School of Physics - Theses
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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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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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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.
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ItemBound states and structural properties of trap-imbalanced fermions( 2016)Ultracold quantum gases, which were only recently realised experimentally, have become one of the most active fields of modern research. This is due to the precision and power of the experiments, as well as the great variety of physical phenomena that they exhibit. In this thesis, the physics of few-body scattering in the strongly-interacting regime is studied. The study of few-body physics allows a better understanding of many-body systems, particularly with strong interactions, which make the usual many-body theoretical techniques untenable. The particular topic of this thesis is few-body scattering in heteronuclear systems, which contain two species of atom with different masses and/or harmonic trapping frequencies. These mass and trap imbalances lead to a variety of interesting physics that is not present in homonuclear systems. Deeply-bound Efimov states with unusual properties appear in systems containing two species of fermions when the ratio of the two species' masses becomes sufficiently large. Other types of deeply bound states also appear above a lower critical mass ratio. We use an implementation of a stochastic variational method to study states of this type under a trap imbalance i.e.\ with two species of fermion with different harmonic trapping frequencies. The stochastic variational method works by randomly generating trial functions, then using a competitive selection scheme to select the best contributions to the approximate variational solution. Using this method, it is shown that the introduction of a trap imbalance has no effect ont the physics of these bound states. Also using this variational method, the effect of trap imbalances on two- and three-body systems, with and without mass imbalances, is studied in detail. It is found that the trap imbalance has the immediate effect of lifting structural and energetic degeneracies between different total angular momentum states of the few-body system. Furthermore, trap imbalances significantly alter the usual physics of the three-fermion system, in which two atoms form a deeply-bound dimer while the third remains unbound. The trap imbalance changes this picture and causes all three atoms to overlap considerably in the ground state, forming a loosely-bound trimer state. Such alterations to the few-body collision properties can have significant effects on the many-body physics of an atomic gas. Thus these results indicate the possibility of additional methods of tuning and control for heteronuclear many-body systems. These results may also be of interest in explicitly few-body experiments, which remain largely unexplored at this time.
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ItemNovel techniques and standards for X-ray spectrometry and tests of QED( 2016)This thesis advances the standards and techniques used in x-ray spectrometry for laboratory sources and advanced sources, such as Electron Beam Ion Traps (EBITs). This impacts upon popular fields including x-ray diffraction, x-ray absorption spectroscopy and advanced quantum mechanics. A new method of removing cosmic ray signals from CCD detectors in a low ux environment is presented. New characterizations of Ti Kß and V Kß, to be used as calibration standards, are presented. Preliminary measurements of lines of He-like Cr lines were performed to test QED. Different types of x-ray spectrometry are important in fields such as testing Quantum Electrodynamics (QED) in the x-ray regime and the study of the local environment of atoms in the solid state through X-ray Absorption Fine Structure (XAFS). Tests of QED require highly accurate and precise determination of x-ray energy from low ux sources, while advances in XAFS studies require the ability to properly determine the validity of results. This research develops techniques and standards in spectrometry for use in XAFS and tests of QED. Further, these techniques are used to test QED. The previous standard method of fitting XAFS did not use measured uncertainties, thus they could not be propagated to the results. Fitting via the minimisation of the statistically valid reduced-χ-squared measure of goodness of fit enables error bars to be propagated. A statistically valid reduced-χ-squared measure of goodness of fit was applied to the previous standard method of XAFS fitting. This improvement also allowed for a more robust interpretation of XAFS fitting parameters and the testing of XAFS theory. This new fitting method was applied to the K-edge XAFS of molybdenum. It was found that the XAFS theory and experiment where inconsistent with each other. New theory is necessary. CCD detectors in a low ux environment have been used for tests of QED. In such situations, distinguishing between signal caused by cosmic rays and a low ux of x-ray photons from the desired source would enable higher precision tests of QED. This work presents a new method of removing the signal caused by cosmic rays from x-ray CCD detector spectra. A new method of calibrating a Johann curved crystal spectrometer is presented, and used to create new characterizations of Ti Kß and V Kß. These characterizations can be used for the calibration of x-ray spectrometers employed for tests of QED. The V Kß peak energy was found to be 5426.962(15) eV. This is an improvement in uncertainty by a factor of 4.7 over prior work. The Ti Kß peak energy was found to be 4931.966(22) eV, an improvement in uncertainty by a factor of 2.6 over the previous best reported result. The limits of the calibration method were tested by measuring Cr Kß, which was on the limit of the calibrated energy range. Preliminary measurements of the w, x, y and z lines of He-like Cr were performed to test QED. The w line energy was measured to be 5682.049(30)(200) eV, a factor of just under two improvement over previous measurements. The energies of the x, y and z lines were found to be 5664.425(73)(200) eV, 5654.232(68)(200) eV and 5626.305(42)(200) eV respectively. These energies had not previously been measured.
