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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ItemFabrication of single crystal diamond membranes for nanophotonics and nanomechanics( 2016)There is an intense interest in exploiting diamond’s remarkable combination of properties in many fields of science and technology. However, diamond fabrication and processing remains a major challenge in the application of diamond as a platform for quantum, nanophotonics, sensing and nanoelectromechanics. These challenges include: difficulty of scalable fabrication of thin single crystal diamond, lack of efficient colour centre-cavity coupling, and immature nanofabrication techniques. Although diamond is well known to display excellent properties for quantum information processing, realization of its potential in various practical applications requires the availability of thin (down to sub-micron thickness), high-quality single crystal diamond membranes. Scalability of such diamond platform for photonic structures is still in its infancy compared with traditional photonic platforms such as silicon-based technology. This thesis seeks the opportunity to overcome these obstacles. Successful outcomes will allow a diamond platform that will accelerate the transition from quantum science to applied quantum technology. The aim of this thesis is to design, fabricate, and characterize single crystal diamond membranes and use these membranes to create high quality factor nanophotonic and nanomechanical devices. In this work, a novel scalable method for the fabrication of single crystal diamond membrane windows down to 300nm thickness has been presented. A suite of integrated photonic structures with optical quality factor for telecom wavelength have been demonstrated. Furthermore the drum head resonators and cantilevers with high mechanical quality factor of in air. The demonstrated single crystal diamond membranes and devices provide immense opportunity toward a platform for wide range of applications that could lead to a significant advancement of quantum technologies.
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ItemMYTHEN CdTe: a new generation state-of-the-art X-ray imaging detector( 2016)MYTHEN is a single photon counting hybrid strip X-ray detector that has found application in X-ray powder diffraction (XRPD) experiments at synchrotrons worldwide. Originally designed to operate with hole collecting silicon sensors, MYTHEN is suited for detecting X-rays above 5 keV. However many PD beamlines have been designed for energies above 50 keV where silicon sensors have an efficiency of only a few percent. In order to adapt MYTHEN to meet these energies, the absorption efficiency of the sensor must be substantially increased. Cadmium-telluride (CdTe) has an absorption efficiency approximately 30 times that of silicon at 50 keV, and is therefore a very promising replacement sensor material candidate. Furthermore, the large dynamic range of the pre-amplifier of MYTHEN and its capability to process charge carriers of either polarity has enabled the characterization of both electron and hole collecting CdTe sensors. A selection of Schottky and ohmic type CdTe MYTHEN test structures have undergone a series of characterization experiments including bias and settings optimization, energy calibration, count rate capability as well as stability tests of bias and radiation induced polarizations. The performance of those systems will be presented and discussed in this thesis. Both, the radiation and bias induced polarization effects remained manageable. The MYTHEN system combined with CdTe sensors has proven to be reliable and stable despite high stress experiments. When biased over an extended period of time, the results of the studies have demonstrated that overdepletion of the sensors allowed the system to remain functional for a period of time 6 fold longer. During the high radiation studies, a count rate loss as well as a shift in threshold were observed, leading to the conclusion that individual charge carriers are been trapped. When applying a high bias as well as high flux, the detector system remained functional for 30 minutes. It was also demonstrated that a brief power cycle resumed normal performance after the system had shown symptoms of either polarization effect. Overall, the polarization effects observed on MYTHEN CdTe strip detector are temporary and show a slower impact than reported in the literature. Generally, a higher bias improved the stability of the detector.
