School of Physics - Theses
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ItemTowards Scalable Fabrication of Plasmonic Devices via Nanoimprint Lithography (NIL)( 2021)Advances in theoretical and computational techniques for advancing concepts in nanophotonics along with major development in nanofabrication technologies has enable the realization of nanoscale optical device with unprecedented ability to control light such as plasmonic metasurface. Fabrication of plasmonic devices, however, has long been relying on conventional lithography technique such as electron beam lithography (EBL), focused ion beam (FIB) milling and photolithography (PL). While these lithography techniques have enable construction of sub-wavelengths nanostructures, their reliance on the interaction between charged particles or light beam with polymer resist during pattern writing process require intricate control systems and usually require high operating and maintenance cost. In fact, for FIB and EBL, the writing time can be very slow depending on the pattern area. These approaches, therefore, are unsuitable for industrial scale production thus hindering commercial uptake of metasurfaces. Therefore, there is a need for an alternative low-cost, high-throughput nano-manufacturing approach to overcome the bottleneck in conventional lithography methods. With this motivation in mind, this thesis focused on utilizing the nanoimprint lithography (NIL) technique for fabrication of the designated plasmonic device. NIL uses pattern transfer approach whereby a predefined pattern on a mould is used to replicate the pattern onto another surface. NIL offers scalability and high-throughput large area manufacturing of nanostructures. In this thesis, the versatility and capability of NIL for producing plasmonic device, particularly plasmonic colours, with a potential of scalability is investigated. This includes the investigation of the dynamic flow of the resist during the nanoimprinting process through various process parameters. With this information, it will be shown that NIL is capable of producing multi height, grayscale-like structures through a careful control of the flow of the resist, without relying on control of charged particle or light beams. This novel technique was then utilised as a method to print the plasmonic colouration device, with ability of controlling the hue and saturation of the resulting colours via tuning of the vertical gap size of the structure. Additionally, an investigation of multispectral, polarisation-selective iridescence plasmonic colour will also be shown in this thesis. This was achieved via the elongation of the spatial dimension of plasmonic structure which results in multiple resonances in both visible and infrared region of the spectrum. Such multispectral system will be very useful especially for optical security device. Finally, a novel plasmonic colour designed to preserve the vividness of the resulting colour under unpolarised ambient lighting condition will be demonstrated in this thesis. This involves production of polarization-independent plasmonic structure featuring symmetric cross structures. It will be shown that this device produced excellent colours with preserved hue and vividness, regardless of polarization state of light
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ItemCharacterisation of sub-radiant modes of metasurfaces( 2016)Metallic nanostructures can be utilised as optical antennas: devices to assist the conversion between localised (near-field) and propagating (far-field) light. Optical antennas possess sub-radiant modes which cannot be easily coupled to propagating fields and have the advantages of longer life-times and higher Q resonances. This project has involved the computational and experimental investigation of the properties of sub-radiant modes of various nanoantennas that form the basis of a metasurface. These modes were found to be excited by off-normally incident light with a specific polarisation or vector beams with radial or azimuthal polarisation. It was also shown through studies of the optical far-field that the angular spectrum of the reflected fields was modified through interaction via a sub-radiant mode.
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ItemFluorescent defects and optical structures in metal oxides( 2016)The emission properties from fluorescent defects and manipulation/control of this light were the main themes of this thesis. It begins with the exploration of the emission properties of a radiating dipole within a spherical nanoparticle (NP) and how it is affected by the surrounding refractive index environment. It is important to address this topic as it can give researchers preliminary insight to their system using relatively simple numerical method and modelling. The radiating dipole is treated classically and the finite-difference time-domain technique is used to solve the electrodynamics of systems with optically relevant environments. This method is not restricted to just the environments presented within, but can be generalised to any arbitrary system. The total radiated power from the dipole can be significantly affected by: the refractive index of the NP, the size of the NP and the surrounding refractive index environment of the NP. Experimentally relevant refractive index distributions were considered in this thesis. The mechanism of visible and infrared fluorescence from optical defects within metal oxides has been highly debated where materials scientists do not have a general consensus on the origin. One aspect that is agreed upon is that the visible emission is due to native point defects within the crystallographic structure. This thesis will focus on the optical characterisation of fluorescence defects in promising optical materials of zinc oxide (ZnO) and titanium dioxide (TiO2). The controversy begins with the abundance of fabrication methods for ZnO and TiO2 that inevitably introduces different defects based on the experimental conditions. The fluorescence characterisation by materials scientists involves measurements that sample many defects, due to a large spot size in commercial set-ups, and therefore individual defect signal is lost. The defects in ZnO and TiO2 explored here will be characterised using confocal microscopy, a high-resolution optical technique. This gives the ability to address individual defects with an appropriate resolution to isolate single defects. The fluorescence defects from a few morphologies of ZnO and TiO2 were shown to exhibit room-temperature single-photon emission and these were furthered characterised by investigating their: photoluminescence spectrum, photodynamics, power saturation and lifetimes. Finally, this thesis explores the control and manipulation of light through a structured environment which is central to the operation of an integrated optical circuit. An important integrated component known as an optical microcavity which confines and traps was explored in this thesis. A deterministic algorithm for an ultrahigh-Q nanobeam cavity was explored within TiO2 that operates at the visible wavelength of λ = 637 nm, the mean fluorescence wavelength of the TiO2 single-photon emitters also presented in this thesis. The electrodynamics of the system was calculated using the finite-difference time-domain method. Preliminary fabrication results were also presented. The trapped light with an optical cavity is routed and connected within an integrated optical circuit using waveguides. An optical waveguide design that exhibits coherent tunnelling adiabatic passage of light was explored. The solving and propagation of optical modes was obtained using a combination of _nite element method and beam propagation method techniques. The design showed robust light transfer despite significant perturbations to the optimised system The archetypal three-waveguide system can be extended to a five-waveguide system where this system acts as a power divider. The three-waveguide system was fabricated into tellurium dioxide, an emerging material for non-linear optical communication applications.
