Unlocking Delocalization: How Much Coupling Strength can Overcome Energy Disorder in Molecular Polaritons?

We investigated the criteria to ensure delocalization exists in molecular polaritons – quasiparticles formed from the collective strong coupling of light and matter that have shown capabilities to modify chemical reactions. Importantly, delocalization, i.e., polaritons possess delocalized wavefunctions, is one of the hallmarks of polaritons which enables energy transport and chemical dynamics. Delocalization in polariton systems has been long assumed to be robust against energy disorder that is ubiquitous in real molecular systems. However, this study reveals that disorder destroys delocalization in polaritons. In order to mitigate the impact of disorder, a collective coupling strength exceeding 3 times the inhomogeneous linewidth is necessary. When the coupling strength is smaller, the delocalization properties of polaritons are significantly compromised both in a static picture and from a dynamic point of view. This observation indicates a more stringent criterion for preserving the unique characteristics of polaritons compared to the conventionally adopted standard (collective coupling strengths larger than photonic and molecular spectral linewidths). This work sheds lights on previous works to explain why the onset of modified dynamics is larger than the strong coupling criteria, and also serves as an important consideration for existing and future polariton studies involving high levels of energy disorder.