Thesis (MSc (Physics))--University of Stellenbosch, 2005. / The extremely high light intensities produced by lasers and the increasing use of lasers highlights the need for measures to prevent damage to materials due to exposure to high intensity laser light. In particular it necessitates the development of systems to protect optical sensors, including the human eye. In this work optical limiters were investigated as a system for protecting sensors. An optical limiter transmits ambient light, but absorbs high intensity light. This makes it ideal for protecting sensors from laser radiation, since it allows the sensor to operate unhindered at design intensities while protecting it from harmful high intensity radiation. There are various mechanisms used for optical limiting, and in this work the nonlinear absorption and the nonlinear index of refraction changes of materials were investigated. A facility was established to measure the nonlinear optical properties of a variety of materials, in order to classify them as possible optical limiters. This entailed creating a so called Zscan setup, which enabled us to measure the nonlinear absorption coefficient and the nonlinear index of refraction of a material. The theory and the design of the setup are discussed and experimental results obtained using this setup are presented. A wide variety of material types were investigated to show the versatility of the experimental setup. These included C60, which was analyzed in solution; ZnO which is a crystal; CdS quantum dots in solution; and poly(dioctyl-fluorene), which is a large polymer molecule, in solution. The materials investigated in this work were chosen based on their known strong nonlinear optical properties. Emphasis was placed on measuring the nonlinear absorption coefficients since it was the dominant optical limiting effect of the materials under investigation. The results obtained displayed the same trends as published results and it shows that the established facility was capable of measuring the nonlinear properties of these samples. The experimental limitations of the setup were determined, and critical experimental parameters were identified for measurements of this nature. Improvements to the experimental facility are suggested to improve the accuracy of future measurements.

Thesis (MSc)--University of Stellenbosch, 2007. / Please refer to full text for abstract

Thesis (MSc (Physics))--University of Stellenbosch, 2005. / A tunable vacuum ultra-violet (VUV) laser source was recently developed for VUV spectroscopy using state selective excitation and total fluorescence detection. The VUV laser source makes use of a four-wave mixing process to provide tunable VUV radiation for the electronic excitation of the molecules. The theory of four-wave mixing, with the emphasis on parameters that are important for our experimental setup to generate efficient tunable VUV radiation is discussed. The experimental setup, and in particular the metal vapor heat-pipe, which provides Mg vapor as the nonlinear medium, is described. New diagnostic equipment described in this work was added to the experimental setup. This equipment was characterized and utilized together with the existing setup. The additional diagnostic equipment introduced enabled us to measure the tunable VUV output of the source (using a VUV monochromator), making it possible to significantly improve the efficiency of the existing tunable VUV laser source.

Thesis (MSc (Physics))--University of Stellenbosch, 2006. / The aim of this thesis is the semi-classical implementation of Wegner’s flow equations and comparison with the well-established Wentzel-Kramers-Brillouin method. We do this by converting operators, in particular the Hamiltonian, into scalar functions, while an isomorphism with the operator product is maintained by the introduction of the Moyal product. A flow equation in terms of these scalar functions is set up and then approximated by expanding it to first order in ~. We apply this method to two potentials, namely the quartic anharmonic oscillator and the symmetric double-well potential. Results obtained via the flow equations are then compared with those obtained from the WKB method.

Thesis (MSc (Physics))--University of Stellenbosch, 2007. / In this thesis we present a relativistic formalism for the description of hypernuclei production via the weak interaction. It will be shown that the cross section can be written as the contraction of a leptonic and hadronic tensor if we model the interaction as a quasifree process. The hadronic tensor is written in a model-independent way in terms of thirteen nuclear structure functions. A Born term model is used to describe the underlying elementary hyperon production process. The bound state wave functions of the hyperon and nucleon are calculated within a relativistic mean-field approximation. Together with the relativistic kinematics a fully relativistic framework for experimental predictions is constructed and a specific cross section calculation is discussed.