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ItemSemiconductor-plasmon interactions( 2016)Modern microelectronic technology has begun to reach the limits of the well known heuristic “Moore’s Law”, the observation that computer power doubles every 18 months due to increases in processor efficiency and fabrication advances. This slowing can be traced to difficulties in increasing the resolution of (diffraction-limited) extreme ultraviolet lithography processes and the critical dimension of processors (defined as the smallest feature size of a fabricated structure; in the latest commercial technology this has reached 14 nanometres) now necessitating consideration of quantum mechanical effects. Clearly, to further advance progress in silicon-based technology, new approaches must be considered as our increasingly-digital world is ever more reliant on connectedness and microtechnology. The field of optoelectronics, in which microelectronic devices are integrated with optical devices, is one that may offer a method to overcome this. Optical circuits have many advantages over their traditional electronic counterparts; the weaker material-interaction of light reduces energy usage and heat generation, while the higher frequency of light enables higher-bandwidth devices. Photonic devices, in which light is able to propagate freely, however, can be orders of magnitude larger this. This dimension mismatch suggests, therefore, that while photonic devices will play a role in advanced optoelectronic technology, at present they are not a complete solution. The field of plasmonics has rapidly matured and may represent a partial solution to this problem. Operating at optical (UV to near-IR) frequencies, plasmonic devices have nanoscale dimensions, in most cases below 100 nm, similar to those of modern complementary metal-oxide-semiconductor (CMOS) devices. A surface plasmon is a coherent, delocalised electron density wave localised to the interface between a metal and a dielectric. Surface plasmons generally occur at visible and near-infrared frequencies, far higher than even state-of-the-art electronic circuits. The small size and high frequency of plasmonic devices therefore suggests them as the ideal means of coupling photonic structures and microelectronic devices. Before this is a reality, however, there are a number of obstacles that need to be overcome with regard to plasmonic technologies. A major issue is that due to the dimensions, and the fact that it is a resonance-based phenomenon, there is no simple way to dynamically modulate the spectral location of a plasmon resonance. Active lasmonic devices would enable all-optical switching, which would operate at far higher frequencies than electronic switches, increasing processor speeds and reducing bottlenecks in optical networks currently enforced by the need to switch information electronically. There are a number of requirements for such an active plasmon-switching device; it must be compact, use minimal energy, operate quickly and at a high frequency, and be easily to implement. In this thesis, plasmonic antenna arrays resonant within the visible spectrum were fabricated on thin films of vanadium dioxide, a phase-change material. Due to the phase-change displaying a significant change in optical properties, the vanadium dioxide permits a modulation of the adjacent plasmonic structures by altering their dielectric environment. I show that by altering the phase of the underlying vanadium dioxide it is possible to modulate the spectral location of the plasmonic resonances supported by the arrays by more than 100 nm within the visible spectrum. By considering the interaction of the plasmon resonances with the thin film optical cavity resonances within the enabling vanadium dioxide layer I demonstrate an additional avenue to further control the resonance’s spectral location. The combinations of these two effects represent a method to create a compact, in-situ, reliable modulation mechanism for plasmonic arrays of any design. A large portion of this work was focussed on the optimisation of the tuning mechanism. Since the phase of vanadium dioxide is temperature-dependent, a numerical investigation of thermoplasmonic heating was undertaken, which required the implementation of a coupled thermal-optical numerical modelling method within the finite element method (FEM). Incorporation of plasmonic structures with high refractive index (RI) substrates (such as silicon) was also considered. Previous research into the interaction between high RI materials and plasmonic structures has tended to focus on much lower RI materials (silicon has an RI around 4 in the visible, while previous works have lingered around RIs of 2.5 and below). This in-depth study culminates in the conception, design, fabrication and characterisation of a plasmonic polarisation switch as a proof-of-principle demonstration of one way in which an optical switch or router could be created. It was shown that by altering the phase of the underlying vanadium dioxide on which specifically-designed nano-antennas were fabricated, the primary polarisation axis of the reflected signal was reversibly modulated. This dynamic metasurface was a novel approach to using plasmonic resonances and has the potential to form the basis of an ultrafast optical switch if the substrate phase were controlled optically. This investigation into the effects of the interaction between an adjacent semiconductorand plasmonic structures is followed by a study into the behaviour of a related system, in which the dimensions of the adjacent semiconductor are reduced to those of the plasmonic structure. The semiconductor of interest comprised a large number of CdSe quantum dots. Electron beam lithography was used to place the quantum dots into dense aggregates of comparable dimensions to those of the adjacent plasmonic structures.The emission from these subwavelength (relative to the excitation wavelength)quantum dot clusters provided an additional avenue for analysis of the coupled system.