Thompson, PG; (2013) Polarisation-Tunable Plasmonic Devices. Doctoral thesis (PhD), UCL (University College London).

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Abstract

This research project aims to investigate the phenomenon of extraordinary optical transmission (EOT) in arrays of asymmetric cruciform apertures. The specific area of EOT examined is the dependence of transmission on incident polarisation. The approach to investigating this phenomenon can be divided into three main sections: fabrication, characterisation and simulations. The method of fabrication involved two main processes: fabrication of Au thin films on CaF2 substrates using thermal evaporation, and the milling of cruciform apertures in the thin films using focused in beam (FIB). A 5 nm adhesion layer of Cr was first thermally evaporated onto the CaF2 substrate followed by an Au film, typically of 30 nm thickness. The unit cells have a lattice constant of 2 μm and typical feature sizes of the apertures are between 160 nm and 1675 nm. The transmission and reflection characteristics are examined using Fourier transform infrared spectroscopy (FTIR) for the Mid IR wavelength range. The dependence of the transmission and reflection on polarisation was investigated. It was found that the arrays display two distinct transmission maxima and a point at which transmission is invariant for all polarisations. The transmission peaks occurred in the wavelength range of 2μm to 6 μm. An ensemble of arrays was fabricated to examine the change in wavelength of the transmission peaks and the invariant point as one of the aperture dimensions was varied. It was found that the transmission wavelength and the wavelength of the invariant point could be tuned by altering the dimensions of the cruciform apertures. Simulations for transmission and reflection were carried out in addition to field profile simulations. Together, these confirmed the assumed processes by which the polarisation dependent transmission maxima were generated, i.e. by localised surface plasmons. One of the arrays more closely examined had transmission peaks at 3.9μm and 5.75μm and an invariant point at 4.46μm. For this array simulations were also used to determine the polarisation state of the transmitted wave. The simulations show that arrays of asymmetric apertures change the incident polarisation such that the transmitted wave becomes elliptically polarised with a major axis that is in general different from the polarisation angle of the incident wave. A maximum shift of around 33! was determined using simulations for a wavelength ! = 3.9μm and an incident polarisation of 60! relative to the x axis. Experimental data was obtained that corroborates these findings.

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