Department of Physics

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Additional degrees of freedom associated with position measurements in non-commutative quantum mechanics
(Stellenbosch : University of Stellenbosch, 2010-12) Rohwer, Christian M.; Scholtz, Frederik G.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Due to the minimal length scale induced by non-commuting co-ordinates, it is not clear a priori what is meant by a position measurement on a non-commutative space. It was shown recently in a paper by Scholtz et al. that it is indeed possible to recover the notion of quantum mechanical position measurements consistently on the non-commutative plane. To do this, it is necessary to introduce weak (non-projective) measurements, formulated in terms of Positive Operator-Valued Measures (POVMs). In this thesis we shall demonstrate, however, that a measurement of position alone in non-commutative space cannot yield complete information about the quantum state of a particle. Indeed, the aforementioned formalism entails a description that is non-local in that it requires knowledge of all orders of positional derivatives through the star product that is used ubiquitously to map operator multiplication onto function multiplication in non-commutative systems. It will be shown that there exist several equivalent local descriptions, which are arrived at via the introduction of additional degrees of freedom. Consequently non-commutative quantum mechanical position measurements necessarily confront us with some additional structure which is necessary (in addition to position) to specify quantum states completely. The remainder of the thesis, based in part on a recent publication (\Noncommutative quantum mechanics { a perspective on structure and spatial extent", C.M. Rohwer, K.G. Zloshchastiev, L. Gouba and F.G. Scholtz, J. Phys. A: Math. Theor. 43 (2010) 345302) will involve investigations into the physical interpretation of these additional degrees of freedom. For one particular local formulation, the corresponding classical theory will be used to demonstrate that the concept of extended, structured objects emerges quite naturally and unavoidably there. This description will be shown to be equivalent to one describing a two-charge harmonically interacting composite in a strong magnetic eld found by Susskind. It will be argued through various applications that these notions also extend naturally to the quantum level, and constraints will be shown to arise there. A further local formulation will be introduced, where the natural interpretation is that of objects located at a point with a certain angular momentum about that point. This again enforces the idea of particles that are not point-like. Both local descriptions are convenient, in that they make explicit the additional structure which is encoded more subtly in the non-local description. Lastly we shall argue that the additional degrees of freedom introduced by local descriptions may also be thought of as gauge degrees of freedom in a gauge-invariant formulation of the theory.
Alpha cluster structure and scattering in 20Ne, 44Ti, 94Mo, 136Te and 212Po
(Stellenbosch : University of Stellenbosch, 2011-03) Ndayishimye, Joram; Wyngaardt, Shaun M.; Perez, S. M.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: We investigate the nuclei 20Ne, 44Ti, 94Mo, 136Te and 212Po using a model of an α-cluster orbiting a closed shell core. A purely phenomenological cluster-core potential is found to provide a successful description of the spectra, B(E2↓) transition strengths, and α-decay rates of the low-lying positive parity states of these nuclei. We then use the same potential as the real part of an optical model potential to describe the α elastic scattering by 16O, 40Ca, 90Zr and 208Pb. The experimental differential cross-section data are reasonably well reproduced with the imaginary potential depth as the only free parameter. The special case of the 8Be system is also analysed.
