Modelling optical and plasmonic responses of 2D materials

Abstract

Bidimensionnal (2D) materials, which are one or a few atoms thick, have been intensively studied since their discovery in 2004 for their unique electronic and optical properties, allowing to foresee applications in various fields. For example, graphene as a transparent conductive material could be integrated in electro-optical devices such as solar panels or smartphones. On the other hand, some transition metal dichalcogenides such as molybdenum diselenide are 2D semiconductors that could replace conventional semiconductors. In these 2D materials, collective electron excitations (plasmons) can also be observed. These phenomena of resonance between light and free electrons are at the origin of an intense confinement of the electromagnetic energy around 2D nanostructures. Such localization opens the path to design optical waveguides much smaller than those in glass or to create highly sensitive biosensors.

To study the optical properties of 2D materials and in particular plasmons, many electrodynamic methods adapted to tri-dimensional materials are commonly used. However, questions arise on the adequate modelling of these atomically thin materials. Should they be considered as homogeneous film with finite thickness or as an infinitely thin sheet? Does the anisotropy of 2D materials play a determining key role in their optical response? In this thesis, some answers are brought by comparing analytically and numerically the different models used. In particular, it is shown that isotropic models are not very well adapted and that anisotropic models of finite and infinitely thin thickness are relatively similar as long as the phase shift of the wave in the 2D film is not too important.

On the other hand, nanostructures of 2D materials can be studied by a quantum approach, considering the real atomic structure of the material and solving the Schrödinger equation in an approximate way. The microscopic dielectric function obtained from these calculations allows to study plasmons in these nanostructures. In this thesis, it is shown that corrugated graphene can sustain plasmons associated with the modification of the topology. Such plasmonic resonances allows these surfaces to exalt the optical response of some molecules such that they can be detected even in extremely small quantities. It is also theoretically proven in the thesis that plasmons can propagate in molybdenum diselenide grain boundaries. The characterization of these materials and the determination of the number of linear defects would thus be possible by the observation of plasmonic resonances.

Date of Award	2022
Original language	English
Awarding Institution	University of Namur
Sponsors	University of Namur
Supervisor	Luc Henrard (Supervisor), Olivier Deparis (Co-Supervisor), Robert Sporken (President), Benoit Hackens (Jury), Jean-Christophe Charlier (Jury) & Pascal Kockaert (Jury)