resonances in small metallic nanoparticles are plasmonic?

TL;DR

This study uses time-dependent density functional theory to analyze the nature of optical resonances in small metallic nanoparticles, distinguishing plasmonic from non-plasmonic modes based on their charge oscillation characteristics.

Contribution

It provides a detailed characterization of resonances in small metallic nanoparticles, identifying features that determine their plasmonic nature, and discusses the stability of these features across simulation parameters.

Findings

Resonances have 'sloshing' and 'inversion' characters.

Charge oscillation patterns differentiate plasmonic from non-plasmonic modes.

Resonance character remains stable despite changes in simulation size.

Abstract

We use time-dependent density functional theory to examine the character of various resonances corresponding to peaks in the optical response of small metallic nanoparticles. Each resonance has both "sloshing" and "inversion" character. The sloshing mode is an oscillation in the occupation of the shells nearest the Fermi energy, transferring charge back and forth from below the Fermi level to above it. It results in oscillation in charge density near the surface of the particle. Inversions monotonically move charge from occupied to unoccupied states, and result in oscillation in charge density in the core of the particle. We also discuss the dependence of the optical response on the size of the simulation grid, noting that the character of resonances appears stable with respect to changes in simulation size, even though the details of the spectrum change. This makes a reliable characterization possible. We consider what characteristics are important in deciding that a resonance is plasmonic.