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

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.

💡 Research Summary

The authors investigate how placing an organic layer on a distributed Bragg reflector (DBR) and coupling it strongly to Bloch surface waves (BSWs) modifies exciton transport. Using a multiscale quantum mechanics/molecular mechanics (QM/MM) approach, they simulate 1 024 methylene‑blue (MeB) molecules in water interacting with a discretized BSW field composed of 120 photonic modes. The electronic ground and first excited states of each molecule are described by DFT/TDDFT (B97/3‑21G), while the light‑matter interaction is treated with a Tavis‑Cummings Hamiltonian. Nuclear motion follows Ehrenfest dynamics, allowing the classical nuclei to evolve on the expectation value of the quantum Hamiltonian.

To mimic the experimental pump‑probe conditions, a single MeB molecule at the centre of a 250 µm periodic DBR surface is initially promoted to its S₁ state, and five independent trajectories are propagated for 200 fs with a 0.5 fs timestep at 300 K (no decay is included because the simulated time is far shorter than the BSW lifetime). The total wavefunction is expressed both in a diabatic product basis (individual molecular excitations and empty photonic modes) and in the adiabatic eigenbasis of the Tavis‑Cummings Hamiltonian, enabling a time‑resolved decomposition of the polaritonic population into photonic and excitonic contributions.

Analysis of the probability density |Ψ(t)|² shows a rapid expansion of the polariton wave packet. The leading edge travels at the lower‑polariton group velocity (≈173 µm ps⁻¹), while a trailing tail remains near the excitation site, indicating a mixture of ballistic and diffusive behavior. To disentangle these contributions, the authors project the wavefunction onto narrow windows of in‑plane wavevector k_z, each characterized by a specific photonic weight. Wave packets with high photonic content (>~0.8) propagate essentially ballistically, maintaining the group‑velocity speed and exhibiting minimal spreading. In contrast, packets with low photonic content (<~0.2) display markedly slower propagation and pronounced spatial broadening, i.e., diffusive transport.

The key mechanistic insight is that thermal molecular vibrations induce non‑adiabatic population transfer between bright (photonic‑mixed) polariton states and dark (predominantly excitonic) states. This vibrationally driven exchange continuously repopulates bright states, providing a pathway for energy and momentum redistribution that manifests as enhanced diffusion when the photonic fraction is small. Thus, the crossover from ballistic to diffusive transport observed experimentally is reproduced and explained at the atomistic level as a vibrationally mediated process rather than a purely disorder‑driven effect.

The study demonstrates that the photonic fraction of Bloch‑surface‑wave polaritons governs the dominant transport regime: high photonic weight yields near‑lossless, fast ballistic motion, whereas low photonic weight makes the system vulnerable to vibrationally induced non‑adiabatic coupling, leading to diffusion. These findings provide a quantitative framework for designing organic‑photonic hybrid devices, suggesting that simultaneous control of cavity photonic properties and molecular vibrational environments can be used to tailor exciton‑polariton propagation distances and speeds.