title: Plasmonic and all-dielectric chiral metamaterials for biomolecule detection
Creators: name: Koyroytsaltis-McQuire, Dominic
Publisher: name: University of Glasgow
Year: 2024

description: Metamaterials have proven themselves revolutionary as a means of bestowing novel properties to materials, for use in a variety of applications. One such area of focus is the enhancement of chiral light-matter interactions. Chirality is a property observed throughout the natural world, which has important consequences in health, environmental and pharmaceutical fields, to name but a few. Metamaterials have been used to enhance the detection of chiral molecules given the inherent weakness in traditional sensing methods. In this thesis, two forms of metamaterial platforms and their optical properties are investigated through a combination of experiment and numerical simulations.

Firstly, the CD spectra of plasmonic gammadions are rationalised and the sensitivity of each resonance to ordered and disordered biomolecule deposits are explored. It is determined that the spectra exhibit localised and delocalised (periodic) resonances. Both types of resonance are shown to display enantiomeric sensitivity, however, only the localised modes show a dependence on the structure of the deposited chiral biomolecular layer. This is reconciled through electromagnetic field analysis, which indicated that the presence of birefringent (ordered) chiral layers can perturb the gammadion’s chiral near fields and act as sources/sinks of optical chirality. As a result, the asymmetric perturbations to the coupling between the nanostructure’s arms are reflected in the CD spectra of the metamaterials.

The second form of metamaterial investigated is composed of silicon S-shaped structures, fabricated to four different heights between 160-240 nm. Both enantiomorphic and racemic arrays are fabricated, and their reflectance and ORD spectra studied. The sources of their spectral features are determined through a numerical method known as multipole decomposition, which is also used to explain the height dependence of the silicon on the spectra. It is determined that at lower silicon thicknesses the magnetic dipole contribution dominates and at higher thicknesses, the changes to their reflectance arise from red shifting of multipole contributions and an increase in magnetic quadrupole character. In addition to traditional Stokes polarimetry, Mueller matrix polarimetry was also performed on the samples. This method allows for a full characterisation of the optical properties of the samples, which were found to be bianisotropic in nature. The MMP data confirmed that the large optical rotations observed in the ORD measurements were a result of the linear birefringence of the samples, highlighting the limitations in standard polarimetry methods which often cannot distinguish between chiral and linear effects.

The sensing capabilities of the silicon structures were also investigated, revealing their refractive index response to be weak. Significant changes to the reflectance and MMP spectra are, however, observed after a layered biomolecule deposition. Numerical simulations were performed, where the layer was approximated as a dipole, to determine if the observed behaviour was a result of the biomolecular charge. Some qualitative agreement in the simulated reflectance spectra was observed, and field analysis revealed that the presence of the charged chiral layer produced an asymmetric change in the field properties of the structures, consistent with the observed MMP data. The model, however, broke down under orthogonal light polarisation, indicating that further refinement is required to confirm the source of the experimental results.