Quantum-mechanical study of optical excitations in nanoscale systems: first-principles description of plasmons, tunneling-induced light emission and ultrastrong light-matter interaction

Abstract

This theoretical thesis applies quantum methodologies to nanophotonic systems in order to investigate the properties of optical excitations in metals, as well as the interaction of matter excitations with optical modes in cavities. Initially, we adopt a first-principles description of electrons in metals to analyze the properties of plasmonic excitations. Specifically, surface plasmons on the Pd(110) surface and in two-dimensional anisotropic metals are investigated. In the two-dimensional system, we notably find collective excitations with a linear dispersion that are called acoustic plasmons and that differ from the conventional plasmon. In the second part of the thesis, we focus on metal-insulator-metal tunneling junctions. We demonstrate the importance of considering the electronic wavefunctions in the full device to accurately model the excitation of plasmons by tunneling electrons and the resulting light emission. Last, we use the framework of cavity quantum electrodynamics to find the appropriate quantum description of the interaction of matter excitations with optical modes in different nanophotonic systems, and show the equivalences of these descriptions with classical models based on coupled harmonic oscillators. These harmonic oscillator models are also applied to analyze experimental results that demonstrate strong and ultrastrong coupling between phonons and infrared modes of a microcavity.