Disentangling enhanced diffusion and ballistic motion of excitons coupled to Bloch surface waves with molecular dynamics simulations

Abstract

Placing an organic material on top of a Bragg mirror can significantly enhance exciton transport. Such enhancement has been attributed to strong coupling between the evanescent Bloch surface waves (BSW) on the mirror, and the excitons in the material. In this regime, the BSW and excitons hybridize into Bloch surface wave polaritons (BSWP), new quasi-particles with both photonic and excitonic character. While recent experiments unveiled a mixed nature of the enhanced transport, the role of the material degrees of freedom in this process remains unclear. To clarify their role, we performed atomistic molecular dynamics simulations of an ensemble of Methylene blue molecules, a prototype organic emitter, strongly coupled to a BSW. In contrast to the established static models of polaritons, even with disorder included, our dynamic simulations reveal a correlation between the photonic content of the BSWP and the nature of the transport. In line with experiment, we find ballistic motion for polaritons with high photonic character, and enhanced diffusion if the photonic content is low. Our simulations furthermore suggest that the diffusion is due to thermally activated vibrations that drive population transfer between the stationary dark states and mobile bright polaritonic states.