School of Physics - Theses
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ItemDetecting and characterising extrasolar planets in reflected light(University of Melbourne, 2009)
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ItemFault-tolerant quantum computation with local interactions(University of Melbourne, 2009)
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ItemMonte Carlo characterisation of narrow photon fields for intensity modulated radiotherapy(University of Melbourne, 2006)
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ItemFocusing of an atomic beam using a TEM01 mode lens(University of Melbourne, 2006)
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ItemThe Panoramic Deep Fields( 2001)The Panoramic Deep Fields are a deep multicolour survey of two ~ 25 ° fields at high galactic latitude. The survey images have been constructed by digitally stacking scans UK Schmidt plates. Deep images (Bj ~23.5) with low contamination have been obtained by subtracting the background from the individual plates scans and using bad pixel rejection during the stacking. The size and depth of the fields allow the accurate statistical measurement of the environments and evolution of galaxies and AGN. The clustering of galaxies and galaxy clusters has been measured from z ~0.4 until the current epoch. The clustering properties of galaxies are strongly correlated with colour and blue U – Bj selected galaxies exhibit weaker clustering than any morphologically selected sample. The weak clustering (ro ≤ 3h -1 Mpc) of blue galaxies implies galaxy colour and stellar population are more strongly correlated with environment than galaxy morphology. Despite the large fields-of-view, the clustering of red galaxies and clusters varies significantly between the two fields and the distribution of clusters is consistent with this being due to large-scale-structure at z ~0.4. The evolution and environments of AGN have been measured at intermediate redshifts with the Panoramic Deep Fields. Photometric redshifts, colour selection and the NVSS have been used to compile a catalogue of ~ 180 0.10 < z< 0.55 radio galaxies. The evolution of the radio galaxy luminosity function is consistent with luminosity evolution parameterised by L (z) ~ L(0) (1+z)3.4. The environments of UBR selected AGN and radio galaxies have been measured at z~0.5 using the Panoramic Deep Field galaxy catalogue. By applying photometric red-shifts and colour selection criteria to the galaxy catalogue, it has been possible to increase the signal-to-noise of the angular correlation function and measure the cross-correlation with specific galaxy types. Most AGN host environments are comparable to the environments of galaxies with the same morphology. However, ~6% of UBR selected AGN are in significantly richer environments. No significant correlation between AGN luminosity and environment was detected in the Panoramic Deep Fields. The richness of AGN environments is not strongly correlated with redshift and the rapid evolution of the AGN luminosity function is not caused by evolution of AGN host environments.
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ItemA spectroscopic and chromatographic study of the photochemical properties of daylight fluorescent paint( 2009)Daylight fluorescent pigments fade rapidly, accompanied by a chronology of colour change. Fluorescence is a photo-physical phenomenon which involves emission of light from an excited state. Fluorescent dyes thus have a high susceptibility of being promoted to an excited state; a characteristic in the case of organic fluorophores which infers vulnerability toward photo-bleaching. Multiple organic fluorescent dyes are routinely incorporated into a given daylight fluorescent pigment, to either additively fluoresce or interact through energy transfer. The organic fluorescent dyes employed invariably differ in photo-stability, and upon loss of each species of fluorophore an abrupt colour change is observed. The collective result of this fading behaviour is that in a short period of time a daylight fluorescent paint layer will be of a different hue, devoid of luminosity. As consequence it is almost impossible to colour match a faded daylight fluorescent paint layer without the hues diverging asynchronously, or ascertain the original palette of a daylight fluorescent artwork after a protracted period of time. The predicament is exacerbated by the fact that there is no standard method in cultural material conservation, of documenting daylight fluorescent colour in a painting photographically or colorimetrically. The objective of this thesis is to investigate the photochemical behaviour of daylight fluorescent pigments, to ensure best practice in the preservation of artworks that contain daylight fluorescent paint. Fluorimetrie and chromatographic analysis of the DayGlo daylight fluorescent pigment range at the constituent dye level, prior to and during an accelerated light ageing program formed the basis of the experimental. Given the limited selection of fluorescent dyes suitable for daylight fluorescent pigment manufacture, it is anticipated that the results attained for the DayGlo range will be applicable to all daylight fluorescent media encountered in cultural material. Experimental data revealed the manner in which the fluorescent dyes behind each DayGlo daylight fluorescent pigment were formulated, and provided explanation for the 1colour changes observed upon fading. A prognosis of when and why a daylight fluorescent palette experiences hue shift and the implications this has for display is presented. Methodology for imaging daylight fluorescence, identification of the constituent fluorescent dyes in a daylight fluorescent pigment and colour matching a daylight fluorescent paint layer are presented and applied in-situ, to case studies possessing a daylight fluorescent palette.
