Title: Light-Matter Interactions in Low-Dimensional Materials
Supervisors
Principal supervisor: Assoc. Prof. Sanshui Xiao
Co-supervisor: Assoc. Prof. Nicolas Stenger
Co-supervisor: Prof. N. Asger Mortensen
Evaluation Board
Professor Peter Uhd Jepsen, DTU Fotonik
Assoc. Prof. Timur Shegai, Chalmers University, Göteborg
Professor Stefan Linden, University of Bonn
Master of the Ceremony
Assoc. Prof. Martijn Wubs
Abstract
The thesis you are about to read is the culmination of my three years as a PhD student in the Structured Electromagnetic Materials group at the Technical University of Denmark (DTU). Contained herein is a compilation of the results obtained from the research projects I have been involved in during these three years. The projects have been focused on experimental characterisation of the optical properties of plasmonic structures and two-dimensional (2D) materials, and especially how we can tailor these properties.
We begin with an introduction to the optical properties of metals. This leads us to the concept of collective electron excitations called plasmons. We especially focus on the localised surface plasmons hosted by nanoscale metallic structures, and we review their properties in single and coupled configurations. We conclude this part with a presentation of the experimental investigation into the interaction between spherical gold nanoparticles and single-crystalline gold substrates, which are known to host electronic surface states. We detect no change, however, in the optical properties from these states, when we compare to a polycrystalline reference substrate. We ascribe this to the weak effect induced by the surface states compared to other factors such as the surface roughness.
Following this, we introduce the family of 2D materials known as the transition metal dichalcogenides (TMDCs). We review their basic electronic and optical properties, and we explore the strong light-matter interaction in these atomically thin materials dominated by electron-hole complexes known as excitons. We also see how it is possible to theoretically model the optical response of such a 2D material on a substrate with the transfer-matrix method, and we use this to extract the dielectric permittivity of WS2 mono- and multilayers.
Having the basic theory in place, we demonstrate how it is possible to manipulate the emission energy of excitons in WSe2 monolayers, encapsulated between flakes of hexagonal boron nitride (hBN), by nanostructuring the dielectric environment. This is explained by the reduced binding energy of the excitons from the dielectric screening, and the change in the material band structure from the presence of the hBN. We find that the emission energy increases with decreasing screening from the hBN, an effect that appears to be dependent on whether the metal atom is molybdenum or tungsten.