Exploring light-matter interactions with graphene

Abstract

Graphene has drawn extraordinary interest from both scientists and the wider public; the idea of an atomically thin material that is staggeringly conductive while also being both strong and flexible, seems impossible. From a scientific perspective, the two dimensional nature of graphene provides an exciting playground to explore new phenomena with the potential for technical applications. The question of how light interacts with graphene underpins these devices and is often set in the context of graphene as a highly transparent material. In this thesis, results from the interaction between light and graphene will be explored across a broad frequency range and methods for manipulating the interaction between light and a graphene layer will be presented.

At infrared frequencies, nanostructured graphene displays plasmonic behaviour. A layer of graphene with a periodically modulated, array-like, conductivity analogous to graphene nano-ribbons is investigated numerically. The total response of this structure was found to arise from the hybridisation between multipolar resonances supported by different areas of the array. By designing the appropriate geometric parameters the resonant absorption of the array is shown to increase by a factor of ~50%.

In addition to displaying interesting linear properties, graphene is a strongly nonlinear material. To enhance a specific nonlinear optical mixing process, third harmonic generation, graphene is combined with a resonant cavity. The measurements of the third harmonic generation from this cavity structure show that integrating a graphene layer with a cavity increases the generated third harmonic power by a factor of 117. Numerical modelling of the cavity structure shows the enhancement mechanism arises from the cavity structure supporting a resonance in the electric field at the wavelengths of both the incident and third harmonic.

Finally, the influence of strain in a graphene layer on the fluorescence collected from a molecule deposited on top of the graphene layer is investigated. Using Raman spectroscopy, the level of local strain was mapped across a graphene sample and confocal microscopy was used to measure the emitted luminescence. The observed correlation between the level of strain and luminescence intensity is supported by a simple model of a dipole near a graphene sheet.