DiG for the future: taming disorder in self-assembled materials with topology

Abstract

Metasurfaces are engineered materials made of subwavelength nanostructures with unique properties and applications that can not be found in nature. For example, they can improve the harvesting of solar energy with potentially far-reaching effects on finding energy alternatives to fossil fuels. Their remarkable applications rest on the careful design and control of their structure, often at the nanometre scale where its components are only thousands of atoms in size. Such control requires complex, time-consuming and expensive techniques, limiting the development of large-scale metasurfaces with potential industrial applications. To circumvent those limitations, bottom-up approaches relying on the self-assembly of nanostructures were suggested. Although these methods allow for the larger-scale fabrication of metasurfaces, they offer little control over the position and orientation of the nanostructures, thus limiting the range of applications that can be obtained with these methods. Interestingly, while disorder is usually perceived as an unavoidable factor of metasurfaces’ fabrication methods, limiting their potential, some specific applications do benefit from it. The ambitious goal of this thesis is to investigate the link between the strength of the optical properties of metasurfaces and their structural disorder characterised using topology, whether it is emerging from their fabrication methods or specifically designed. We first focus on a type of metasurface made of self-assembled nanocubes deposited on a metallic substrate coated by a nanometre-thin dielectric layer. This setup exhibits particularly strong plasmonic-based optical modes with many applications based on the intense electromagnetic field localised underneath the nanocubes, such as creating ultra-fast quantum systems. In particular, we show how high refractive index dielectric nanocubes can extend the region of applications of standard metallic nanocubes to the blue near-UV, while reducing the inherent losses due to the Joule’s effect. Additionally, we demonstrate that the optical properties of this type of metasurfaces can be dynamically tuned upon including them into a thermotropic liquid crystal cell. Finally, we consider metasurfaces made of gold nanodisks, whose collective modes strongly depend on the positional disorder of the nanodisks. Using topology-inspired tools, we build a measure of disorder independent from a reference ordered lattice that is more accurate than standard statistical measures to describe correlated disordered metasurfaces. Using these tools, we were able to successfully design and fabricate metasurfaces with a controlled amount of disorder.

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Identifiers

ORCID for Tristan Madeleine: orcid.org/0000-0001-7655-8367

ORCID for Giampaolo D'Alessandro: orcid.org/0000-0001-9166-9356

ORCID for Jacek Brodzki: orcid.org/0000-0002-4524-1081

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