Chirality in diffractive plasmonic nanostructures

Student thesis: Doctoral ThesisPhD

Abstract

Historically, chirality was mostly studied in molecules over which control is naturally limited. However, the ability to produce artificial, chiral structures with feature sizes smaller than the wavelength of light, allows to execute hitherto unseen control over geometrical parameters which govern the emergence and evolution of the research around chirality.

The presence of chirality in molecules can be measured with light. However, the resulting chiral optical response is typically very weak. Interestingly, it is possible to combine artificially created nanostructures and chiral molecules to enhance the chiral optical response of the latter. Nanostructures which are specifically tailored to enhance the response of a certain molecule are therefore needed.

However, such nanostructures also present a predicament: If the nanostructures themselves are chiral, they will overshadow the chiral optical response of the molecule. If the nanostructures are achiral (non-chiral) they cannot be spectrally tailored to the molecule as they do not yield any chiral optical response in the far-field.

A promising solution is offered by diffractive nanostructures which have been shown to yield extremely large responses. However, previous studies have been limited in scale and a physical explanation for the observed behaviour is still elusive. Developing an experimental apparatus to optically characterise various nanogratings, supported by numerical simulations and mathematical models, this thesis provides both an explanation and the means for chiral sensing in diffracted beams. The apparatus enables the detection of chirality through circular intensity measurements of the light diffracted by such nanogratings. Starting with a proof-of-principle study of higher-order diffraction spectroscopy, various nanograting designs and their far-field responses are investigated. Consequently, the potential of such nanogratings to enhance the response of chiral molecules is demonstrated. Of special interest are racemic nanogratings, i.e. consisting of an equal number of both chiral configurations. The reason for this lies in the fact that racemic nanogratings yield no net chiral optical response but, by means of diffraction spectroscopy, it is possible to unveil such a behaviour in the far-field which is the first time this has been reported. Although the far-field chiral optical response of a racemic nanograting accumulates to a net zero, individual diffracted beams do yield non-zero values and hence can now be spectrally tailored to that of the molecules.

The results of a proof-of-principle study with chiral molecules demonstrate the potential of artificially created nanogratings to become a platform to detect chirality in molecular films with light. The presented findings contribute to the advancement in the design and characterisation of chiral materials.
Date of Award29 May 2019
LanguageEnglish
Awarding Institution
  • University of Bath
SupervisorVentsislav Valev (Supervisor) & William Wadsworth (Supervisor)

Cite this

Chirality in diffractive plasmonic nanostructures
Kuppe, C. (Author). 29 May 2019

Student thesis: Doctoral ThesisPhD