Notararigo, Valentina; (2019) Quantum optical signatures of coherent vibronic dynamics in bio-inspired light harvesting systems. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The study of quantum phenomena in biology has received significant attention in the last decade. One of the problems of most interest is the understanding of quantum effects during the first steps of photosynthesis. Ultrafast two-dimensional electronic spectroscopy has revealed that pigment-protein complexes responsible for light- harvesting and charge separation in photosynthetic organisms can support quantum coherent dynamics in the excited state, for up to few hundreds of femtoseconds. The leading hypothesis on the mechanisms supporting this coherent behaviour is quan- tum interactions between electronic and some specific vibrational motions in the excited state. This hypothesis, however, awaits unambiguous confirmation. Among the most powerful techniques to investigate the quantum behaviour of an emitter is the study of quantum optical properties of the light it emits. This thesis de- velops theoretical studies showing that frequency-filtered and time-resolved photon counting statistics of the light emitted by a prototype photosynthetic unit can give important insight into the quantum coherent nature and the mechanisms underlying excited state dynamics in single photosynthetic complexes. By developing a pertur- bative and efficient approach to the computation of frequency- and time- resolved photon correlation functions, we show that such correlations have the potential to give unambiguous signatures of coherence contributions to the steady state emis- sion. For a light-harvesting unit emitting in free space, the signatures of excited state coherence manifest themselves as non-trivial antibunching. This feature can- not be probed by measuring unfiltered photon correlations. We then consider the situation in which a prototype energy transfer unit is embedded in an optical cavity such that its emission rate is enhanced. In this case we observed a rich behaviour of the frequency-filtered, second-order photon correlations that allows a clear distinc- tion of coherence contributions, and their variation, depending of the electronic and vibrational interactions in the system of interest.

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