Analysis and applications of the generalised Dyson mapping
(Stellenbosch : Stellenbosch University, 2004-12) Snyman, Izak; Geyer, H. B.; Scholtz, Frederik G.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: In this thesis, generalized Dyson boson-fermion mappings are considered. These are techniques used in the analysis of the quantum many-body problem, and are instances of so-called boson expansion methods. A generalized Dyson boson-fermion mapping, or a Dyson mapping for short, is a one-to-one linear but non-unitary operator that can be applied to vectors representing the states of a many-fermion system. A vector representing a fermion system maps onto a vector that is most naturally interpreted as representing a state of a many-body system that contains both bosons and fermions. The motivation for doing such a mapping is the hope that the mapping will reveal some property of the system that simplifies its analysis and that was hidden in the original form. The aims of this thesis are 1. to review the theory of generalized Dyson boson-fermion mappings, 2. by considering a tutorial example, to demonstrate that it is feasible to implement the theory and 3. to find a useful application for a generalized Dyson boson-fermion mapping, by considering a non-trivial model, namely the Richardson model for superconductivity. The realization of the first two aims mainly involve the collecting together of ideas that have already appeared in the literature, into one coherent text. Some subtle points that were treated only briefly due to space restrictions in the journal publications where the theory was first expounded, are elaborated on in the present work. On the other hand, the analysis of the Richardson Hamiltonian that uses a Dyson mapping, goes beyond what has already appeared in the literature. It is the first time that a boson expansion technique is implemented for a system where the roles of both collective and non-collective fermion pairs are important. (The Dyson mapping associates bosons with Cooper pairs, while the fermions not bound in Cooper pairs result in fermions being present in the mapped system as well.) What is found is that the Dyson mapping uncovers non-trivial properties of the system. These properties aid the construction of time-independent perturbation expansions for the stationary states of the system, as well as time-dependent expansions for transition amplitudes between states. The time-independent expansions agree with results that other authors obtained through methods other than boson expansions. The time-dependent expansions, that one would be hard-pressed to develop without a Dyson mapping, might in future prove useful in understanding aspects of the dynamics of ultracold fermi gases, when time-dependent magnetic fields are used to vary the atom-atom interaction strenght.
The analysis of multi-clustering in heavy nuclei
(Stellenbosch : Stellenbosch University., 2020-04) Carolus, Timothy Gary; Wyngaardt, Shaun M.; Malaza, Vusi D.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The binary cluster model is the simplest mathematical model which is used to describe a system of strongly interacting, and highly correlated nuclear matter. This study will serve as a means to better understand the phenomenon of nuclear clustering beyond a 208P b-alpha cluster core system. Theoretically predicted observables, such as the cluster decay half-life, positive parity cluster states, and the reduced electric dipole transition (B(E2)) are compared to the available experimentally observed quantities. The results show that the tested systems for various cluster-core configurations, of both a fixed parent nuclei and fixed stable core favoured the phenomenological description of their interaction. A numerical optimization procedure is applied to the core- cluster potential of the binary cluster formalism and is then used as an analytical tool in order to predict the possibility of higher modes of cluster decay in 216Rn.
Analyzing power and cross section distributions of the 12C (p,pα)8Be cluster knockout reaction at an incident energy of 100 MeV
(Stellenbosch : University of Stellenbosch, 2010-03) Mabiala, Justin; Cowley, A. A.; Fortsch, S. V.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The (p, pα) reaction on 12C was investigated experimentally using polarized incident protons of 100 MeV. Coincident data, which were obtained at ten quasifree angle pairs for proton angles ranging from 25◦ to 110◦, were analyzed in terms of the distorted-wave impulse approximation (DWIA). Calculated energy-sharing cross section and analyzing power distributions reproduce the data reasonably well. The observed agreement allows the extraction of distorted momentum distributions from experimental data. These distributions are very consistent over a wide range of angle pairs at which cross section energy-sharing distributions vary considerably. Since measurements of analyzing powers were made, spin-orbit distortions were included in the DWIA calculations. The effects of spin-orbit distortions were found to be very small near zero recoil momentum and did not destroy the validity of the factorization approximation where the two-body p-α cross section enters as a multiplicative factor in the three-body (p, pα) cross section expression. Spectroscopic factors derived from the data are fairly consistent with the trend of the theoretical predictions. Analyzing power data also follow the trend of free p-4He scattering data, and comparisons with DWIA predictions are in reasonable agreement. The theory reproduces also very well analyzing power angular distributions of the projectile-cluster two-body scattering at large angular momentum of the residual nucleus. This indicates that a quasifree knockout mechanism dominates the reaction. The two-body interaction response between the projectile and the α cluster was found to resemble the scattering of protons from a free α particle to a remarkable degree, the present results strongly imply the existence of preformed α clusters in 12C.
