Coherent and incoherent dynamics in excitonic energy transfer: correlated fluctuations and off-resonance effects

TL;DR

This paper develops a polaron-based master equation to analyze how environmental interactions, correlations, and energy mismatches influence the transition between coherent and incoherent energy transfer in coupled two-level systems.

Contribution

It introduces a unified theoretical framework that captures both weak and strong system-bath couplings, enabling detailed study of energy transfer regimes and environmental effects.

Findings

Correlation in fluctuations can protect coherence during transfer.

Increasing temperature induces a crossover from coherent to incoherent transfer.

Energy mismatch suppresses coherence and alters transfer dynamics.

Abstract

We study the nature of the energy transfer process within a pair of coupled two-level systems (donor and acceptor) subject to interactions with the surrounding environment. Going beyond a standard weak-coupling approach, we derive a master equation within the polaron representation that allows for investigation of both weak and strong system-bath couplings, as well as reliable interpolation between these two limits. With this theory, we are then able to explore both coherent and incoherent regimes of energy transfer within the donor-acceptor pair. We elucidate how the degree of correlation in the donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the range of the environment frequency distribution impact upon the energy transfer dynamics. In the resonant case (no energy mismatch) we describe in detail how a crossover from coherent to incoherent transfer dynamics occurs with increasing temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also explore how fluctuation correlations are able to protect coherence in the energy transfer process. We show that a strict crossover criterion is harder to define when off-resonance, though we find qualitatively similar population dynamics to the resonant case with increasing temperature, while the amplitude of coherent population oscillations also becomes suppressed with growing site energy mismatch.