Tuning optical properties in random arrays of plasmon resonant nanoparticles
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Small conductive particles show a resonant behaviour at wavelengths where bulk or thin film samples have no features. This resonance is caused by the collective oscillation of the free electrons in the particle and is called localised surface plasmon resonance. It is influenced by the shape of the particle, the surrounding medium and particle interaction. I studied shape, matrix and interaction effects of metallic and metal-like particles in various systems with the aim to rationally tune the resonance to specific wavelengths for different applications. Dilute samples of small LaB6 particles were studied with regard to their NIR blocking performance. My analysis showed that they are more efficient than the alternative materials ITO and ATO. This is mainly due to the position of the LaBg particle resonance, which lies precisely in the spectral region which needs to be blocked (around 1 /mi). I was able to model the optical properties of the window samples, using a dilute quasi-static approach for anisotropic particles. Different embedding matrices and particle interaction have also an influence on the localised surface plasmon resonance. An example for a combination of matrix and interaction effects is a self-assembled gold particle him with organic linkers. Structural effects were especially important in these films, as was verified by electron microscopy. The optical properties were successfully modeled, using a two level effective medium approximation. A different way to tune the resonance is to change the shell thickness to core size ratio in metallic nanoshells. The resulting spectral shift, though, is limited by experimental realities for the metal coating and the onset of scattering for larger particles. The shell has two resonances, of which the low energy one can be tuned by the ratio mentioned above. This resonance also shows a different electric field profile to the normal dipole (and high energy shell) resonance. The field pattern also highlights a strong field gradient across the external shell interface and along the incident polarisation direction. The properties were calculated using Mie theory and the quasi-static approximation. Finally, the far and near-field optical properties of thin silver films with randomly distributed holes were studied. They showed an enhanced absorption, due to coupling of the incident light into surface plasmon polaritons by the holes. Whereas the films did not show the enhanced transmission, which occurs in regular hole arrays, they still might provide some insight in the processes involved by helping to exclude some possible explanations.
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