Application of the relativistic random-phase and distorted wave impulse approximations to quasielastic proton-nucleus scattering
(Stellenbosch : University of Stellenbosch, 2010-12) Van Niekerk, David Douglas; Van der Ventel, B. I. S.; Hillhouse, G. C.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: In this dissertation a fully relativistic model for polarized inclusive quasielastic proton-nucleus scattering is developed. Using a standard relativistic impulse approximation (RIA) treatment of quasielastic scattering and a two-body SPVAT form of the current operator, it is shown how the behaviour of projectile and target can be decoupled. Subsequently, different models for projectile and target can be adopted and combined to examine a variety of relativistic effects. The most simplistic model of the target is provided by a mean-field nuclear matter approximation to the relativistic meson-nucleon model, quantum hadrodynamics (QHD). Here relativistic effects manifest as an effective mass, which is lower than the free mass, of the constituent nucleons. This model is improved upon by including many-body correlations through medium-modification of meson propagators in the relativistic random-phase approximation (RPA). Since it is generally accepted that the strong nuclear force and the extended range of the nuclear potential lead to distortion effects on the projectile and ejectile (seen as a modulation of the wave functions), our formalism is geared towards the use of relativistic distorted waves (RDWIA). The distorted waves are written as partial wave expansions and are solutions to the Dirac equation with potentials. The inclusion of distortions, however, greatly increases the computational burden and we show how a number of analytical and numerical techniques can be used to facilitate the process of calculation. It is also shown how the standard relativistic plane wave treatment (RPWIA) can, instead, be easily employed to obtain a baseline for determining the impact of distortions. A calculation is performed for the reaction 40Ca(!p, !p !) at a beam energy of 500 MeV. Here it is found that the effect of correlations on the RPWIA calculation can be seen as a quenching of the cross section that is expected to become more pronounced at lower energies or for higher density targets. A RDWIA calculation shows additional reduction and if target correlations are included this effect is enhanced. To our knowledge this is the first calculation that attempts to include both these effects (RPA and RDWIA) in the context of quasielastic proton-nucleus scattering.
Aspects of femtosecond pulse shape transfer via difference frequency mixing
(Stellenbosch : Stellenbosch University, 2015-12) Botha, Gerda Nicolene; Uys, Hermann; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: We investigate the generation of shaped femtosecond pulses in the infrared spectral regime via a nonlinear process called di erence frequency mixing. First we develop a detailed model of the process, incorporating pulse propagation during di erence frequency mixing, in the slowly varying envelope approximation. Di erence frequency mixing (DFM) is numerically simulated for several wavelengths, nonlinear crystals and Type I and Type II frequency mixing. The di erent factors in uencing the shape transfer e ciency of a shaped pulse to a di erent wavelength regime, as well as the conversion e ciency, is identi ed and investigated thoroughly by doing a parametric study. The numerical modeling demonstrates that the e ciency, with which a shaped pulse in the near-infrared is transferred to another wavelength regime, depends strongly on the refractive index of the nonlinear medium for the interacting pulses and is optimal when the velocity of the generated pulse equals that of the shaped input pulse. We show that it is possible to control the temporal pulse duration of the generated pulse by using speci c input angles and so manipulating the e ective refractive index of the nonlinear material for the input and generated pulses. It was found that it is possible to temporally broaden or narrow the generated pulse relative to the input pulses. We compare the developed numerical model to experimental measurements. A liquid crystal spatial light modulator (SLM), inserted in a 4f setup, is used to generate the shaped pulses. Experimentally we demonstrate high- delity shape transfer by mixing 795 nm and 398 nm femtosecond pulses in a BBO crystal. The temporal broadening and narrowing of the generated pulse is also shown and compared to the numerical simulations showing excellent agreement with measured results.
Aspects of quantum field theories
(Stellenbosch : Stellenbosch University, 1997) Van Biljon, Andrew; Scholtz, Frederik G.; Stellenbosch University. Faculty of Science. Department of Physics.