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ItemAstigmatic phase retrieval of lightfields with helical wavefronts( 2012)The controlled use of coherent radiation has led to the development of a wide range of imaging methods in which aspects of the phase are enhanced through diffraction and propagation. A mathematical description of the propagation of light allows us to determine the properties of an optical wavefield in any plane. When a sample is illuminated with coherent planar illumination and its diffracted wavefield is recorded in the far-field of propagation, a direct inverse calculation of the phase can be quickly performed through computational means – the fast Fourier transform. Algorithmic processing is required, however, because only the intensity of the diffracted wavefield can be recorded. To determine structural information about the sample, some other information must be known about the experimental system. What is known, and how it is processed computationally, has led to the development and successful application of a broad spectrum of phase reconstruction iterative algorithms. Vortices in lightfields have a helical structure to their wavefront, at the core of which exists, necessarily, a screw-discontinuity to their phase. They have a characteristic intensity distribution comprising a radially symmetric bright ring around a dark core which, for either handedness of the rotation of the vortex, appears identical. Observation of a vortex is, therefore, ambiguous in its ability to determine its true direction of rotation. The ubiquitous presence of vortices in all lightfields hinder the success of phase reconstruction methods based on planar illumination and, if successful, render any reconstruction of the phase non-unique, due to the ambiguity associated to their helicity. The presence of a controlled spherical phase distortion can break the symmetry of the appearance of the vortices and, hence, remove the ambiguity from the system and drive algorithms to a solution. For the pathological case of an on-axis vortex, however, spherical distortion will not break the radial symmetry. The astigmatic phase retrieval method separates the spherical distortion into cylindrical distortion in two orthogonal directions. This form of phase distortion breaks the symmetry of a vortex allowing a unique determination of the phase. The incorporation of such use of cylindrical distortion into an iterative phase reconstruction algorithm forms the basis for the astigmatic phase retrieval (APR) method. Presented in this thesis is the creation and propagation of lightfields with helical wavefronts, produced through simulation and experiment. Observation of the effects of cylindrical distortion on vortices is explored in detail, particularly for split high-charge vortices where their positions can inform the type and strength of the applied phase distortion. Experimentally, onaxis vortices are created and distorted for the purposes of astigmatic phase retrieval in both visible light and X-ray wavefields. This thesis presents the first experimental demonstration of the astigmatic phase retrieval (APR) method, successfully applied optically with a simple test sample. The method is also applied to lightfields with helical wavefronts. The successful unambiguous reconstruction of on-axis chargeone and charge-two visible light vortices are presented, which is the first experimental demonstration on the unique phase reconstruction of an on-axis vortex from intensity measurements alone. Experiments are then performed to apply the method to vortices created in X-ray wavefields. The parameters of the experiment and the data have not, however, allowed for a successful reconstruction in this case. It is demonstrated through extensive simulation analysis that the APR method is a fast and robust imaging method. It is also shown that, through observation of the error metric, experimental parameters can be corrected or even determined, making the method successful even if there is no a priori knowledge of the experimental system. The application of the APR method as a general imaging technique for use in high-resolution X-ray diffraction experiments is, therefore, is a logical extension of the work of this thesis.
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ItemErbium for optical modulation and quantum computation( 2017)Erbium (Er) is a lanthanide element, mainly used in its trivalent ionic form (Er3+), as an active dopant in optical devices, for light amplification or generation. The luminescence of Er3+ lies within the conventional wavelength band, 1530-1565 nm, for fiber-optic communication. The low noise, linear response, and stability of the optical gain provided by the Er3+ luminescence are ideal for applications in photonic systems that operate in the fiber-optic frequency. While much research has been done to understand the Er3+ luminescence in various lasing media, few studies have been conducted to tap the potential of Er for applications other than amplifiers or lasers. This thesis delves into two new areas, namely optical modulation and quantum computation, where the Er3+ luminescence may be able to be applied in a novel way. By incorporating Er3+ into a switchable optical material, an optical modulator could potentially be made that is capable of not only switching but also amplifying signal transmission or sustaining the signal intensity from propagation losses. This integrated approach could reduce device footprint and latency for on-chip as well as synchronous applications. Successful integration of Er, however, has never been demonstrated in conventional optical modulators because their reliance on electro-optic effects conflicts with the carrier-sensitive mechanism of the Er3+ luminescence. The compatibility between Er and a recently advocated optical material, namely vanadium dioxide (VO2), is examined in the first part of this thesis. VO2 exhibits a hysteretic, bistable phase transition that is accompanied by a high-contrast optical switching in infrared, including the fiber-optic, wavelength band. The phase transition can be triggered thermally as well as optically. When triggered optically, it can occur in picosecond timescale, making VO2 a promising material for ultrafast optical switching applications. Experimental characterizations of the Er3+ luminescence and the optical switching were performed on selectively prepared thin-film samples of VO2. The Er3+ luminescence could be observed after the samples were implanted with Er and then annealed between 800*C and 1000*C. The optical switching could also be measured in the implanted and annealed samples as they were