Abstract
Nonlinear optical processes provide a valuable technique for probing properties of chiral structures, i.e. those lacking mirror symmetry.Such structures are encountered frequently throughout organic chemistry, pharmacology, and biology, with a recent growing emphasis on sensitive characterisation of small quantities of chiral molecules.
It has been previously shown that chiroptical effects such as circular dichroism and optical rotation are significantly enhanced in second-harmonic generation.
Additionally, plasmonic nanostructures can create local regions of high intensity chiral electromagnetic fields that can further enhance chiral-optical (chiroptical) interactions with molecules attached to the material surface.
In this thesis, chiroptical and nonlinear optical properties of effective dielectric media are reviewed in the context of both natural materials and plasmonic metamaterials.
Using these principles, three novel chiral analysis schemes are demonstrated experimentally. Each experiment is designed for the characterisation of a unique plasmonic nanomaterial, all making use of enhanced nonlinear chiroptical effects.
First, plasmonic structures with dimensions comparable to the wavelength of incident light are studied. In these structures, electric field hotspots form, leading to small regions of enhanced chiral and nonlinear optical interactions. A theoretical description making use of a modal analysis and group theory is found to be a very good match to experimental results. Following this, the effects of anisotropy on nonlinear chiroptical measurements of a planar nanomaterial are examined. The structures are of sub-wavelength dimensions, however exhibit strong anisotropy that can contribute to chiroptical effects. It is found that specific experimental configurations can be exploited to separate the contributions from structural chirality and anisotropy, allowing pure chiral information to be obtained from highly anisotropic materials. Finally, anisotropic nanostructures are dispersed in liquid, allowing the study of orientationally-isotropic samples. In this case, nonlinear scattering is measured, and found to exhibit strong chiroptical effects. This is the first experimental report of circular intensity difference in second-harmonic scattering, and demonstrates significantly enhanced sensitivity when compared to widely used, linear characterisation techniques.
By designing new experimental schemes considering the unique challenges associated with each nanomaterial geometry, previously unobserved optical properties of the materials are revealed. Such tailored experimental methods pave the way for the further optimisation of chiral nanomaterials designed for applications in nanorobotics, photonic devices, and chemical sensing.
| Date of Award | 4 Jan 2019 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Ventsislav Valev (Supervisor) & Daniel Wolverson (Supervisor) |