ENGLISH ABSTRACT: In this study project we give a general introduction to quantum field theories. In the first chapter we revise the operator formalism of quantum mechanics as well as the second quantization scheme that is used to describe many-particle systems. In the second chapter we develop the idea of path integrals within a quantum mechanics framework. We then apply path integral formalism developed in chapter two to introduce quantum field theories. We describe a field theory for scalar fields in chapter three and then a field theory for fermion fields in chapter four as well as the renormalization techniques in chapter five. In chapter six we show how scattering amplitudes are related to Green's functions which are derived from the path integral formalism. In the last chapter we give a brief introduction to gauge field theories.
Basic concepts of random matrix theory
(Stellenbosch : University of Stellenbosch, 2005-12) Van Zyl, Alexis J.; Scholtz, Frederik G.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
It was Wigner that in the 1950’s first introduced the idea of modelling physical reality with an ensemble of random matrices while studying the energy levels of heavy atomic nuclei. Since then, the field of Random Matrix Theory has grown tremendously, with applications ranging from fluctuations on the economic markets to M-theory. It is the purpose of this thesis to discuss the basic concepts of Random Matrix Theory, using the ensembles of random matrices originally introduced by Wigner, the Gaussian ensembles, as a starting point. As Random Matrix Theory is classically concerned with the statistical properties of levels sequences, we start with a brief introduction to the statistical analysis of a level sequence before getting to the introduction of the Gaussian ensembles. With the ensembles defined, we move on to the statistical properties that they predict. In the light of these predictions, a few of the classical applications of Random Matrix Theory are discussed, and as an example of some of the important concepts, the Anderson model of localization is investigated in some detail.
Bayesian parameter estimation for discrete data spectra
(Stellenbosch : Stellenbosch University, 2017-12) Wang, Li; Eggers, H. C.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT : Discrete spectra are ubiquitous in physics; for example nuclear physics, laser physics and experimental high energy physics measure integer counts in the form of particles in dependence of angle, wavelength, energy etc. Bayesian parameter estimation ( tting a function with free parameters to the data) is a sophisticated framework which can handle cases of sparse data as well as input of pertinent background information into the data analysis in the form of a prior probability. Bayesian comparison of competing models and functions takes into account all possible parameter values rather than just the best t values. We rst review the general statistical basis of data analysis, focusing in particular on the Poisson, Negative Binomial and associated distributions. After introducing the conceptual shift and basic relations of the Bayesian approach, we show how these distributions can be combined with arbitrary model functions and data counts to yield two general discrete likelihoods. While we keep an eye on the asymptotic behaviour as useful analytical checks, we then introduce and review the theoretical basis for Markov Chain Monte Carlo numerical methods and show how these are applied in practice in the Metropolis-Hastings and Nested Sampling algorithms. We proceed to apply these to a number of simple situations based on simulation of a background plus two or three Gaussian peaks with both Poisson and Negative Binomial likelihoods, and discuss how to select models based on numerical outputs.
Biofilament interacting with molecular motors
(Stellenbosch : Stellenbosch University, 2015-12) Meylahn, Janusz Martin; Muller-Nedebock, Kristian K.; Touchette, Hugo; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: We study molecular motors moving along a filament or polymer using two different mathematical models in which motors are idealised as springs. In the first model we study the average and the fluctuations of the motor stretch by modelling the motion of the motors along the filament using a simple stochastic differential equation with linear friction. We use the notion of stochastic resetting to explicitly include the attachment and detachment dynamics of the motors to and from the filament and study the fluctuations around the most probable value of the mean stretch using methods from large deviation theory. The second model uses methods from field theory to model a dynamic network consisting of a single polymer and many molecular motors. In this case, we develop techniques to include the bias motion of the molecular motors in a weighting factor for the formation of specific networks rather than in the dynamical constraints of the partition function which allows us to study the steady-state of the network using a self-consistency argument and a saddlepoint approximation.