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ItemMono-X searches for simplified models of dark matter( 2016)The identity of dark matter remains one of the big open questions in particle physics; while much is known about its distribution throughout the Universe, very little is understood about its particle nature. In particular, a small but non-zero coupling to the Standard Model (SM) sector has not yet been ruled out. WIMP-type dark matter (DM), with weak-scale mass and couplings, may therefore be produced in proton collisions with the Large Hadron Collider (LHC), and detected by the ATLAS experiment. Several collider searches are presented, which utilise the mono-X+ MET (missing transverse energy) topology, wherein DM (the presence of which is inferred through the observation of missing transverse energy) is produced in association with some object X. The mono-jet process has the largest cross section, however mono-boson analyses, the focus of this thesis, have other advantages. The mono-Z(l+l−) channel benefits from the straightforward identification of charged leptons within the detector and removal of the multi-jet background, while the mono-W/Z(jj) channel is able to utilise the growing collection of electroweak boson identification techniques which exploit the two-prong substructure of a large-radius jet. This thesis describes two ATLAS analyses that seek to constrain both Effective Field Theory (EFT) models and simplified models of DM. The ATLAS mono-Z(ll) analysis uses 20.3 fb−1 of data produced at 8 TeV and selects events with a leptonically-decaying Z boson produced back-to-back with a large amount of MET. A cut-and-count method finds that no excess above the SM prediction is observed, and so constraints are calculated for the suppression scale Λ of the EFTs, and for the quark-DM-mediator coupling of a simplified model with a scalar mediator exchanged in the t-channel. The ATLAS mono-W/Z(jj) analysis uses the first 3.2 fb−1 of data produced at 13 TeV, and selects events with a single large-radius jet produced in association with MET. A profile likelihood fit of the SM background estimation and data is used to extract a limit on the signal strength for a vector mediator s-channel simplified model, and converted to a limit on the suppression scale Λ for a ZZχχ contact operator. A reinterpretation of Run I results from ATLAS for three common simplified models is also presented, including a comparison of the results from the mono-jet, mono-Z(l+l−) and mono-W/Z(jj) channels. Limits on the model coupling strengths are discussed. The strongest constraints are obtained with the mono-jet channel, however the leptonic mono-Z channel is able to remove the large multi-jet background to attain limits that are weaker by only a factor of a few. It is essential that the reconstruction of objects within the ATLAS detector, along with their energy measurement and calibration, is well understood and that the performance is optimised. Along with a general discussion of the relevant objects in the detector (leptons, jets and MET), the in situ measurement of corrections to the energy scale of hadronically-decaying tau leptons is described.
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ItemOptically and electrically driven single-photon emission in silicon carbide( 2016)Single-photon sources are considered a key component for future quantum technologies. Their applications in bio-imaging and quantum metrology aside, SPSs are the building block for quantum cryptography and linear optical quantum computing. To significantly enhance their practicality and reduce their cost, some single-photon sources can be driven electrically requiring nothing more than a simple battery. The focus of this work lies on the realisation of such an electrically driven single-photon source, a single-photon emitting diode, in silicon carbide, a wide-bandgap semiconductor that is recently emerging as a versatile platform for photonic and quantum technology. Here, optically and electrically driven single-photon generation in the different polytypes of SiC is investigated. At first, optically active defects that exist at the interface between SiC and SiO2 were discovered and analysed, and a robust method to produce them on a single defect level, i.e. in a density low enough to individually address single defects, is presented. We found that these defects were ultra-bright single-photon emitters in the visible spectral region with fully polarised emission and short lifetimes. With these favourable properties, the defects were then integrated into PN junction diodes and were found to be highly suitable for electrically driven single-photon emission. For the first time, electrically driven single-photon emission under pulsed excitation at room temperature was demonstrated from a defect in a wide bandgap semiconductor. Finally, the fabrication of ultra-thin free standing membranes and the integration of single defects into these membranes, and also into cavity structures to enhance the emission properties, is presented. The origin of the single-photon emitters described in this work is related to the oxidation of the SiC surface, but the atomic structure remains unknown. Given the ease of defect creation and integration into different device architectures, and their excellent properties, the single-photon emitters described in this work are viable candidates for next generation single-photon sources.
