School of Physics - Theses
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ItemSpatial adiabatic passage techniques in mesoscopic quantum electronic systems( 2010)Adiabatic passage techniques have been used for coherent population transfer in quantum optical systems. Coherent Tunnelling Adiabatic Passage (CTAP) extends one such technique, the well known STIRAP protocol, into a spatial context. It transports a particle coherently using a counter-intuitive coupling sequence of tun- nel matrix elements. Working in the spatial regime affords us the opportunity to tailor the Hilbert space by controlling the spatial location of states and hence the topology of the resulting quantum network. These techniques provide opportunities for observing and exploring new physics and are applicable to the success of quan- tum information and other quantum technologies. This thesis looks at extensions of adiabatic passage on a number of topics. Particularly, the Alternating coupling scheme variant of the CTAP protocol (ACTAP), and its use in a number of proposed devices, is examined. We show that the ACTAP protocol is robust to variations in device parameters away from ideal conditions. We look at the modelling of a semi- realistic device using phosphorus donors buried in silicon using industry-standard semiconductor device modelling software in combination with quantum mechanical calculations to incorporate parameters needed when considering experimental imple- mentations of this device. Through these calculations we estimate the time required for adiabatic operation of a five donor ACTAP device is approximately 70ns, within measured spin and estimated charge relaxation times. The use of ACTAP in an electron interferometer geometry is then explored. This device shows an interesting interplay between adiabatic and non-adiabatic behaviour, including regimes which are analogous to the electrostatic Aharonov-Bohm effect. An extension of this de- vice, with two coupled interferometers, is explored, motivated by Hardy’s Paradox. Finally the effect of continual measurement of one of the CTAP sites on the fidelity of the protocol is examined. Phenomenological models of decoherence in a three site CTAP device are investigated. We also look at how both periodic and random changes in the on-site energy of one of the dots throughout the CTAP process, as would be induced by a measurement device such as an SET, will affect the fidelity. We link the models developed here with the characteristics of real experimental de- vices. These calculations show that the effect on the fidelity depends on which site is measured and the frequency of the measurement signal.
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ItemShaped electron bunches from ultracold plasma( 2010)This thesis presents the development of a new apparatus, and imaging tech- niques, used to produce shaped cold electron bunches from ultracold plasma (UCP). Due to their low temperature, cold electron bunches from UCP have the potential to provide a compact electron source with sufficient brightness and coherence to enable single-shot ultrafast diffractive imaging of nano-scale samples, such as bio-molecules. To create the electron bunch, a cold (70 μK) cloud of 108 rubidium-85 atoms was prepared in a magneto optical trap (MOT). Some of the atoms were excited to an intermediate state using a laser with a spatially varying in- tensity profile, then ionised by a second laser pulse. The shaped excitation laser allowed two-dimensional control of the electron density. The ionisation laser could, in principle, also be spatially varying, to allow three dimensional manipulation of the electron bunch shape. Due to Coulomb repulsion betweeen electrons within a bunch, the brightness of the source is critically dependent on its initial shape, and therefore the shape of the atom cloud. Uniform density bunches are particularly desirable due to their linear space-charge expansion. To optimise the source through production of uniform bunches, knowledge of the spatial density distribution of atoms in the cloud is required. Conventional imaging techniques for cold atoms are either technically demanding or destructive to the atom cloud, and are unsuitable for this application, providing direct motivation for the development of new methods. I have developed two new imaging techniques for use in shaped electron bunch production. The new techniques are particularly suited to our application, but are applicable to the cold atom research community in general, offer- ing several advantages over conventional methods. The first method adapts a phase-contrast imaging technique to measure the spatial distribution of atoms in a specific excited state. The second approach allows single-shot imaging of inhomogeneous atom clouds; that is, where both the density and refractive index may be spatially varying. The method uses a perturbative approach in conjunction with phase retrieval based on the transport of in- tensity equation. This technique is also potentially valuable for studies of atomic coherence effects in cold atoms and was demonstrated using spatially modulated electromagnetically induced transparency. In collaboration with other members of our research group and in parallel with the imaging research, I also designed and constructed a new appara- tus to produce UCP. The experiment first produced electron bunches in late 2009, and using the imaging techniques I developed, the results of the first shaped electron bunches from UCP are presented here. Due to the low tem- perature of the electrons, such shaped bunches can only be produced and observed using a cold electron source. Conventional thermal sources, includ- ing photoemission and field emission sources, produce hot electrons whose high temperature immediately diffuses any initial structure. The effects of increased electron temperature on the quality of the bunch shape are inves- tigated here, resulting in the conclusion that cold electrons are essential for observing, and thus optimising, shaped bunches. Arbitrarily shaped bunches are demonstrated for the first time, using an intensity shaped excitation laser beam followed by uniform ionisation of the excited atom distribution. The laser intensity profile is adjusted using a spatial light modulator (SLM). Variations in atomic density which would degrade the bunch shape are measured using the imaging techniques developed. Adjustment of the excitation laser intensity to compensate for atomic density is shown to produce uniform bunches in two dimensions. Quantitative analysis of the acuity of the bunch edge provides an upper limit to the electron temperature of T = 35 K. Unlike photoemission sources, the electron bunch shaping mechanism demonstrated here can easily be generalised to three dimensions. In addition, field ionisa- tion of Rydberg atoms is observed to play an important role in the ionisation process, and suggested as a future avenue of research.