Bound states near the interface of a distorted graphene sheet and a superconductor
(Stellenbosch : Stellenbosch University, 2011-12) Van Zyl, Hendrik Jacobus Rust; Snyman, Izak; Scholtz, Frederik G.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The goal of this thesis is to investigate the effects of distorting a graphene lattice and connect- ing this distorted graphene sheet to a superconductor. At low energies the possible excitation states in graphene are restricted to two distinct regions in momentum space called valleys. Many electronic applications are possible if one can design a graphene system where excitations can be forced to occupy a single valley in a controllable way. Investigating the spectrum of the distorted graphene sheet reveals that, if the chemical potential is chosen to coincide with a bulk Landau level, the normal-superconductor interface always supports propagating modes in both directions. Excitations from opposite valleys travel in opposite directions along the interface. The spectrum of a distorted graphene sheet terminated by an armchair edge, in contrast, is dis- persionless. We verify this insulating nature of the armchair edge for finite samples by numerical means. Furthermore, we verify previous analytical results pertaining to a graphene sheet with NS interface and an applied perpendicular real magnetic field numerically. In the process, it is shown that considering graphene sheets of perfect width is not necessary, as long as the width a few magnetic lengths away from the interface is well-defined. By then considering a finite graphene sheet, terminated by armchair edges, that is distorted and connected to a superconductor, we find bound states near the NS interface that can be changed by distorting the graphene lattice further.
Cave front tracking experiment
(Stellenbosch : Stellenbosch University, 2016-12) Green, Mark; Muller-Nedebock, Kristian; Boonzaaier, Leandro; Lynch, Richard; Stellenbosch University. Faculty of Science. Department of Physics
ENGLISH ABSTRACT : Seismic tomography, a procedure to trace seismic wave velocity variation, has been used for more than 100 years to “see through rock”. Initially the recording of seismic waves from natural sources (earthquakes) using seismographs, provided the first evidence of the crust - mantle boundary. Further development of recording techniques and use of explosive seismic sources were employed for the delineation of the geological structures of mineral deposits (primarily oil and gas) as well as detailing the earth - mantle location and geometry. Ultra sonic techniques have been applied in the laboratory experiments to characterise rock sample velocities and its relationship to changes in temperature and stress. Active seismic tomography for hydrocarbon resources determination as well as for the earth structure utilized both reflection and refraction techniques. During the early 60’s seismic techniques were employed in active mines to investigate rock properties for geotechnical purposes. The objective of the Cave Front Tracking Experiment was to investigate whether active seismic tomography could potentially be used to track the cave back progression of a block cave mine. A small scale experiment was conducted to study the variations in seismic travel times associated with ray path refraction caused by stress changes and changes in the rock fabric induced by sub-level mining. A piezoelectric transducer was employed as an active seismic source and a seismic recording system was installed to monitor a volume of rock through which sub-level mining was propergated. Implementation involved the deployment of 11 seismic detectors and the recording of pre-stacked data. The experimental details and hardware characteristics are discussed, which includes technical problems encountered, the process and techniques of significant data stacking in order to recover weak signals generated by a piezoelectric seismic source and travel time variation calculation using signal cross-correlation. Signal travel time variation measurements were made over the course of a year and a half (2013-14), however only five months are investigated in detail and related to mining activities. Finite difference numerical modelling was also employed in order to create a better understanding of the sub-level mining process and its effect on seismic signal delay change. Technical problems associated with the signal processing and signal behavior interacting with an advancing mining front were identified and a number of techniques were employed to overcome problems faced by routine cave front estimation in a working mine environment. The field testing of a prototype impact source that is pneumatically driven and an eccentric rotating mass device is discussed.