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ItemThe study of high redshift galaxies in hierarchical galaxy formation models: epoch of reionisation and galaxy clustering( 2016)Advanced observations of the high redshift Universe will provide important information on the question of how the Universe evolves. The observations will shed light on the early galaxy formation process and the astrophysical properties during the Epoch of Reionisation (EoR). To interpret the observations detailed modelling is required. In this thesis, we study high redshift galaxies using semi-analytical galaxy formation models, GALFORM and Meraxes. We start by studying the clustering of Lyman-break galaxies (LBGs) at z ~ 4. Using GALFORM we predict, for the first time using a semi-analytical model with feedback from active galactic nuclei (AGN), the angular correlation function (ACF) of LBGs and compare the predicted results with new measurements of the ACF from surveys including the Hubble eXtreme Deep Field (XDF) and Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) field. We investigate the dependence of clustering amplitude on luminosity. Using the halo occupation distribu- tion (HOD) predicted by the model, we analyse the connection between galaxies and their dark matter haloes, and effect of AGN feedback on the HOD. We find that al- though the predicted ACFs recover the trend of increased clustering with luminosity, the corresponding dependence is weaker than expected observationally which is con- sistent with existing studies. We find that for the detection limits of the XDF field, central LBGs at z ~ 4 predominantly reside in haloes of mass ~ 10^11 − 10^12 M_{sol}/h that satellites reside in larger haloes of mass ~ 10^12 − 10^13 M_{sol}/h . This model predicts fewer bright satellite LBGs at z ~ 4 than is inferred from measurements of the ACF at small scales. We also find evidence that AGN feedback affects the HOD of central LBGs in massive haloes. We then study the clustering properties of LBGs at z ~ 6 and z ~ 7 using Meraxes. We predict the ACF and compare the results with the measure- ments of the ACF from the same fields. We also investigate the clustering dependence on luminosity, and the evolution of galaxy bias during reionisation. We find that the predicted ACFs are in good agreement with recent observational measurements. We confirm the dependence of clustering on luminosity, implying that the relation between luminosity and dark matter halo mass remains valid at high redshift, with more massive dark matter haloes hosting brighter galaxies. Additionally, the predicted galaxy bias at fixed luminosity is found to increase with redshift, in agreement with observations. We find that LBGs of the rest-frame UV magnitude < −19.4 at 6 < z < 7 reside in dark matter haloes of mean mass ~ 10^11.1 − 10^11.5 M_{sol} , and that this dark matter halo mass does not evolve significantly during reionisation. Finally, we study the cross-correlation between 21-cm emissions and galaxies, which is sensitive to the astrophysical properties of galaxies during the EoR. Using the semi-numerical scheme combined with GALFORM, we study the evolution of the cross-power spectrum, cross-correlation function and cross-correlation coefficient between 21cm emission and galaxies. We find that including different feedback processes changes the cross-power spectrum shape and amplitude. In particular, the feature in the cross-power spectrum corresponding to the size of ionized regions is significantly affected by supernovae feedback. We calculate predicted observational uncertainties of the cross-correlation coefficient based on specifications of the Murchi- son Widefield Array (MWA) combined with galaxy surveys of varying area and depth. We find that the cross-power spectrum could be detected with a galaxy survey with a galaxy redshift error less than 0.1 over several square degrees.
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ItemPhysics beyond the standard model( 2016)In this Thesis we present a collection of original bodies of work pertaining to a number of theoretical/phenomenological questions of the Standard Model, as studied from a "bottom-up" perspective. In Chapter 2: Higgs Sector, we consider the implications of extending the Standard Model Higgs sector by a very light (100 MeV < $m_s$ < $m_h/2$) real singlet scalar field. We identify the regions of parameter space which experiments at the Large Hadron Collider are uniquely sensitive to. There is a strong focus on low background displaced decay signatures. In Chapter 3: Naturalness, we show how a Higgs mass sensitivity measure can be rigorously derived from Bayesian probability theory. We use this measure to constrain the masses of various fermionic and scalar gauge multiplets, obtaining naturalness bounds of O(1-100) TeV. In Chapter 4: Neutrino Mass, we write down the minimal UV completions for all the Standard Model dimension 7 operators which might be responsible for the radiative generation of Majorana neutrino masses. A detailed collider study of a one-loop realisation is performed. In Chapter 5: Baryon Asymmetry of the Universe, we present a proof that the three-flavour Type I seesaw model cannot provide an explanation for neutrino masses and the baryon asymmetry of the Universe via hierarchical leptogenesis without introducing a Higgs naturalness problem. We then describe a minimal extension (the "$\nu$2HDM") which can avoid this conclusion. In Chapter 6: Strong CP Problem, we describe a very minimal model (the "$\nu$DFSZ") which can explain neutrino masses, the baryon asymmetry of the Universe, the strong CP problem, and dark matter, without introducing a naturalness problem for the Higgs. This model serves as an existence proof that weakly coupled high scale physics can naturally explain phenomenological shortcomings of the Standard Model. Lastly, in Chapter 7: Dark Matter, we consider the implications of a class of self-interacting "plasma dark matter" models for direct detection experiments. A number of qualitatively unique signatures (when compared to single component collisionless dark matter) are identified. We emphasise the prediction for a signal which modulates with sidereal day.