The application of chiral metamaterials for sensing and active plasmonics

Gilroy, Cameron (2021) The application of chiral metamaterials for sensing and active plasmonics. PhD thesis, University of Glasgow.

Abstract

Chirality, the property of asymmetry, is ubiquitous in nature, observed in the handednesses of molecules to the twist of galaxies. This work concern artificial chiral metamaterials, periodic arrays of chiral metal nanostructures. The extraordinary properties that they exhibit are determined not only by the materials from which they are constructed, as in traditional materials, but by the size, shape and spacing of nanostructures which constitute them. The chirality of the structures mean that they strongly interact with the chiral manifestation of light, circularly polarised light. It is this interaction that is the theme that runs through this work. Two applications of chiral metamaterials are investigated here. The first is their use for the detection of chiral biological material. Arrays of gammadion nanostructures are created using state of the art electron beam lithography fabrication techniques in the James Watt Nanofabrication Centre. The effect of deformations to the nanostructure geometry and morphology on their interaction with circularly polarised light is investigated. In particular focus is the effect on the circular dichroism (CD) spectra and levels of enhanced optical chirality, a parameter which has been related to the enhancement of the interaction of light with chiral molecules. It is shown here that whilst geometric changes do not have a significant effect on CD spectra, surface roughness on the surface of the nanostructure can support optical chirality ‘hotspots’. These enhancements are, however, not as large as those observed for electric field enhancements commonly exploited for surface enhanced Raman spectroscopy. It is subsequently shown through numerical simulation that the sensing capabilities of the gammadion nanostructure is correlated to its thickness. This is reconciled within a framework of an interference mechanism for the “dissipation” of optical chirality into chiral material currents. The presence of a chiral dielectric can cause an asymmetric change to the phase difference between two spatially separated chiral modes resulting in asymmetric changes to their chiral optical properties. The second application is in the field of active plasmonics, which allows for the properties of metamaterials to be tuned post-fabrication by changing their size and shape using an external stimulus. Current approaches can require complex fabrication techniques which are inherently high cost. Here, a simple and novel approach is described which allows the chiral optical properties, namely the optical rotatory dispersion spectra, to be controlled electrically through a flexoelectric effect. The flexoelectric element, a lead zirconate titanate film, does not require high temperature processing and so can be integrated with polymer-based material.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
• Supervisor's Name: Kadodwala, Professor Malcolm
• Date of Award: 2021
• Depositing User: Theses Team
• Unique ID: glathesis:2021-82336
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 28 Jul 2021 11:40
• Last Modified: 28 Jul 2021 13:17
• Thesis DOI: 10.5525/gla.thesis.82336
• URI: https://theses.gla.ac.uk/id/eprint/82336

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