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ItemNovel single photon emitters based on color centers in diamond( 2010)Exploitation of emerging quantum technologies requires efficient fabrication of key building blocks. Single photon sources are one of these fundamental constituents that are presently pushing the bounds of existing materials and fabrication techniques. Color centers in diamond are very attractive in this respect since they are the only photostable solid-state single photon emitters operating at room temperature known to date.
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ItemStudies of the electrical and optical properties of defects in ion implanted silicon( 2010)Ion implantation is a technique commonly used in the fabrication of semiconductor devices. This process creates lattice damage which can adversely affect the device efficiency. Conversely, implantation damage in more recent years has allowed for the possibility of new silicon based optoelectronic devices. However, the exact conditions that cause particular defects to accumulate and dissociate are extremely complex. The ion species, implantation energy and fluence, angle of implantation, pre-existing impurities and defect structures, and processing temperatures are some of the main factors known to play a key role in damage accumulation and evolution. This thesis investigates aspects of the accumulation, migration and dissociation of defects created by the implantation of the light ions, hydrogen and helium, and dopants, boron and phosphorus, into Czochralski-grown silicon. The identification and monitoring of particular defects using the electrical characterisation technique, deep-level transient spectroscopy (DLTS) and the optical characterisation technique, photoluminescence (PL) is central to this investigation. Damage accumulation as a function of implantation angle in relation to the silicon [100] channelled-axis has been studied using DLTS. For the first time depth profiles of specific stable electrically active charge traps created by the channelled implantation of light ions are measured. Specifically, traps created in the implantation of 70 keV hydrogen and 500 keV helium as well as 450 keV phosphorus, have been profiled. These profiles have been directly compared to the expected vacancy profiles simulated using the Monte Carlo style code Crystal-TRIM. The depth correlation between experiment and simulation show remarkable agreement. The results presented provide a further understanding of defect formation and the role of dynamic annealing during channelled implantation. Defect accumulation and stability in the vicinity of an oxide has also been investigated using DLTS. For the first time the bulk and interface defect concentrations have been obtained and compared for atomic and dimer phosphorus implanted though a 50 nm insulating surface oxide. Thermal treatment in the temperature range of 200-400◦C demonstrates the annealing characteristics of the bulk and interface defects which are thought to be influenced by the presence of hydrogen in the oxide-silicon interface region. The bulk defect identified as the vacancy-oxygen-hydrogen centre was found to be the dominant defect after a 300◦C anneal. The results presented provide new understanding of defect evolution and minimisation in the near-interface region of a metal-oxide-semiconductor capacitor. Enhanced band-edge luminescence, through the creation of boron-related dislocation networks, has been widely reported and proposed by Louren¸co, Ng and Homewood et al. as a method for the creation of an all-silicon light emitting diode (LED). In this thesis PL has been used to monitor the band-edge PL of furnace and excimer-laser annealed samples implanted with 30 keV boron. Through the series of measurements presented it is shown that the proposed model of Louren¸co et al. is not consistent with some of our experimental data. The combination of our results and those of other studies discussed signifies the lack of consensus in the understanding of how the luminescence of the proposed LED device arises and how it can be optimised. Finally, the interstitial-silicon related W and X-centres and the boron related Y –centre have been monitored using PL, and a new understanding of the boron and interstitialsilicon diffusion and clustering at low temperatures has been obtained. The presented results provide new evidence of clustering and diffusion effects for anneals in the temperature range of 175-400◦C where W-centre formation is maximised but which also have bearing on defect interactions during the initial temperature-ramp stages of anneals performed at much higher peak temperatures. Also, a fabrication technique has been proposed which would provide a means of conveniently position optical centres, particularly the W-centre, away from the majority of implantation related damage but within electrical contact of the surface. This technique could be useful in the fabrication of an all-silicon optoelectronic device which could operate at temperatures up to and exceeding 80 K.
