Optically driven nanostructures with strong vibronic coupling

Abstract

Many developments in modern physics rely on the understanding of quantum systems that are strongly coupled to complex, structured environments. In particular, quantum emitters such as single molecules and quantum dots are of great interest in many applications in photonics, solar energy and nano-electronics. In many of these situations, the traditional tools of quantum optics break down, requiring a non-perturbative approach to system-environment coupling. Understanding strongly-coupled open quantum systems is still an active area of research, with many powerful numerical and analytical techniques being developed. In this thesis, we attempt to combine the approaches of strongly-coupled open quantum systems with the study of natural and artificial solar energy conversion. Light-harvesting systems are highly non-equilibrium in nature, since they interact with multiple environments at different temperatures - a regime which is difficult to treat with many cutting edge open systems techniques. Starting from the simplest possible model, that of a single transition in a molecule, we uncover some common inconsistencies that get made in the literature and develop further the notion of \textit{environmental non-additivity}. We show that this effect, where the coupling of a quantum system to one environment affects how it couples to another environment, is a general property of many non-equilibrium models. We see that for sufficient phonon-coupling it can directly lead to population inversion in a two-level emitter under incoherent thermal driving. After analysing the two-level case, we then expand the model to include two interacting emitters. This builds on the previous work in the literature on excitonic energy transfer, in order to more realistically evaluate the effectiveness of these systems within a quantum photocell context. Using non-perturbative open systems techniques, we demonstrate a striking diversion from the weak-coupling theory in the resultant dynamics and steady-states. Finally, we derive a novel model for a molecular photocell, which allows us to explore the interplay between strongly-coupled vibrations, photon absorption and molecule-electrode coupling. This setting gives rise to an array of rich non-additive phenomena, which ultimately determines the photocell performance. We identify regimes where strong phonon-coupling enhances photocurrent.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Tobias Galla (Supervisor) & Ahsan Nazir (Supervisor)