Light-Matter Interactions from First Principles

Our understanding of light-matter interactions enables a wide range of the technologiesthat define modern life. Examples include lasers, solar cells and light emitting diodes, all of which are likely to play a key role in our transition towards a more sustainable society. In this thesis, light-matter interactions are studied using first-principles quantum mechanical calculations. It is demonstrated how such calculations can be used to both interpret novel experimental results and actively drive materials discovery for opto-electronic applications. The thesis both presents new methodologies, as well as novel applications of existing methodologies.
The quantum nature of light has traditionally been neglected in the context of condensed matter physics and quantum chemistry. However, it can become important when the electromagnetic environment is structured in such a way as to host resonant or confined modes of light. A particularly interesting version of this is how resonant electromagnetic environments can be used to alter the electronic structure of materials, even in the absence of actual light. This happens as a result of the altered quantum fluctuations of the electromagnetic field, and the effect is directly related to the fundamental quantum nature of both light and matter. This thesis presents a novel methodology combining Macroscopic Quantum Electrodynamics with different flavors of Density Functional Theory. These results represent a step towards a fully first-principles description of quantum light-matter interactions in arbitrary electromagnetic environments.
Cavity-QED with color centers in 2D hexagonal Boron Nitride is also investigated. This investigation is carried out using a combination of the tensor network based TEMPO algorithm for time propagation of open quantum systems and a first-principles description of the electron-phonon coupling. This investigation shows that the fine structure of the electron-phonon coupling’s spectral density manifests as clear signatures of non-Markovianity in the dynamics of the coupled cavity-defect system. Furthermore, clear signs of hybridisation between the light-matter polaritons and the phonon modes of the environment are found.
Another focus of this thesis is materials discovery for nano-photonic- and optoelectronic applications. Using a combination of high-throughput computational screening, accurate many-body perturbation theory, experimental synthesis as well as near-and far-field optical characterization, Boron-Phosphide is identified as an overlooked high-performance material for near-UV dielectric nano-photonics.
The thesis also addresses the potential of indirect band gap semiconductors for photovoltaic applications. Combing a computationally efficient approximation of phononassisted new compounds with potential use in future thin-film solar cells are identified. New results from the Computational 2D Materials Database (C2DB) project are also presented. Comparison with full phonon calculations validates the Center and Boundary Protocol as a efficient tool to evaluate dynamical stability of relaxed 2D atomic structures. Furthermore, it is shown that the protocol can be iteratively applied to generate dynamically stable structures from unstable initial structures via systematic dislocations of the lattice. Finally, the existing data in the C2DB is harnessed to train a machine learning model for dynamical stability prediction. This model performs very well, showing an excellent receiver operating characteristic (ROC) curve with an area under the curve (AUC) of 0.9. The results show that the integration of such classification models into the workflows can drastically reduce computational efforts in high-throughput studies.
Furthermore, the elementary electronic excitations of layered PtSe₂ were unravelled using both momentum-resolved Electron Energy Loss Spectroscopy and first-principles calculations. Additionally, it was shown how to turn theoretical considerations about the momentum-dependence of the Coulomb interaction matrix elements into actionable spectroscopic insight for 2D materials. This could potentially lead to new possibilities in spectroscopy.
Finally, the thesis presents results from a large collaboration seeking to use van der Waals (vdW) heterostructures under illumination of electrons from a Transition Electron Microscope as a source of energy tunable X-ray radiation. It is shown that the energy of the emission can be tuned both via the incoming electron energy and the configuration of the heterostructure. This opens the door to highly tunable sources of X-ray radiation with very small physical footprints compared to existing technologies.