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Computational modelling of photosynthetic excitons coupled to a microcavity

Turunen, Ilmari LU (2022) FYTM03 20212
Department of Physics
Computational Biology and Biological Physics - Undergoing reorganization
Department of Astronomy and Theoretical Physics - Undergoing reorganization
Abstract
Cavity quantum electrodynamics (QED) is a theory that is used for describing the interaction between matter and cavity modes. Not until relatively recently, cavity QED has been extended to the study of photosynthetic light-harvesting complexes (LHCs). Multiple phenomena unique to strongly cavity-coupled molecular systems have also been observed in LHCs in cavities. Of such phenomena, one is the formation of highly coherent states delocalized among thousands of molecules. These states have a partial light character, and are known as molecular polaritons. In this thesis, I explore the formation of polaritons by computationally modelling the light-harvesting complex 2 (LH2) of the purple bacterium Rhodopseudomonas acidophila in a microcavity.... (More)
Cavity quantum electrodynamics (QED) is a theory that is used for describing the interaction between matter and cavity modes. Not until relatively recently, cavity QED has been extended to the study of photosynthetic light-harvesting complexes (LHCs). Multiple phenomena unique to strongly cavity-coupled molecular systems have also been observed in LHCs in cavities. Of such phenomena, one is the formation of highly coherent states delocalized among thousands of molecules. These states have a partial light character, and are known as molecular polaritons. In this thesis, I explore the formation of polaritons by computationally modelling the light-harvesting complex 2 (LH2) of the purple bacterium Rhodopseudomonas acidophila in a microcavity. I also show the possibility of energy transfer between noninteracting LH2s via coupling to a cavity, providing a potentially new way in which photosynthetic energy transfer pathways could be artificially optimized. (Less)
Popular Abstract
The world around us is a colorful place to behold. Behind this colorful splendor is a fine interplay between light and matter, in which some materials absorb certain wavelengths of light which we perceive as colors that other materials do not. Absorption of light can be properly understood through the framework of quantum mechanics: in an absorption event, a quantum of light -- a photon -- is absorbed by electrons in a molecule, after which the molecule gains the energy of the absorbed photon. The molecule is then said to be in an excited state. In the case of plants and algae and some microbes that are capable of photosynthesis, the characteristic green hue is caused by special light-absorbing pigments, the chlorophylls. The wavelengths... (More)
The world around us is a colorful place to behold. Behind this colorful splendor is a fine interplay between light and matter, in which some materials absorb certain wavelengths of light which we perceive as colors that other materials do not. Absorption of light can be properly understood through the framework of quantum mechanics: in an absorption event, a quantum of light -- a photon -- is absorbed by electrons in a molecule, after which the molecule gains the energy of the absorbed photon. The molecule is then said to be in an excited state. In the case of plants and algae and some microbes that are capable of photosynthesis, the characteristic green hue is caused by special light-absorbing pigments, the chlorophylls. The wavelengths of light that are absorbed by these pigments are crucial for making photosynthesis and consequently most of life on earth possible.

Multiple chlorophyll molecules close to each other can interact as a group, leading to striking changes in the wavelengths of light that are absorbed. In such circumstances it is no longer possible to determine with certainty which of the chlorophylls partake in the absorption event -- it is as if all of the chlorophylls act as a single excited entity. Such collective quantum states of matter are called excitons. It turns out that there is a photosynthetic group of bacteria, the purple bacteria, which harness a special light absorbing structure called LH2. The wavelengths that the LH2 absorbs can only be explained using the theory of excitons, giving a hint of the importance of quantum mechanics in biology.

The story does not end with excitons, however. Over the last few years, researchers have become increasingly more interested in investigating the properties of pigments in cavities, that is, between two mirrors. When light of suitable wavelength is shone into the cavity, the light not only excites the molecules in the cavity but also partakes in the excited state itself. Such an excited state, which is partially light and partially matter, is known as a polariton. A polariton is a highly delocalized state, meaning that all of the excited molecules in the cavity are a part of it. It is like a tensed bowstring, which remains tensed by the action of light.

In my thesis, I explore these light-matter states in the case of the LH2 of the purple bacterium Rhodopseudomonas acidophila. I explain the observed absorption properties of the LH2 inside a cavity with computational models. I also take a step further and investigate what happens after the excitation of polaritons: how do they finally break down and where does their energy transfer? It is already known that polaritons harness many interesting properties, such as the capability of enhancing chemical reactions. Furthermore, the quantum properties of groups of molecules are known to be enhanced in cavities. In addition to validating more well known properties of polaritons involving LH2s, I show that the delocalized nature of the polaritons enable energy to be transferred between spatially separated LH2s. My thesis might thus offer tools for not only obtaining a clearer understanding on the significance of quantum mechanics in biology, but also for potential applications related to utilizing photosynthesis -- perhaps in solar powered technologies of the future. (Less)
Please use this url to cite or link to this publication:
author
Turunen, Ilmari LU
supervisor
organization
course
FYTM03 20212
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Cavity QED, LH2, Chlorophyll, Exciton, Polariton, Transition dipole moment, Absorbance
language
English
id
9072305
date added to LUP
2022-01-26 09:52:08
date last changed
2022-01-26 09:52:08
@misc{9072305,
  abstract     = {{Cavity quantum electrodynamics (QED) is a theory that is used for describing the interaction between matter and cavity modes. Not until relatively recently, cavity QED has been extended to the study of photosynthetic light-harvesting complexes (LHCs). Multiple phenomena unique to strongly cavity-coupled molecular systems have also been observed in LHCs in cavities. Of such phenomena, one is the formation of highly coherent states delocalized among thousands of molecules. These states have a partial light character, and are known as molecular polaritons. In this thesis, I explore the formation of polaritons by computationally modelling the light-harvesting complex 2 (LH2) of the purple bacterium Rhodopseudomonas acidophila in a microcavity. I also show the possibility of energy transfer between noninteracting LH2s via coupling to a cavity, providing a potentially new way in which photosynthetic energy transfer pathways could be artificially optimized.}},
  author       = {{Turunen, Ilmari}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Computational modelling of photosynthetic excitons coupled to a microcavity}},
  year         = {{2022}},
}