Characterisation of heavy mineral sands and soils by radiometry and its use in mineral benefication and agriculture
(Stellenbosch : University of Stellenbosch, 2011-03) De Villiers, Dawid; Stander, J. A.; Rozendaal, Abraham ; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: Radioactivity is well known and well understood, but its usefulness in industrial applications to optimise processes or increase economic viability is not yet fully utilised by many industries. This study focuses on the measurement of natural radioactivity and its application in heavy mineral separation and vineyard soil classification. The gamma radiometry set-up consisted of a high purity germanium detector, a Marinelli beaker as sample container and associated electronics. It was calibrated for laboratorybased measurements by minimising the background radiation with the use of lead castle and energy and resolution calibrations. Furthermore, detection parameters were optimised; these included the counting time, the selection of gamma rays used for analysis of a sample, the peak area calculation for the detector dead time and the detector efficiency. Given that the samples had different densities and volumes, the detector efficiency had to be corrected for volume and density effects. After implementation of the corrections and optimisations the detection system was tested and found able to accurately measure radioactivity concentrations. The systematic measurement errors for 238U were 5.1 % in the case of the heavy mineral sands and 34.3 % for the vineyard soils, 4.5 % for the 232Th concentrations and 4.7 % for 40K concentrations. Statistical errors were kept below 2 %. The application of radiometry has not been done before at any South African heavy mineral separation plant. For this reason radiometry is suggested as an easier, faster and cheaper alternative to X-ray fluorescence (XRF) for effective implementation of grade control for zircon to improve the cost benefit of the heavy mineral separation process. Zircon is an example of a heavy mineral that is worldwide in demand with a consumption of more than a million tonnes per year. It is used in a wide range of industrial applications and products that include tiles, sanitary ware and plasma displays. South Africa is the second largest producer of zircon in the world and also has the second largest reserve of available zircon, making this mineral a viable source of income for several years to come. Radioactivity, in the form of uranium and thorium, and other impurities such as iron oxide and titanium oxide are found in the crystal lattice of zircon. For it to be a sellable product, the sum of the uranium and thorium concentrations must be less than 500 parts per million for prime or first grade zircon and less than 1000 parts per million for second grade zircon. At present the concentrations of uranium and thorium in zircon concentrates are measured on a whole rock basis by XRF during and at the end of the processing cycle before the final products are ready to be shipped. This is not an ideal situation as the grab samples are taken periodically and are not necessarily representative of the stream or final assignment and has resulted in significant losses by the producer. The solution is to accurately measure the uranium and thorium concentrations fast or immediately, preferably the measurements must be made online and in real time so that processing decisions can be implemented quickly to optimise the final product. Heavy mineral sand samples were obtained from the various separation processes in a Mineral Separation Plant and their 238U and 232Th concentrations determined. The results indicated that the samples’ uranium to thorium ratios together with their total concentrations can be used to differentiate between the samples (i.e. separation processes). The measurement results were compared with those obtained with XRF. The correlations with radiometry were excellent for the uranium (r2 = 0.992), thorium (r2 = 0.998) and total concentrations (r2 = 0.998). Radiometric measurements were also conducted by decreasing the counting time from 3600 s to 1 s to investigate its effect on the accuracy of the results. Correlations between the different times and 3600 s ranged from excellent to good. The obtained results are then used to recommend that radiometry is used in a Mineral Separation Plant to verify that the zircon and zirkwa meet the specifications, to optimise the entrance feed and the other separation processes and to monitor the tailings streams. Finally the practical aspects of the implementation of radiometry are discussed. As a second application was radiometry applied in an agricultural pilot study to demonstrate the applicability of radiometry as a possible useful tool in soil classification. The creation of a vineyard is a long term and expensive investment and its yield and quality will be influenced by many factors such as the type of soil, viticultural preparations and climate. Information on the different soil types in a vineyard is therefore indispensable for the optimisation of land use with respect to vine cultivar, wine quality and production. Soil samples were obtained from Kanonkop, Simonsig and Spier vineyards and their 238U, 232Th and 40K concentrations determined, assuming that fertilisers would have no effect on the results. The difference in 40K concentrations were related to the clay fraction of the soil and demonstrated that the Kanonkop and Simonsig soils are fine-grained and clay-rich compared to the sandy coarse-grained soils of Spier. The uranium and thorium concentrations were indicators of whether the mineralogy of the soil is the same as the underlying bedrock as well as soil maturity. The measurement results were compared with those obtained with XRF. The correlation with radiometry were poor for the uranium concentrations (r2 = 0.314), as many of the samples concentrations were below the XRF detection limit. The correlations were excellent for both thorium (r2 = 0.985) and potassium (r2 = 0.999). As a positive result from the findings of the study was an in-situ measurement performed by Newman et al. for the radiometric mapping of a Simonsig vineyard for soil classification.
