Quantum theory of spontaneous and stimulated emission of surface plasmons

TL;DR

This paper develops a quantum framework for surface plasmon emission and interaction, accurately modeling spontaneous emission rates and clarifying the limitations of quantum approaches in lossy media.

Contribution

It introduces a general quantization scheme for surface waves on interfaces, deriving energy expressions without specific dielectric models and analyzing emission processes.

Findings

Quantum formalism matches Green's tensor results in non-lossy media.

Stimulated emission probability is negligible near the dispersion asymptote.

The approach clarifies limitations of quantum models for lossy surface plasmons.

Abstract

We introduce a quantization scheme that can be applied to surface waves propagating along a plane interface. An important result is the derivation of the energy of the surface wave for dispersive non-lossy media without invoking any specific model for the dielectric constant. Working in Coulomb's gauge, we use a modal representation of the fields. Each mode can be associated with a quantum harmonic oscillator. We have applied the formalism to derive quantum-mechanically the spontaneous emission rate of surface plasmon by a two-level system. The result is in very good agreement with Green's tensor approach in the non-lossy case. Green's approach allows also to account for losses, so that the limitations of a quantum approach of surface plasmons are clearly defined. Finally, the issue of stimulated versus spontaneous emission has been addressed. Because of the increasing density of states near the asymptote of the dispersion relation, it is quantitatively shown that the stimulated emission probability is too small to obtain gain in this frequency region.