Simplified description of an active medium in quantum plasmonics

TL;DR

This paper develops a simplified theoretical model for a four-level gain medium in quantum plasmonics, showing it can often be reduced to an effective two-level system under certain conditions.

Contribution

It introduces a Lindblad equation-based approach to model four-level systems and identifies conditions under which they can be simplified to two-level systems.

Findings

Four-level systems can be effectively reduced to two-level systems.

The reduction depends on the ratio of Rabi frequency to relaxation rate.

The simplified models reproduce key properties of the full four-level system.

Abstract

In order to simplify the theoretical description of spasers, a gain medium is commonly represented by a two-level system. A realistic model, however, should have four levels. By using the Lindblad equations we develop a description of such a system and show that depending on ratios of the Rabi frequency and the rate of relaxation of the polarization, a four-level system may be reduced to one of two effective two-level systems that reproduce the key properties of a four-level system.