Thesis (MSc (Physics))--Stellenbosch University, 2009. / ENGLISH ABSTRACT: This thesis aims to comprehensively discuss ultrafast electron di raction and its role in temporally resolving ultrafast dynamics on the molecular level. Theory on electron pulses and electron pulse propagation will be covered, but the main focus will be on the method, equipment and experimental setup required to generate sub-picosecond electron pulses, which are needed to perform time resolved experiments. The design and construction of an electron gun needed to produce the electron pulses will be shown in detail, while preliminary pulse characterization experiments will also be illustrated. An introduction into the theory of electron diffraction patterns and how to interpret these diffraction patterns will conclude the thesis. / AFRIKAANSE OPSOMMING: Hierdie tesis het ten doel om ultravinnige elektrondi raksie deeglik te bespreek asook die rol wat dit speel om ultravinnige tyd-dinamika op 'n molekulêre vlak op te los. Die teorie van elektonpulse en die voortplanting van elektronpulse sal gedek word, maar die fokus sal op die metode, gereedskap en eksperimentele opstelling wees wat benodig is om sub-pikosekonde elektronpulse te genereer. Die ontwerp en konstruksie van 'n elektrongeweer, wat benodig word om elektronpulse te produseer, sal in detail bespreek word, terwyl aanvanklike pulskarakterisasie eksperimente ook illustreer sal word. 'n Inleiding tot die teorie van elektrondi raksie patrone en hoe om hulle te interpreteer sal die tesis afsluit.

Thesis (MSc (Physics. Institute for Laser Research))--University of Stellenbosch, 2005. / This thesis consists of two parts: a discussion on diode-pumped solid-state lasers and a detailed description of the development of a diode-end-pumped Nd:YLF laser. A background motivation, which places this research area in perspective, is also given. Part One introduces solid-state lasers and their physics. The focus is on the Nd3+ active ion and describes its energy level structure as a typical four-level solid-state laser. An overview of optical pump sources for solid-state lasers is given, focussing on the construction, operation and advantages of diode lasers. It is motivated that diode-end-pumping solid-state lasers produce laser systems with the highest efficiency and diffraction limited beam quality. It is, however, emphasised that power scaling of diode-end-pumped solid-state lasers is problematic due to localised heat generation in the solid-state laser medium. The adverse effect of heat generation on the laser performance is also described. In the design of diode-endpumped solid-state lasers, the management of thermal effects is suggested as the approach to scale the output power of these lasers. Part Two of the thesis describes the design and the results of a novel high-power diode-end-pumped solid-state laser developed at the Laser Research Institute. The description of the design is split into three components: the laser material, the pump source and the laser resonator. The choice of laser material is motivated in detail, focussing on Nd:YLF’s advantage of having a very weak thermal lens when operated on the σ-polarization at 1053 nm. Its disadvantage of having a low fracture limit is also highlighted, but the approach to power scale it to the multi-10-watt level, with the use of low doping concentration, a low absorption pump wavelength, and a large pump beam, is described. It is further shown that this approach led to the development of a novel laser resonator, which allows a large fundamental mode in the laser material to match the large pump beam, and it can compensate for the astigmatic thermal lens in Nd:YLF. The experimental results of the high-power diode-end-pumped Nd:YLF laser are presented, showing the influence of doping concentration, output coupling efficiency and resonator adjustments on the continuous wave and Q-switched laser performance. It is shown that the optimum laser parameters were determined, resulting in the Nd:YLF laser producing more than 26 W of continuous wave output power with a close to diffraction limited beam quality (M2 < 1.4), and more than 3 mJ of energy per pulse at a repetition rate of 6 kHz when Q-switched. It is concluded that the powerscaling concept proved to be efficient and that further power scaling would be possible with this scheme.

