Nanoscale metamaterials tailored for optical and mechanical applications

Abstract

Metamaterials have been exploited to show a number of exotic effects, in particular for longer wavelengths, from infrared to microwaves. Extending their response to shorter wavelengths requires structuring on the nanoscale which is made possible with increasing advances of fabrication techniques. The rigid pattern of metamaterials, however, meant that their response can only be observed for a narrow wavelength range. The aim of this project was to extend the functionality of metamaterials to manipulate visible and infrared light and to demonstrate wavelength tuneability. Utilising liquid crystals, with their attractive optical properties and easily controllable nature, was the main method towards achieving adaptive metamaterials. While typically the optical properties of liquid crystals are employed in applications, this work went beyond that and exploited their elastic properties. A liquid crystal layer, coupled to a mechanical metasurface, was shown to remove the limits of the stiction forces present at the nanoscale. New liquid crystal loaded metamaterials, made of nanostructured zigzag bridges, were fabricated and characterised to better understand the interactions taking place and to improve the functionality of future devices. The zigzag design was then explored separately in a project investigating its selective dependence on the spatial coherence of light. Computational modelling of the geometry was first completed and then compared with the experimental results. A large discrepancy between the experimental and modelled spectra for the zigzag metamaterial design was found, namely a split resonance being experimentally observed while the model predicted a single resonance peak. The split resonance was then successfully simulated for the case of incoherent incident light. In order to understand this feature further, variations of the zigzag geometry were fabricated and analysed. The final nanoscale design explored for investigating the optical properties of metamaterials included arrays of asymmetric slits. Optical activity upon reflection from a metasurface with equivalent, larger slits was demonstrated in the earlier work for microwave wavelengths. Samples with pairs of both symmetric and asymmetric slits were fabricated to obtain data from both reference and active samples. The presence of optical activity was then demonstrated for the asymmetric samples at optical wavelengths, in line with the theoretical predictions. The nanostructured metamaterials simulated, fabricated and experimentally characterised for this thesis, contribute to demonstrating the exciting potential of nanoscale metamaterials for photonic components and other groundbreaking technologies.

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Identifiers

ORCID for Thomas Frank: orcid.org/0000-0003-4677-4055

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