Osley, EJ; (2013) Tunable field enhancement in plasmonic nanostructures. Doctoral thesis (PhD), UCL (University College London).

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Abstract

Metallic nanostructures that contain bound geometries will support localised surface plasmon (LSP) resonances if they are illuminated with light of appropriate frequency. These LSP resonances result in a concentration of the electric field of the incident light into a volume which is smaller than the photon wavelength. Certain geometries that support LSP resonances are sensitive to the polarisation of incident light, and the enhanced electromagnetic field can therefore be tuned in situ by adjusting this polarisation. We have investigated polarisation tunable LSP field enhancement by observing, in the linear regime, the interaction of an asymmetric cruciform aperture structure with a chemical bond and, in the non-linear regime, the second harmonic generation (SHG) produced by three metallic nanostructures. Numerical simulations implementing rigorous coupled-wave analysis (RCWA) were used to find asymmetric cruciform aperture dimensions that produced LSP resonances when illuminated with light of a wavelength between 2 μm and 8 μm. Arrays of these apertures were fabricated in a 35 nm thick gold film on a transparent calcium fluoride (CaF_{2}) substrate. The fabrication methods used to create the apertures were either focused ion beam (FIB) milling, or electron beam lithography (EBL) with argon ion milling, of the gold film. Fourier transform infrared spectroscopy (FTIR) was used to measure the transmission and reflection spectra of these plasmonic nanostructures. The apertures were coated with poly(methyl methacrylate) (PMMA), which has a local absorption maximum at 5.784 μm created by the stretching of its carbonyl bonds. The transmission and reflection spectra of the PMMA-coated apertures were measured using FTIR. The interaction of the LSP and molecular resonances was shown to form an asymmetric Fano resonance at the carbonyl bond wavelength. We found that this Fano resonance can be tuned in situ by rotating the polarisation of incident light. A classical mechanical oscillator model was developed to interpret the reflection and transmission spectrum in terms of the interference of the LSP and molecular resonances. A quantum mechanical model was also developed and used to predict the absorption spectrum of the system. This quantum mechanical model provides information on the physical interactions within the system, and predicts a near-field mediated interaction between the plasmon and molecular resonances. Nonlinear optical measurements were made using an SHG microscope, which allowed the location of near-field SHG hotspots to be determined. Three geometries were measured using this technique using fundamental wavelengths of 800 nm or 1 μm. The first geometry, a chiral star structure, was found to display dichroic SHG that was dependent on the handedness of the incident circularly-polarised fundamental light. The second, a `windmill' structure, was used to investigate the dependence of near-field SHG on the linear polarisation of fundamental light; the ablation of these metallic windmill structures by the fundamental demonstrates that laser ablation of patterned surfaces is dependent on the LSP resonance of the constituent structures. Finally, the spatial dependence of SHG produced by a cruciform aperture structure in a gold film illuminated by linearly polarised light was observed. SHG intensity was found to be greatest along the axis of the cruciform which was perpendicular to the incident E field polarisation.

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