Light-harvesting efficiency cannot depend on optical coherence in the absence of orientational order

TL;DR

This paper demonstrates that optical coherence cannot be used to control light-harvesting efficiency in disordered systems or over long timescales, emphasizing the importance of orientational order for such control.

Contribution

It establishes that optical coherence control of light-harvesting efficiency is ineffective in disordered or long-timescale systems, clarifying the conditions for coherent control.

Findings

Optical coherence does not influence efficiency in disordered systems.

Polarization control is ineffective without orientational order.

Spectral-phase control is lost when averaging over long times.

Abstract

The coherence of light has been proposed as a quantum-mechanical control for enhancing light-harvesting efficiency. In particular, optical coherence can be manipulated by changing either the polarization state or spectral phase of the light. Here, we show that, in weak light, light-harvesting efficiency cannot be controlled using any form of optical coherence in molecular light-harvesting systems and, more broadly, those comprising orientationally disordered sub-units and operating on longer-than-ultrafast timescales. Under those conditions, optical coherence does not affect light-harvesting efficiency, meaning that it cannot be used for control. Specifically, polarization-state control is lost in disordered samples or when the molecules reorient on the timescales of the light-harvesting, and spectral-phase control is lost when the efficiency is time-averaged for longer than the optical coherence time. In practice, efficiency is always averaged over long times, meaning that coherent optical control is only possible through polarisation in systems with orientational order.