A characterization of beam shaping devices and a tunable Raman laser
(Stellenbosch : University of Stellenbosch, 2003-04) Du Plessis, Anton; Rohwer, Erich G.; Von Bergmann, H. M.; University of Stellenbosch. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The efficient manipulation of various nonlinear optical processes frequently requires the shaping of the laser beams used for these processes. Three beam shaping techniques were investigated in this thesis. The focussing of Gaussian laser beams was investigated analytically, in order to efficiently manipulate the focussed beam characteristics. The beam-shaping characteristics of a diffractive optical element (DOE) was investigated numerically, which illustrates the beamshaping capability of the DOE, and identifies the critical parameters in experimental situations. The use of a waveguide as beam shaping device was investigated analytically and experimentally, and characterized for use with the available tunable laser sources. A Raman laser, or Raman shifter, employs stimulated vibrational Raman scattering to generate laser radiation at shifted frequencies. The waveguide was successfully applied as a beam shaping device in the Raman laser system, for optimisation of the process. The Raman laser system was investigated experimentally and characterized for use with the available tunable laser sources. The successful generation of laser radiation at shifted frequencies illustrates the usefulness of the system for generating tunable red-shifted frequencies. The results of this work allow the simple and efficient application of the Raman laser to generate laser radiation at shifted frequencies, in particular tunable infrared laser radiation which is desirable for molecular spectroscopy.
Characterization of the pre-eminent 4-α cluster state candidate in 16O
(Stellenbosch : Stellenbosch University, 2015-12) Li, Kevin Ching Wei; Papka, Paul; Neveling, Retief; Adsley, Philip; Stellenbosch University. Faculty of Sciences. Dept. of Physics.
ENGLISH ABSTRACT: Please see full text for abstract
Characterization of thulium doped fiber for mid infrared laser applications
(Stellenbosch : University of Stellenbosch, 2011-03) Ndebeka, Wilfrid Innocent; Rohwer, Erich G.; Schwoerer, Heinrich; University of Stellenbosch. Faculty of Science. Dept. of Physics.
Please refer to full text to view abstract.
Charge carrier effects in free standing Si membranes investigated by linear and second harmonic optical techniques
(Stellenbosch : Stellenbosch University, 2016-12) Ndebeka, Wilfrid Innocent; Rohwer, Erich G.; Neethling, Pieter N.; Steenkamp, Christine M.; Stafast, Herbert; Stellenbosch University. Faculty of Science. Dept. of Physics
ENGLISH ABSTRACT : Silicon (Si) based materials have been of technological importance throughout the years and today silicon remains a leading material in microelectronics industries. The silicon/silicon dioxide (Si/SiO2) interface has been one of the most studied systems in the realms of material and condensed matter physics. Despite the great outpouring research and huge technological success, there has been little effort to construct a comprehensive, unified microphysical model of the Si/SiO2 interface. The optical properties, both linear and nonlinear, of the interface region are of interest since they vary considerably from those of the adjacent bulk phases. Second harmonic generation is used as a versatile tool to investigate surfaces and buried interfaces of centrosymmetric materials such as Si. Specifically, the electric field induced second harmonic (EFISH) generation process is used as a valuable technique to investigate the mechanisms of charge transfer and trap generation and population, both by electrons and holes being pumped across the interface. In this work, the transmission of the fundamental laser beam (90 fs pulse trains at 80MHz repetition rate, 800 nm) through 10 m to 30 m thick silicon (Si) membranes as well as simultaneous measurements of the electric field induced second harmonic (EFISH) generated in reflection from and transmission through the Si membranes are reported for the first time. The transmission of the laser power through Si membranes, irradiated at an incident angle of 45 , shows an increase with increased power, then reaches a maximum, and finally continues to decrease considerably as the incident power is increased. A similar behavior is observed with the EFISH signals in transmission of the Si membranes. The nonlinear behavior of the fundamental transmitted signal is attributed to free charge carrier absorption (FCA) in Si. The experimental setup and these results are discussed in this thesis.
