Tuning the Coherent Propagation of Organic Exciton-Polaritons through the Cavity Q-factor

TL;DR

This study demonstrates that the Q-factor of an optical cavity influences the transition from ballistic to diffusive polariton propagation in organic materials, enabling control over exciton transport dynamics.

Contribution

The paper reveals how the cavity Q-factor affects polariton propagation by controlling the lifetime of bright states, bridging the gap between lifetime and group velocity effects.

Findings

Propagation initially dominated by ballistic motion at group velocities.

Cavity decay and dark state population transfer cause wavepacket contraction.

Propagation continues as a slower diffusion process, tunable by Q-factor.

Abstract

Transport of excitons in organic materials can be enhanced through polariton formation when the interaction strength between these excitons and the confined light modes of an optical resonator exceeds their decay rates. While the polariton lifetime is determined by the Q(uality)-factor of the optical resonator, the polariton group velocity is not. Instead, the latter is solely determined by the polariton dispersion. Yet, experiments suggest that the Q-factor also controls the polariton propagation velocity. To understand this observation, we performed molecular dynamics simulations of Rhodamine chromophores strongly coupled to Fabry-P\'erot cavities with various Q-factors. Our results suggest that propagation in the aforementioned experiments is initially dominated by ballistic motion of upper polariton states at their group velocities, which leads to a rapid expansion of the wavepacket. Cavity decay in combination with non-adiabatic population transfer into dark states, rapidly depletes these bright states, causing the wavepacket to contract. However, because population transfer is reversible, propagation continues, but as a diffusion process, at lower velocity. By controlling the lifetime of bright states, the Q-factor determines the duration of the ballistic phase and the diffusion coefficient in the diffusive regime. Thus, polariton propagation in organic microcavities can be effectively tuned through the Q-factor.