Quantum plasmonics: An energy perspective to quantify nonclassical effects

TL;DR

This paper introduces a nonclassical-impact parameter (NCI) to quantify nonclassical effects in plasmonic systems from an energy perspective, relating it to fundamental resonance parameters and comparing models.

Contribution

It proposes a new energy-based metric (NCI) to measure nonclassical effects in plasmonics, linking classical and quantum models and analyzing different microscopic levels.

Findings

NCI relates directly to loss function and quality factor.

NCI effectively characterizes nonclassical effects in plasmon waveguides.

The relation between hydrodynamic Drude model and RPA is formally established.

Abstract

Plasmons are commonly interpreted with classical electrodynamics, while nonclassical effects may influence the dynamics of plasmon resonances as the plasmon confinement is approaching the few--nanometer scale. However, an unambiguous approach to quantify the degree of nonclassical dynamics remains. We propose a nonclassical-impact parameter (NCI) to characterize the degree of nonclassical effects from an energy perspective, i.e. which fraction of the total electromagnetic energy is attributed to classical electrodynamic terms and which fraction is correspondingly to be assigned to nonclassical degrees of freedom? We show that the NCI relates directly to two fundamental parameters of plasmon resonances: the loss function and the quality factor. Guided by the NCI, we discuss the nonclassical effects of plasmon waveguiding modes of metallic slab waveguides, and highlight the general features of the nonclassical effects at different microscopic levels by contrasting the numerical results from the semi-classical hydrodynamic Drude model (HDM) and the microscopic random-phase approximation (RPA). The formal relation between the HDM and the RPA is also established for metals by exploring the limit of an infinite work function.