Cavity-induced modifications of molecular structure in the strong coupling regime

TL;DR

This paper presents a first-principles model of molecules in strong cavity coupling, revealing limitations of traditional approximations and showing how collective effects influence molecular structure and observables.

Contribution

It introduces a comprehensive model accounting for electronic and nuclear degrees of freedom, challenging the Born-Oppenheimer approximation in strong coupling regimes.

Findings

Dark states are affected by collective strong coupling.

Ground-state properties depend on collective coupling strength.

Limitations of two-level models in describing rovibrational effects.

Abstract

In most theoretical descriptions of collective strong coupling of organic molecules to a cavity mode, the molecules are modeled as simple two-level systems. This picture fails to describe the rich structure provided by their internal rovibrational (nuclear) degrees of freedom. We investigate a first-principles model that fully takes into account both electronic and nuclear degrees of freedom, allowing an exploration of the phenomenon of strong coupling from an entirely new perspective. First, we demonstrate the limitations of applicability of the Born-Oppenheimer approximation in strongly coupled molecule-cavity structures. For the case of two molecules, we also show how dark states, which within the two-level picture are effectively decoupled from the cavity, are indeed affected by the formation of collective strong coupling. Finally, we discuss ground-state modifications in the ultra-strong coupling regime and show that some molecular observables are affected by the collective coupling strength, while others only depend on the single-molecule coupling constant.