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ItemNovel quantum technology based on atom-cavity physics( 2010)Cavity quantum electrodynamics explores the dynamics and manipulation of quantized electromagnetic fields and atomic systems coherently coupled inside an electromagnetic resonator or cavity. High-quality, small-volume cavities have been exploited as the critical component in numerous promising schemes for developing quantum technology in this era of quantum revolution. Within this context, this thesis investigates various prospective quantum technology based on atom-cavity systems, from new optical-cavity designs, through to cavity-enhanced single-photon sources in diamond for secure communication, quantum controls for photon pulse-shaping, qubit preparation and implementing faster quantum gates, and finally novel metamaterials built of cavity-arrays. The discussion begins with a proposal of novel slot-waveguide cavity designs for visible wavelength operations. Optimization to minimize cavity mode volumes in these structures is conducted, showing that they are capable of generating stronger atom-photon couplings than state-of-the-art cubic-wavelength cavities. The use of the diamond colour centres are then reviewed with original work carried out to show that spectral bandwidth/purity, emission rate, quantum efficiency and reliability can be improved by the use of cavities. These results improve the prospects of the centres for practical applications in long-distance, high-speed quantum communication. Next, a dynamic control for modifying effective cavity properties in a coupled-cavity arrangement is introduced. The scheme can be applied to perform reversible switching of single photons into and out of cavities, and temporal shaping and time-encoding of single photons. The reversible photon-switching control is applied to implement a two-qubit quantum gate, which by breaking the time-bandwidth limitation, outperforms comparable passive devices. Finally, we present a novel metamaterial design based on cavity-arrays and coupled atom-cavity systems, which can be used to engineer anomalous light response and complex waveguiding properties. In particular, as an introductory application, demonstrations of superlensing effects, which are pertinent for subwavelength imaging, are presented.
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ItemQuantitative studies of x-ray optical coherence( 2010)The continued development of synchrotron sources, especially over the previous two decades, has allowed the new field of coherent X-ray optics to flourish. Rapid advancement in the field is set to continue, with the recent completion of the new X-ray free electron laser (XFEL) source at the Stanford Linear Accelerator Centre. Unlike visible wavelength lasers, XFEL and synchrotron sources are not fully coherent, meaning that coherence based imaging techniques are not at present fully optimised for use with these sources. A pre-requisite for the optimisation of coherence based imaging techniques, is the full characterisation of the coherence properties of the incident beam used in these techniques. The task of characterising the coherence properties of an optical wavefield was first investigated by Zernicke in 1938. His use of Young’s double slit experiment remains to this day a popular means of performing a basic measurement of the coherence properties of a wavefield. A limitation of such measurement techniques arises because the coherence properties of an incident wavefield are fully described in terms of a four-dimensional correlation function. This means that a one-dimensional measurement such as a Young’s double slit type experiment fails to provide sufficient information about the wavefield to fully characterise the beam. Over the past 15 years, there has been a development of non-interferometric methods of coherence measurement which seek to obtain a full characterisation of the four-dimensional correlation function. The author develops a non-interferometric iterative method for mapping the correlation function, which is based upon the analysis of the partially coherent wavefield in terms of its coherent modes. A computational method for numerically extracting the coherent modes of a partially coherent wavefield is presented, which is followed by the development of an iterative scheme for experimentally determining the form of these modes. The iterative scheme is then quantitatively evaluated for both experimental and simulated data, and its performance represents a significant advancement towards the goal of achieving a fully general characterisation of optical coherence.