Charge dynamics in hybrid and organic-inorganic light harvesting thin films followed with femtosecond transient absorption spectroscopy
(Stellenbosch : Stellenbosch University, 2017-12) Minda, Iulia; Schwoerer, Heinrich; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT : In order to bridge the gap between traditional sources of electricity and the increasing global demand for it, as a society we must move towards renewable sources of energy such as solar radiation. Photovoltaic devices (PVs) harness solar power and convert it to electrical power. In order to be commercially viable, they need to be efficient, cost effective, simple to fabricate and environmentally friendly. To address these requirements, the class of emerging PVs arose, which includes dye sensitised solar cells (DSSCs) and perovskite solar cells. Femtosecond transient absorption spectroscopy (TAS) is an experimental technique which allows us to follow the ultrafast photoinduced charge dynamics in real time in light harvesting thin films and PVs. By assigning time and rate constants to various processes governing the charge generation and extraction in solar cells, we construct charge dynamics models, and therefore learn the fundamental photophysics reasons behind what makes the power conversion efficiencies (PCEs) of some solar cells superior. In particular, this study focused on the charge transfer processes in indoline dye (DN216) sensitised electrodeposited ZnO solar cells, and the charge recombination dynamics in FA0.85MA0.15PbI2.55Br0.45 perovskite thin films. To construct the simplest fully consistent charge dynamics models, we match the visible and near-infrared spectroscopic signals of our samples to the allowed electronic transitions, and follow their temporal evolutions on the femtosecond and picosecond time scales. From our measured time and rate constants we observed that ZnO based DSSCs are less efficient than their TiO2 counterparts because the electron injection from the photoexcited indoline dye into the ZnO CB doesn’t just occur directly (< 200 fs), but also stepwise via neutral (∼ 2 ps) and ionic (∼ 10 ps) intermediate charge transfer states, resulting from surface trap states characteristic of electrodeposited ZnO. Moreover, FA0.85MA0.15PbI2.55Br0.45 is an excellent hybrid photoabsorber in record efficiency perovskite solar cells because even at high charge carrier densities of 1019 cm−3 , the third order non-radiative Auger recombination mechanism is not dominant. Furthermore we determined the associated geminate, non-geminate and Auger recombination rate constants as A = 5 × 109 s −1 , B = 10−10 s −1 cm3 and C = 50 × 10−32 s −1 cm6 .
Cluster model analysis of exotic decay in actinide nuclei
(Stellenbosch : Stellenbosch University, 2014-12) Du Toit, Erasmus Johannes; Wyngaardt, Shaun M.; Perez, Sandro M.; Stellenbosch University. Faculty of Science. Dept. of Physics.
ENGLISH ABSTRACT: The binary cluster model is used to investigate the properties of exotic structures and decays in various nuclei. A simple method is described to choose the possible clusters forming within the nucleus, by assuming the nucleus consists of a mixture of up to four different core-cluster pairs. A phenomenological potential is then used, with optimized parameters, to describe the even-even 222-232Th nuclei within the binary cluster model, by calculating exotic decay half-lives, reduced electromagnetic transition probabilities, and energy spectra. After finding that all experimentally observed heavy ion emissions are predicted with the model and calculated structure observables are reproduced within good agreement of the experimentally measured values, the model was extended to include the even-even 230-234U, 236-240Pu and 222-224Ra nuclei. We found that almost all experimentally observed heavy ion emissions are predicted within the model with reasonable accuracy in the calculated halflives, and found good agreement in the other calculated structure observables.

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