Thesis (MSc (Physics))--University of Stellenbosch, 2005. / The goal of this thesis is the development and implementation of a non-perturbative solution method for Wegner’s flow equations. We show that a parameterization of the flowing Hamiltonian in terms of a scalar function allows the flow equation to be rewritten as a nonlinear partial differential equation. The implementation is non-perturbative in that the derivation of the PDE is based on an expansion controlled by the size of the system rather than the coupling constant. We apply this method to the Lipkin model and obtain very accurate results for the spectrum, expectation values and eigenstates for all values of the coupling and in the thermodynamic limit. New aspects of the phase structure, made apparent by this non-perturbative treatment, are also investigated. The Dicke model is treated using a two-step diagonalization procedure which illustrates how an effective Hamiltonian may be constructed and subsequently solved within this framework.

Thesis (MSc)--Stellenbosch University, 2004. / 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. / AFRIKAANSE OPSOMMING: In hierdie tesis word veralgemeende Dyson boson-fermion-afbeeldings ondersoek. Hierdie afbeeldings word gebruik in die analise van die kwantum veeldeeltjie probleem, en is voorbeelde van sogenaamde boson-uitbreidingstegnieke. 'n Veralgemeende Dyson bosonfermion- afbeelding, of kortweg 'n Dyson afbeelding, is 'n een-tot-een, lineêre maar nie-unitêre operator wat inwerk op vektore wat toestande verteenwoordig van 'n veel-fermion sisteem. 'n Vektor wat 'n fermionsisteem verteenwoordig word so afgebeeld op 'n vektor waarvoor die mees natuurlike interpretasie is dat dit 'n toestand verteenwoordig van 'n sisteem waarin beide bosone en fermione aanwesig is. So 'n afbeelding word gewoonlik gemaak in die hoop dat eienskappe van die sisteem, wat versteek was in die oorspronklike weergawe, voor-die-hand-liggend is na die afbeelding. Hierdie tesis het ten doel 1. om die teorie van veralgemeende Dyson boson-fermion-afbeeldings te hersien, 2. om 'n eenvoudige voorbeeld deur te werk, en so te demonstreer dat die teorie sonder moeite geïmplimenteer kan word en 3. om 'n nuttige toepassing te vind vir 'n veralgemeende Dyson boson-fermion-afbeelding deur 'n nie-triviale model, naamlik die Richardson model vir supergeleiding, te ondersoek. Die eerste twee van hierdie doelwitte behels hoofsaaklik dat idees wat reeds in die literatuur verskyn het, saamgevat word in een koherente teks. Sommige subtiele punte wat, vanwee beperkte ruimte, slegs kortliks bespreek is in die joernaalartikels waarin die teorie oorspronklik verskyn het, word in hierdie tesis meer breedvoering bespreek. Daarteenoor verteenwoordig die analise van die Richardson model met behulp van 'n Dyson afbeelding 'n nuwe bydra. Dit is naamlik die eerste keer dat 'n bosonuitbreiding ingespan word vir 'n sisteem waar sowel kollektiewe as nie-kollektiewe fermionpare 'n belangrike rol speel. (Die Dyson afbeelding assosieer bosone met die oorspronklike sisteem se Cooper pare, terwyl die fermione wat in die oorspronklike sisteem nie tot Cooper pare gebind is nie, sorg dat daar ook fermione teenwoordig is in die afgebeelde sisteem.) Ons vind dat die Dyson afbeelding nie-triviale eienskappe van die sisteem aan die lig bring. Hierdie eienskappe is nuttig vir die konstruksie van beide tyd-onafhanklike steuringsreekse vir die stasionêre toestande van die sisteem en vir tyd-afhanklike steuringsreekse vir oorgangsamplitudes tussen toestande. Die tyd-onafhanklike uitbreidings stem ooreen met resultate wat ander outeurs afgelei het sonder die gebruik van 'n Dyson afbeelding. Die tyd-afhanklike uitbreidings, wat kwalik afgelei kan word sonder 'n Dyson afbeelding, mag vorentoe nuttig wees om aspekte van die dinamika van baie koue Fermi gasse te verstaan, wanneer tydafhanklike magneetvelde gebruik word om die inter-atoomwisselwerking te manipuleer.

