Nonequilibrium steady-state picture of incoherent light-induced excitation harvesting

TL;DR

This paper develops a theoretical framework and numerical scheme to analyze the nonequilibrium steady state of excitation harvesting in molecular aggregates under weak incoherent light, revealing how trapping time influences coherence and excitation distribution.

Contribution

It introduces a comprehensive model and numerical method to describe excitation harvesting in molecular systems under incoherent light, linking steady-state and time-dependent behaviors.

Findings

Long trapping times lead to NESS similar to excited-state equilibrium.

Stationary coherences originate from excitation-environment entanglement.

Properties of NESS can be derived from time-dependent dynamics for certain conditions.

Abstract

We formulate a comprehensive theoretical description of excitation harvesting in molecular aggregates photoexcited by weak incoherent radiation. An efficient numerical scheme that respects the continuity equation for excitation fluxes is developed to compute the nonequilibrium steady state (NESS) arising from the interplay between excitation generation, excitation relaxation, dephasing, trapping at the load, and recombination. The NESS is most conveniently described in the so-called preferred basis in which the steady-state excitonic density matrix is diagonal. The NESS properties are examined by relating the preferred-basis description to the descriptions in the site or excitonic bases. Focusing on a model photosynthetic dimer, we find that the NESS in the limit of long trapping time is quite similar to the excited-state equilibrium in which the stationary coherences originate from the excitation-environment entanglement. For shorter trapping times, we demonstrate how the properties of the NESS can be extracted from the time-dependent description of an incoherently driven but unloaded dimer. This relation between stationary and time-dependent pictures is valid, provided that the trapping time is longer than the decay time of dynamic coherences accessible in femtosecond spectroscopy experiments.