thermally heated up and then cooled down past the critical temperature of the phase transition. The Er-implanted samples, however, were found to have broader hysteresis and lower switching contrasts than the pure VO2 samples. It is concluded that although Er-implanted VO2 could probably work as a combined optical switch and amplifier, the poorer switching qualities do not guarantee that a device based on the material could provide better utility than a separated system of optical switches and Er amplifiers. The Er3+ luminescence could also be utilized for quantum frequency conversion, for implementation in interconnects that interface superconducting quantum computers to a fiber-optic quantum network. For two superconducting quantum computers to be able to communicate over a fiber-optic quantum network, the frequency of the signals transmitted from either computer needs to be converted into the fiber-optic frequency, and then back into the microwave frequency upon receipt at the other computer. Early proposals suggested that the interconnect be at least comprised of Er3+ ions, a microwave resonator, and an optical resonator. The realization of this system has been attempted recently in basic experiments, but the conversion efficiency was found to be too low. The weak couplings between the Er3+ ions and the two resonators were identified as one of the main reasons for the low conversion efficiency. One way to mitigate the weak coupling in the microwave part is to have a superconducting flux qubit bridge the interaction between the Er3+ ions and the microwave resonator. The second part of this thesis presents a theoretical and simulation study of the dynamics of a coupled system consisting of Er3+ ions, a superconducting microwave resonator, an optical resonator, and a superconducting flux qubit. It is shown that quantum information can be exchanged between the Er3+ ions and the microwave resonator with a high fidelity via the qubit coupling, and the exchange process is controllable by changing the frequency of the qubit. The frequency conversion between the microwave and the optical regime is shown to be infeasible to be realized at the limit where the number of optical excitations (n) is much less than the number of ions (N). A high-efficiency frequency down-conversion is demonstrated to be achievable in the case where there is no decoherence, and both n and N are small. However, the time it takes to complete the down-conversion is very long, leaving the efficiency prone to decoherence. It is argued that for the frequency conversion to be able to be accomplished in a typical decoherent environment, both n and N need to be large. The study of the dynamics, in this case, is left for future research.
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ItemUltrafast spectroscopy of nanostructures( 2017)This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.
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ItemNon-invasive imaging techniques for the investigation of cultural objects( 2017)Laser speckle is used in cultural materials conservation to study changes in the physical condition and movement on the surface and within the structure of objects. This thesis investigates three speckle methods, Laser Speckle Contrast Imaging (LSCI), Dynamic Speckle Imaging (DSI) and Electronic Speckle Pattern Interferometry (ESPI) to: image obscured text, examine paint application methods, record physical transformations during paint drying processes, and monitor canvas strain due to climate variations in- situ. It extends the boundaries of use of speckle-based methods in conservation through a set of four case studies that cover a broad range of cultural conservation problems faced in Australian collections. These studies were contextualised within an examination the properties of viscoelastic materials, a mathematical description of speckle, an explanation of the speckle-based methods and the experimental equipment and design considerations that provide information about the challenges and considerations required when deploying speckle-based methods. In the first case study, LSCI was used to image obscured text on books that had been repaired with layers of paper. The quality of the results, which was dependent on the weight of the paper, colour of the subsurface medium, and temperature of the object, was similar to that of Infrared Photography. In the second case study LSCI was used to monitor the curing process of modified alkyd resins over 120 hours to investigate whether the application method used by the artist was robust to typical ageing conditions. The results correlated well with traditional gravimetric results, which only provide information about the mass changes to the samples with no spatial information. LSCI was able to demonstrate where and when the alkyd resin surface was moving, most likely due to off gassing processes. The third case study used DSI to monitor the drying process of artists’ acrylic paints, allowing for the novel non-invasive detection of when the paint was touch dry, a task that normally involves destructive methods or has only been applied to rapid drying industrial paints. The final case study demonstrated the in-situ application of ESPI to monitor two archetypical colonial easel paintings in assessing physical movement due to variations in the display environment. ESPI was used in-situ to record changes in the strain of the canvas. Furthermore, it was shown that DSI analysis could be applied to ESPI data to monitor for vibrational disturbances that can decorrelate results The speckle-based techniques used in this thesis were shown to satisfy six criteria for preferred conservation methods: all information is gathered non-invasively and can be analysed in a variety of ways; off the shelf optical components are used, thus it is highly suitable for fields in which specialised optical components are not available; no prior analysis about the sample is required; experimental challenges can be diagnosed and overcome through the analysis of data sets and/or computer simulations; the techniques can be applied to the investigation of a range of materials with different optical properties; and multiple speckle-based methods can be combined to provide complementary information. The case-studies explored in this thesis further demonstrated that speckle is capable of non-invasive characterisation of many physical properties and can be applied both in the laboratory and in-situ. Through the assessment of theory and case studies this work has extended the application of speckle analysis within conservation studies, delivering practical outcomes to aid conservation efforts.