Thesis (MSc)--Stellenbosch University, 2000. / ENGLISH ABSTRACT: The radionuclide 75Se (T1/2= 119.8 days) decays to the stable isotope 75As by 100 % electron capture. Applications of this radioisotope include its use as a radiotracer in agricultural investigations and as a potential irradiation source in high-activity brachytherapy. The application of this isotope as a radiotracer in clinical studies has been limited due to a lack of availability of carrier-free 75Se. In this study the excitation functions for the production of carrier-free 75Se, other selenium radioisotopes and radiocontaminants produced via natBr(p,x) nuclear reactions were measured by the use of the stacked foil technique. Three separate stacks were assembled from aluminium and copper monitor foils and potassium bromide tablets. The latter were prepared by pressing KBr salt into tablets with a thickness of 200 mg/cm2 and a diameter of 20 mm, which are sandwiched between thin Al foils. The monitor foils, with a diameter of 19 mm, were punched from foil sheets. The stacks were irradiated at proton beam energies of 100.9 MeV, 66.8 MeV and 40.4 MeV respectively at the National Accelerator Centre cyclotron facility. After bombardment the KBr tablets were sealed in plastic bags to prevent the loss of any krypton isotopes produced. The gamma-ray spectra of the induced radioactivity in each of the target foils were recorded by means of a Ge-detector coupled with a SILENA 16-k multichannel analyser. The measured excitation functions for the above-mentioned isotopes are presented. Where applicable the cross-section values are compared with previously published values, while the others are presented as new data. Theoretical calculations by means of the computer code ALICE (IPPE) were also done for comparison. / AFRIKAANSE OPSOMMING: Die radionuklied 75Se (T1/2= 119.8 dae) verval deur 100 % elektronvangs na die stabiele isotoop 75As. Toepassings vir hierdie radioisotoop sluit in sy gebruik as 'n radiospoormiddel in landbou-ondersoeke en as 'n potensiële stralingsbron in hoë aktiwiteit bragiterapie. Die aanwending van hierdie isotoop as 'n radiospoorder in kliniese studies word egter beperk deur die gebrek aan beskikbaarheid van draer-vry 75Se. In hierdie studie is die opwekkingskrommes vir die produksie van draer-vry 75Se, ander selenium radioistope en radiokontaminante bepaal deur die natBr(p,x) kernreaksies te meet met behulp van die foelie-stapel metode. Drie verskillende stapels, bestaande uit aluminium- en koper-monitor foelies en kalium-bromied tablette, is saamgestel. Laasgenoemde is voorberei deur KBr sout saam te pers tot tablette met 'n dikte van 200 mg/cm2 en 'n deursnit van 20 mm en wat omsluit is met dun Al-foelies. Die monitor-foelies, met 'n deursnit van 19 mm, is gepons uit foelieplate. Die stapels is bestraal met protonbundels met energieë van onderskeidelik 100.9 MeV, 66.8 MeV en 40.4 MeV by die Nasionale Versneller Sentrum, Faure, se siklotron-fasiliteit. Na bombardering is die KBr tablette verseël in plastieksakkies om verliese van enige vervaardigde kripton isotope te verhoed. Die gamma-straal spektra van die geinduseeerde radioaktiwiteit in elke teiken is opgeneem met behulp van 'n Ge-detektor wat gekoppel was aan 'n SILENA 16-k multikanaal-analiseerder. Bogenoemde isotope se gemete opwekkingskrommes word aangebied. Waar van toepassing word die kansvlakwaardes vergelyk met vorige gepubliseerde waardes terwyl ander aangebied word as die eerste data bekend. 'n Vergelyking met die teoreties berekende kansvlakwaardes van die rekenaar kode ALICE (IPPE) word ook gedoen.