The Struggles of Light Bound in Matter: Modelling Optical Excitations in Nanostructures

This thesis addresses three different topics on the interaction between light and matter in open quantum systems. The first topic concerns two-dimensional spectroscopy and the spectral contributions from different nonlinear action signals with a focus on how to differentiate between them. The second topic concerns the thermalization of excited electrons in hot-carrier solar cells with a focus on how to optimize the quantum efficiency of extraction.
The third topic concerns quantum heat engines with two related foci: the fundamental nature of heat and work flow in quantum thermodynamics, as well as the relation between quantum-coherent dynamics and a thermodynamic quantum advantage compared to classical model systems. The thesis comprises four papers.

In Paper I we identify the qualitative differences between the ''true'' nonlinear spectral contributions and nonlinear incoherent mixing signals.

In Paper II we model the thermalization of excited carriers in a hot-carrier solar cell and quantify the second order Coulomb scattering in the systems.

In Paper III we compare two different definitions of work and heat flow in a three-level maser and salvage the second law of thermodynamics in the conventional definitions of work and heat.

In Paper IV we investigate the advantage of a quantum model over its classical counterpart in terms of the Thermodynamic Uncertainty Relation and illustrate how the steady-state quantum coherence is insufficient to describe the nature of this advantage.