Disentangling Enhanced Diffusion and Ballistic Motion of Excitons Coupled to Bloch Surface Waves with Molecular...

Placing an organic material on top of a Bragg mirror can significantly enhance the 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 quasiparticles 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 dye molecules strongly coupled to a BSW. The simulations reveal a correlation between the photonic content of the BSWP and the nature of the transport. In line with the 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 (i) excitation energy disorder of the molecules and (ii) thermally activated vibrations that drive population transfer between the stationary dark states and mobile bright polaritonic states. Importantly, the transition to diffusion at a low photonic content cannot be fully captured by static models of polaritons, underscoring the importance of dynamical effects (thermal disorder and nonadiabatic coupling) on transport of organic polaritons.