The prospect of coherent dynamics and excitonic delocalization across several light-harvesting structures in photosynthetic membranes is of considerable interest, but challenging to explore experimentally. Here we demonstrate theoretically that the e
xcitonic delocalization across extended domains involving several light-harvesting complexes can lead to unambiguous signatures in the optical response, specifically, linear absorption spectra. We characterize, under experimentally established conditions of molecular assembly and protein-induced inhomogeneities, the optical absorption in these arrays from polarized and unpolarized excitation, and demonstrate that it can be used as a diagnostic tool to determine the coherent coupling among iso-energetic light-harvesting structures. The knowledge of these couplings would then provide further insight into the dynamical properties of transfer, such as facilitating the accurate determination of Forster rates.
The early steps of photosynthesis involve the photo-excitation of reaction centres (RCs) and light-harvesting (LH) units. Here, we show that the --historically overlooked-- excitonic delocalisation across RC and LH pigments results in a redistributio
n of dipole strengths that benefits the absorption cross section of the optical bands associated with the RC of several species. While we prove that this redistribution is robust to the microscopic details of the dephasing between these units in the purple bacterium Rhodospirillum rubrum, we are able to show that the redistribution witnesses a more fragile, but persistent, coherent population dynamics which directs excitations from the LH towards the RC units under incoherent illumination and physiological conditions. Stochastic optimisation allows us to delineate clear guidelines and develop simple analytic expressions, in order to achieve directed coherent population dynamics in artificial nano-structures.
Natural and artificial light harvesting processes have recently gained new interest. Signatures of long lasting coherence in spectroscopic signals of biological systems have been repeatedly observed, albeit their origin is a matter of ongoing debate,
as it is unclear how the loss of coherence due to interaction with the noisy environments in such systems is averted. Here we report experimental and theoretical verification of coherent exciton-vibrational (vibronic) coupling as the origin of long-lasting coherence in an artificial light harvester, a molecular J-aggregate. In this macroscopically aligned tubular system, polarization controlled 2D spectroscopy delivers an uncongested and specific optical response as an ideal foundation for an in-depth theoretical description. We derive analytical expressions that show under which general conditions vibronic coupling leads to prolonged excited-state coherence.
Based entirely upon actual experimental observations on electron-phonon coupling, we develop a theoretical framework to show that the lowest energy band of the Fenna- Matthews-Olson (FMO) complex exhibits observable features due to the quantum nature
of the vibrational manifolds present in its chromophores. The study of linear spectra provides us with the basis to understand the dynamical features arising from the vibronic structure in non-linear spectra in a progressive fashion, starting from a microscopic model to finally performing an inhomogenous average. We show that the discreteness of the vibronic structure can be witnessed by probing the diagonal peaks of the non-linear spectra by means of a relative phase shift in the waiting time resolved signal. Moreover, we demonstrate the photon-echo and non-rephasing paths are sensitive to different harmonics in the vibrational manifold when static disorder is taken into account. Supported by analytical and numerical calculations, we show that nondiagonal resonances in the 2D spectra in the waiting time, further capture the discreteness of vibrations through a modulation of the amplitude without any effect in the signal intrinsic frequency. This fact generates a signal that is highly sensitive to correlations in the static disorder of the excitonic energy albeit protected against dephasing due to inhomogeneities of the vibrational ensemble.
The statistics of photons emitted by single multilevel systems is investigated with emphasis on the nonrenewal characteristics of the photon-arrival times. We consider the correlation between consecutive interphoton times and present closed form expr
essions for the corresponding multiple moment analysis. Based on the moments a memory measure is proposed which provides an easy way of gaging the non-renewal statistics. Monte-Carlo simulations demonstrate that the experimental verification of non-renewal statistics is feasible.
The study of how photosynthetic organisms convert light offers insight not only into natures evolutionary process, but may also give clues as to how best to design and manipulate artificial photosynthetic systems -- and also how far we can drive natu
ral photosynthetic systems beyond normal operating conditions, so that they can harvest energy for us under otherwise extreme conditions. In addition to its interest from a basic scientific perspective, therefore, the goal to develop a deep quantitative understanding of photosynthesis offers the potential payoff of enhancing our current arsenal of alternative energy sources for the future. In the following Chapter, we consider the trade-off between dynamics, structure and function of light harvesting membranes in Rps. Photometricum purple bacteria, as a model to highlight the priorities that arise when photosynthetic organisms adapt to deal with the ever-changing natural environment conditions.
