Doctor of Philosophy

dissertationThis dissertation describes our work on design, fabrication and characterization of plasmonic metamaterials and tapered structures, with primary focus on their applications at terahertz (THz) frequencies. The phenomena associated with these structures rely on surface plasmon polaritons (SPPs), which may allow for high field enhancement and tight field confinement. We have investigated the underlying mechanisms of these structures and used that knowledge to develop unique and practical applications. We first studied two-dimensional periodic and random lattices based on aperture arrays, and modified the model to describe the effective dielectric response of the perforated metallic medium. Using two layers of the perforated stainless steel films, we demonstrated the emergence of an additional resonance and reproduced the transmission spectra using the effective dielectric model of the single-layer medium. Also, we improved the filtering performance of the multilayer periodic aperture arrays by adjusting the relative distance and angle between the layers, and demonstrated its application as a high quality bandpass filter. Then, we examined the transmission properties of graphite and carbon nanotube (CNT) films, and then the same films perforated with periodically distributed aperture arrays. The extracted dielectric constants of the graphite and CNT films demonstrate their availability for THz surface plasmonic devices. Moreover, we developed a narrow band/multiband THz detector in which the photoconductive antenna was surrounded by periodically corrugated gratings. This detector not only enhanced the sensitivity of detection at the specific frequencies, but also efficiently collected the radiation within the structure area, which obviated the need for a substrate lens. Finally, we improved the concentration properties of conically tapered apertures. Based on the optimal taper angle we determined, we introduced various modifications to the individual tapered aperture, e.g., to form an array and insert a gap spacing, and further enhanced the concentration capabilities and realized complete broadband transmission. Based on these studies and results, we are currently extending our work towards development of more reconfigurable and active devices that could enrich the available pool of THz and optical devices. Furthermore, such THz devices have great promise for the development of THz systems level applications and even a THz-based world in the future

Sensing water accumulation and transport in proton exchange membrane fuel cells with terahertz radiation

Fuel cells are like batteries in the sense that they are electrochemical cells whose main components are two electrodes (anode and cathode) and an electrolyte material. They differ from most batteries as they require a continuous stream of fuel and oxidant, generating electricity and heat for as long as these are supplied. Perfluorinated sulfonic-acid ionomers such as Nafion are the most common proton exchange membrane material (solid electrolyte) whose structure underpins its unique water and chemical/mechanical stability properties. Pure hydrogen and air are typically used as the fuel and oxidant, respectively, and by-products are water and waste heat. Due to their high efficiency, low temperature operation and capacity to quickly vary their output to meet shifting demands, these fuel cells are attractive to the automobile industry, although they can also be used for stationary power production. Water management is a prominent issue in proton exchange membrane fuel cell technology. Strategies in this topic must maintain a delicate balance between adequate hydration levels in the Nafion proton electrolyte membrane to maximise proton conductivity, and minimal flooding, which hinders mass transport to active sites. The complex nature of water transport in these fuel cells can be investigated via in situ or ex situ diagnostics with visualisation techniques such as neutron imaging or optical diagnostics. Despite the wealth of information provided by these techniques, they suffer from issues such as limited availability, excessive cost, limited sensitivity, and penetration depth. Terahertz radiation has been growing in popularity for contactless and non-destructive testing across various industrial sectors, including pharmaceutical coating analysis, defect identification, and gas pipeline monitoring. The ability of terahertz waves to penetrate through dielectric materials such as plastics or ceramics combined with strong attenuation by liquid water provides the necessary contrast to image water presence in proton exchange membrane fuel cells and their components. Motivated by the recent commercial availability of a compact terahertz source and video-rate terahertz camera, a simple terahertz imaging system in transmission geometry was realised. First, as a first step towards flooding inspection in an operating fuel cell, the feasibility of the imaging system for visualising and quantifying liquid water during an ambient air desorption process for Nafion membranes of a wide range of thicknesses – NRE-212 (50 µm), N-115 (127µm), N-117 (180 µm) and N-1110 (254 µm) was investigated. It was demonstrated that the imaging system was able to quantify liquid water in the 25-500 µm thickness range, estimate membrane weight change related to liquid water desorption, which correlated well against simultaneous gravimetric analysis and visualise the room temperature liquid water desorption process of a partially hydrated Nafion N-117 membrane. Further work consisted in imaging water build-up inside an operating proton exchange membrane fuel cell using the terahertz imaging system, combined with high-resolution optical imaging. Using a custom-built, laboratory-scale, terahertz, and optically transparent fuel cell, two-phase flow phenomena of water accumulation and transport, such as membrane hydration, main droplet occurrence, water pool formation, growth, and eventual flush out by gases were imaged. Results of the terahertz agree with simultaneous optical imaging and electrochemical readings. To demonstrate the potential used of the proposed imaging modality, the effect of air gas flow rates on flooding was demonstrated

Propagation of terahertz radiation in non-homogeneous materials and structures

The work undertaken is concerned with looking at how terahertz frequency radiation (here defined as 300 GHz -10 THz) propagates through media which have a random structure ("non-homogeneous materials"). Materials of this type are important in a wide range of applications, but are of particular interest in security and surveillance. Propagation of terahertz radiation through non-homogeneous materials is not well understood: both interference and scattering effects become important in this spectral range, where the wavelength and size and separation of the scattering centres are often commensurable. A simple model, which uses the phase change of a wave to describe its transmission through media having relatively small changes in refractive index is developed and compared with both exact theories and experimentally obtained measurements. Overall, a satisfactory agreement between the experimental data for transmission through arrays of cylinders, textiles and powders is seen. It is well known that pulses of terahertz radiation from optoelectronic sources have a complex shape. Post detection signal processing routines can be used to clean up the experimentally determined signals. The development of such algorithms is described, before they are applied to experimental results to determine: the minimum size of gaps between slabs to mimic voids in media; and the response of various compounds to a sharply terminated input pulse. The investigation of scattering from random structures requires the construction of a spectrometer having the capability to measure THz pulses scattered at different angles. Such a system ideally requires fibre-fed detection schemes to be used. The construction of a scattering spectrometer is described and its performance outlined. Pulses of terahertz which have been scattered by a sample of interest can be reconstructed, using methods from conventional tomography, to produce images of the phantom under test. Such measurements are outlined here. To our knowledge, this is the first time that tomography has been undertaken using a fixed sample and rotating detector arrangement