# Exciton-plasmon energy exchange drives the transition to strong coupling regime

## TL;DR

This paper introduces a model for exciton-plasmon interactions based on energy exchange, revealing how collective effects and local field enhancements drive the transition to strong coupling in plasmonic systems.

## Contribution

It provides an analytical framework linking QE-plasmon coupling to cooperative energy transfer and defines a universal saturation value for ensemble coupling.

## Key findings

- Ensemble QE-plasmon coupling saturates to a universal value.
- Analytical enhancement factor for single QE near a metal tip.
- Model explains transition to strong coupling in plasmonic structures.

## Abstract

We present a model for exciton-plasmon coupling based on an energy exchange mechanism between quantum emitters (QE) and localized surface plasmons in metal-dielectric structures. Plasmonic correlations between QEs give rise to a collective state exchanging its energy cooperatively with a resonant plasmon mode. By defining carefully the plasmon mode volume for a QE ensemble, we obtain a relation between QE-plasmon coupling and a cooperative energy transfer rate that is expressed in terms of local fields. For a single QE near a sharp metal tip, we find analytically the enhancement factor for QE-plasmon coupling relative to QE coupling to a cavity mode. For QEs distributed in an extended region enclosing a plasmonic structure, we find that the ensemble QE-plasmon coupling saturates to a universal value independent of system size and shape, consistent with the experiment.