# Heuristic Modeling of Strong Coupling in Plasmonic Resonators

## TL;DR

This paper introduces a heuristic classical modeling approach for plasmonic strong coupling that incorporates excitonic line broadening, enabling better interpretation of experiments and design predictions involving single or multiple emitters.

## Contribution

The paper presents a simple, intuitive classical model that accounts for line broadening in plasmonic strong coupling, improving experimental analysis and design accuracy.

## Key findings

- Model accurately predicts spectra and emitter densities.
- Discriminates between systems capable of strong coupling and those that are not.
- Matches well with recent experimental results.

## Abstract

Strong coupling of plasmonic excitations and dipolar emitters, such as organic molecules, have been studied extensively in the last years. The questions whether strong coupling can be achieved with a single molecule only and how this is unambiguously proven are still under debate. A critical issue of plasmonic in contrast to photonic systems is additional excitonic line broadening, which is often neglected when modeling such systems. This has led to too optimistic design predictions or incorrect interpretation of ambiguous experimental data, for example in models relying on Maxwell solvers without self-consistent incorporation of line broadening effects. In this paper, we present a heuristic modeling approach for strongly coupled systems based on plasmonic nanoparticles and dipolar emitters that accounts for such broadening and elucidates on recent experiments with single emitters. We explicitly focus on a clear and intuitive classical description that utilizes well-established methods, easy to use within typical Maxwell solvers. The heuristic model (i) provides experimentally relevant numbers like emitter densities and spectra (ii) allows to discriminate systems, which can reach the strong coupling regime from those, which can not (iii) allows to identify optimization routes and (iv) nicely matches with experimental findings. In particular, we employ an approach related to quasi normal modes and extinction simulations where the excitonic system is represented by a frequency dependent permittivity. As examples, we investigate two configurations with many, but also single emitters, which have been studied in recent experiments.