The role of dark and grey states in polaritonic photophysics

Abstract

Strong light-matter interactions give rise to hybrid states known as polaritons, which combine the properties of photons and molecular excitations. Remarkably, these interactions can alter the properties of organic molecules without any physical or chemical modification, simply through their intense coupling with light. In addition to bright polaritonic states, strongly coupled systems also contain dark states, which lack photonic character, and grey states, which possess only minute photonic character. This thesis explores several aspects of photophysics in strongly coupled light-matter systems. It places particular focus on the roles of dark and grey states. By comparing optical cavities of different mode orders but identical cavity energies, it was found that the intensity of lower polariton emission is inversely proportional to the number of dark states comprising the exciton reservoir. A kinetic model was developed to describe lower polaritonic emission quantum yield. This model revealed that the population of grey states increases with the energy overlap between the exciton reservoir and the lower polariton. Furthermore, a quantitative emission model was established by combining the source term and transfer matrix methods. This model successfully reproduces the spectral, angular, and polarization characteristics of experimental polaritonic emission. Together, these findings offer new insights into how dark and grey states shape energy relaxation and light emission in molecular polariton systems. They also provide predictive modeling frameworks for guiding the future design and optimization of polaritonic photonic devices.