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ItemQuantum information engineering: concepts to quantum technologies( 2007-11)This thesis investigates several broad areas related to the effective implementation of quantum information processing, from large scale quantum algorithms and error correction, through to system identification and characterization techniques, efficient designs for quantum computing architectures and the design of small devices which utilize quantum effects. The discussion begins with the introduction of a quantum circuit appropriate for implementing Shor’s factoring algorithm on Linear Nearest Neighbor qubit arrays such as the Kane phosphorus in silicon system. Detailed numerical sim- ulations are then presented, demonstrating the sensitivity of the circuit under coherent quantum errors. The concepts of Quantum Error Correction and Fault-tolerant computation are reviewed with original work carried out to show the relative robustness and practicality of Fault-tolerant computation for logical state preparation. Methods of intrinsic system identification and characterization are proposed. Protocols for characterizing both the confinement of a multi-level system to the qubit subspace and the Hamiltonian dynamics governing two-qubit interactions are presented as well as a brief review of characterization techniques already developed for single qubit dynamics. (For complete abstract open document)
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ItemA generalised holographic approach to coherent diffractive imaging( 2013)This thesis examines methods for obtaining the full complex wave-field from measured probability intensities in microscopy experiments, otherwise known as the phase problem. We present a new theoretical framework for solving the phase problem. From this, previously intractable problems may be solved via direct methods. In addition, we present a method for greatly increasing the potential output of single particle diffraction measurements in free-electron laser science (FELS) experiments. This method extracts many single particle diffraction measurements from a single many particle diffraction measurement. We investigate the feasibility for performing coherent diffractive imaging (CDI) in the scanning transmission electron microscopy (STEM) geometry using standard methods. Despite some success in ideal cases, we find that the inversions (from the measured diffraction intensities to the complex wave-field) are very sensitive to imperfections in the data. Consequently we investigate the degree to which various sources of error, such as detector noise and spatial incoherence, affect the convergence properties of the inversions. Difficulties in the application of single-shot CDI in STEM may be overcome by measuring many diffraction patterns from the same sample and combining these data for a ptychographic method. We performed a ptychographic reconstruction of a complex sample transmission function. This was done by combining several diffraction measurements from overlapping regions of a boron-nitride cone. We present the subsequent atomic resolution retrieval and the algorithm used. We develop and implement new holographic algorithm to obtain a high resolution reconstruction of the complex exit surface wave formed by a gnat’s wing. In doing so we derive a new error metric applicable in both holographic and non-linear CDI. It is also found to be a more reliable measure for the fidelity of the retrieval. By extending the previous holographic method to accommodate diffraction data taken in the near-field (or the Fresnel regime) we find that previous restrictions on the illumination and the position of the sample in the beam are greatly reduced. We demonstrate the experimental feasibility of this method by recombining the complex exit surface wave emanating from a microfibre. Here the specimen was illuminated by a plane wave while the diffraction data was taken in a plane centimetres from the object. We show how the requirement for full coherence of the imaging system can be removed by incorporating the partially coherent modes of the imaging probe into the formalism of the holographic method. A new iterative linear algorithm is developed, extending the applicability of the algorithm to higher resolutions and greatly increasing the computational speed of the retrieval. We present a direct single-shot sub-Angström retrieval from the edge structure of a CeO2 nano-particle, using electrons. This retrieval is improved by the inclusion of the entire autocorrelation function of the exit surface wave. Using the new iterative linear algorithm we are able to include a subsequent iteration, correcting for the non-linear contribution to the autocorrelation function by the object. Finally, we use the techniques developed in this thesis to extend the new iterative linear algorithm to include ptychographic data-sets.