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ItemAtomic resolution imaging in two and three dimensions( 2010)This thesis explores theoretical aspects of scanning transmission electron microscopy (STEM) and the comparison of simulation with experiment. The long standing contrast mismatch problem between theory and experiment in conventional high resolution transmission electron microscopy (HRTEM) is examined using the principle of reciprocity and bright field scanning transmission electron microscopy (BFSTEM). It is found that quantitative agreement between theoretical and experimental images is possible provided that theory suitably accounts for the spatial incoherence of the source, and that experimental images are placed on an absolute scale with respect to the incident beam current. Agreement between theory and experimental image contrast is found to be independent of specimen thickness and probe defocus. Core-loss electron energy-loss spectroscopy (EELS) is a powerful experimental tool with the potential to provide atomic-resolution information about the electronic structure at defects and interfaces in materials and nanostructures. Interpretation, however, is nonintuitive due to the nonlocal ionization potential. Novel improvements in microscope design and operating environment have enabled two dimensional chemical maps. This has permitted a more thorough theoretical analysis. This thesis compares experimental STEM EELS images of LaMnO3, BiSrMnO3 and Si samples to the relevant theoretical simulations. Image features which at first appear counter intuitive are discussed and explained with the accompanying theoretical simulations. It is demonstrated, using a sample of SrTiO3, that more direct interpretation of atomic resolution chemical maps is possible when using energy dispersive x-ray spectroscopy (EDS) in STEM. This thesis considers extending chemical mapping in STEM EELS to three dimensions using depth sectioning. It explores, theoretically, the feasibility to depth section zone-axis aligned crystals that contain embedded impurities. In STEM EELS this is found to be possible for point defects but not for larger extended objects such as nanoparticles. The theory describing the mechanism by which contrast is obtained in elastic scanning confocal electron microscopy (SCEM) is developed. It is shown that there is no first order phase contrast in SCEM and thus low image contrast. Finally, energy filtered scanning transmission electron microscopy (EFSCEM) is developed theoretically. The fundamental equation describing image formation is derived and an efficient computation method is developed to allow the rapid calculation of EFSCEM images.
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ItemQuasar microimaging( 2010)Observations of gravitationally microlensed quasars offer a unique opportunity to probe quasar structure on extremely small scales. In this thesis, we conduct extensive microlensing simulations and compare with observational data to constrain quasar accretion discs, and conduct preliminary probes of broad emission line region structure. This analysis is done using a new single-epoch imaging technique that requires very little telescope time, and yet produces results that are comparable to those obtained from long-term monitoring campaigns. We begin by exploring the impact of variable smooth matter percentage and source size on microlensing simulations. Adding a smooth matter component affects minimum and saddle point images differently, broadening the magnification distribution for the saddle point image significantly. However, increasing the radius of the background source washes out this difference. The observation of suppressed saddle point images can therefore only be explained by microlensing with a smooth matter component if the background source is sufficiently small. We use these simulations, in combination with I-band imaging of the lensed quasar MG 0414+0534 to constrain the radius of the quasar source. This demonstrates the viability of a single-epoch imaging method for constraining quasar structure. This technique is then expanded to single-epoch multi-band observations, in order to constrain the radial profile of quasar accretion discs as a function of observed wavelength. We present new Magellan observations of two gravitationally lensed quasars: MG 0414+0534 and SDSS J0924+0219. We also analyse two epochs of Q2237+0305 data obtained from the literature. Our results are compared with four fidicial accretion disc models. At the 95 per cent level, only SDSS J0924+0219 is inconsistent with any of the accretion disc models. When we combine the results from all three quasars -- a first step towards assembling a statistical sample -- we find that the two steepest accretion disc models are ruled out with 68 per cent confidence. In addition, we are also able to use our microlensing simulations to constrain the smooth matter percentages in the lenses at the image positions. In both MG 0414+0534 and SDSS J0924+0219 we find smooth matter percentages that are inconsistent with zero. A smooth matter percentage of approximately 50 per cent is preferred in MG 0414+0534, and approximately 80 per cent in SDSS J0924+0219. Q2237+0305 is usually assumed to have a smooth matter percentage of zero at the image positions, as they lie in the bulge of the lensing galaxy. Though our measurement is consistent with a zero smooth matter percentage, there is a peak in the probability distribution at a value 20 per cent. This is perhaps a hint of additional intervening structures along the line of sight to the background quasar. We test the sensitivity of our accretion disc constraints to a range of modelling parameters. These include errors in lens modelling, Bayesian prior probability selection, errors in observational data, and the microlens mass function. Constraints on the power-law index relating source radius to observed wavelength are found to be relatively unaffected by changes in the modelling parameters. Constraints on source radii are found to be more strongly affected. Finally, the broad emission line region of Q2237+0305 is examined. Gemini IFU observations are presented clearly showing differential microlensing across the velocity profile of the CIII] emission line. To analyse this signature, we present three simple broad emission line region models: a biconical outflow, a Keplerian disc, and spherical infall. A method is developed to compare the shapes of simulated flux ratio spectra with the observed spectrum. We are unable to discriminate between the biconical outflow and Keplerian disc models when we average over all viewing angles and orientations. The spherical infall model, however, does not fit the observed data. We also find that for the non-spherically symmetric geometries, low inclination angles are essentially incompatible with the observations. This analysis offers hope that with sufficiently high signal-to-noise observations, differential microlensing signatures may allow us to constrain the geometry and kinematics of this poorly understood portion of quasar structure.