The effects of strong environmental coupling on light-harvesting systems

Abstract

Inspired by the observation of long lived coherences in photosynthetic complexes, organic molecules have become prominent candidates for light-harvesting devices that utilise coherent effects to enhance efficiency. An example that we study in this thesis is dark state protection, whereby coherent effects lead to an eigenstate decoupling from the electromagnetic field. Excitons transferred to this dark state by vibrational processes cannot recombine and reradiate, breaking detailed balance and increasing efficiency.

Organic molecules typically have strong coupling to their vibrational environments. The central aim of this thesis is to understand the effects of this on the coherent efficiency enhancements procured by these light-harvesting systems. Moreover, it is known that systems with both strong vibrational coupling and weak light-matter coupling, typical of organic molecules in sunlight, display non-additive behaviour of the two environments. Therefore, the roles of vibrational coupling alone and in the presence of light-matter interactions are distinct.

In this thesis we theoretically study strong vibrational coupling effects using the standard polaron and variational polaron transformations in conjunction with Redfield theory. We do so in a number of topical light-harvesting systems: a single optical dipole in free-space; two coupled dipoles in free space, and hundreds of billions of dipoles in a cavity. Starting from first principles derivations of the Hamiltonians, with explicit discussion of the effects of gauge-relative approximations, we explore the role of realistically strong vibrational coupling on coherences and light-harvesting efficiency. Throughout, we relate the calculations to experimentally observable spectra to bridge the gap to experimental realisation. Similarly, we also design experiments where coherent efficiency enhancements could be unambiguously observed by controlling the coherence of the exciting light source.