| dc.description.abstract | The key question that this thesis aims to answer is “can a tuneable bandpass optical filter for the mid-infrared regime be made by combining extraordinary optical transmission (EOT) arrays and phase-change materials (PCMs)?”.
It is proposed that such devices may be useful for a wide range of applications where the ability to dynamically change the transmissive (or reflective) properties of a filter is required, include multispectral sensing/imaging and signal modulation amongst others. Current multispectral imaging systems are mainly dependant on either multiple sets of lenses and sensors, or multiple mechanically-exchanged filters exposed through in-sequence; the use of a single, dynamically tuneable phase-change EOT-based filter opens avenues to reducing systems’ size, cost and complexity.
The EOT effect is observed with arrays of sub-wavelength-sized holes in thin plasmonic metal (e.g. gold) films, with the transmission peak position dependent on the array geometry and surrounding materials’ optical properties: a PCM layer on top of the array allows shifting of the transmission peak position by switching the PCM phase (and its refractive index) via heat pulses.
Specific areas studied in this thesis include the use of different fabrication methods to make phase-change EOT transmission filters for the mid-infrared regime, including electron-beam lithography-based techniques and a novel (and much faster) approach of direct patterning via laser ablation. Tuneable filters for use in various parts of the optical spectrum, especially the mid and long-wave infrared range, were designed, simulated, fabricated and characterised. Good performance was obtained for phase-change EOT filters over a wide range of array pitch sizes. EOT arrays designed for the mid-infrared range and fabricated via wet-etching and measured with FTIR spectroscopy produced very similar spectra to those of finite-element simulations with peak transmittance of Q-factors between 5-6 and a peak transmittance of ~0.8. Laser-ablated arrays showed a similar (though not quite so good) performance, due mainly to slight irregularities in the positioning of holes in the array.
The addition of a phase-change layer, specifically Ge2Sb2Te5, to the EOT arrays resulted in a shift in the wavelength of the peak transmission, with the amount of shift depending on the phase-state (crystalline, amorphous, or mixed-state) of the phase-change layer, so demonstrating the ability for dynamic tuning of the filter response by switching of the phase-change layer. An important requirement for proper and prolonged operation of the filter devices was found to be the use of a thin dielectric barrier layer (here Si3N4, between the plasmonic film and the phase-change layer, to prevent inter-diffusion between the two: reflection cavities of Ge2Sb2Te5 on unpatterned gold layers were created to investigate this effect, with resonance features of a 20 nm layer being destroyed without a barrier layer present and the complete assimilation of gold and phase-change layers evident with cross-section TEM imaging. | en_GB |
