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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Lyman alpha absorption systems

Rauch, Michael January 1992 (has links)
No description available.
2

Quark and gluon jets at LEP

Parsons, Mark Ian January 1994 (has links)
Using the large number of hadronic decays of the Z° particle recorded by the ALEPH detector at LEP between 1990 and 1992, a study of the properties of a sample of these events has been made. High transverse lepton tagging has been used to identify the quark jets in three jet events. Evidence is shown that the angular distribution of the identified quark jets is described well by the theory of quantum chromodynamics. Using a log-likelihood statistical test a confidence level of 97% is obtained for the data to be in good agreement with JETSET Monte Carlo. An Abelian scalar gluon model is excluded. A separate study using symmetric three jet events and the same quark jet identification method is presented. This study compares quark and gluon jets in a configuration designed to minimise experimental biases. Gluon jets, when compared with a mixed flavour quark jet sample, are found to have a higher average charged track multiplicity ratio N<SUB>g</SUB>/N<SUB>q</SUB> = 1.62 ± 0.051 (the error is predominantly statistical). Evidence is also presented that they have a wider jet profile and that their particle energy spectrum is softer.
3

Investigations relating to optical imaging in partially coherent light

Kintner, Eric C. January 1975 (has links)
No description available.
4

Nonlinear guided waves in fibre optics

Ryder, Elaine January 1993 (has links)
Optical fibres are widely used in optical communication systems because they can transmit signals in the form of extremely short pulses of quasi-monochromatic light over large distances with high intensities and negligible attenuation. A fibre that is monomode and axisymmetric can support both left- and right-handed circularly polarised modes having the same dispersion relation. The evolution equations are coupled nonlinear Schröinger equations, the cubic terms being introduced by the nonlinear response of the dielectric material at the high optical intensities required. In this thesis we analyse signal propagation in axisymmetric fibres both for a fibre with dielectric properties which vary gradually, but significantly, along the fibre and for a fibre which is curved and twisted but with material properties assumed not to vary along the fibre. For fibres with axial inhomogeneities, we identify two regimes. When the axial variations occur on length scales comparable with nonlinear evolution effects, the governing equations are found to be coupled nonlinear Schröinger equations with variable coefficients. Whereas for more rapid axial variations it is found that the evolution equations have constant coefficients, defined as appropriate averages of those associated with each cross-section. The results of numerical experiments show that a sech-envelope pulse and a more general initial pulse lose little amplitude even after propagating through many periods of an axial inhomogeneity of significant amplitude. For a curved and twisted fibre, it is found that the pulse evolution is governed by a coupled pair of cubic Schröinger equations with linear cross coupling terms have coefficients related to the local curvature and torsion of the fibre. These coefficients are not, in general, constant. However for the case of constant torsion and constant radius of curvature which is comparable to the nonlinear evolution length, numerical evidence is presented which shows that a nominally non-distorting pulse is unstable but the onset of instability is delayed for larger values of torsion.
5

The BaBar light pulser system

Clark, Philip James January 2000 (has links)
The B<sub>A</sub>B<sub>AR </sub>experiment at the PEP-II <i>e<sup>+</sup>e<sup>-</sup></i> collider at SLAC in California started taking data in May 1999. The aim of the experiment is to study <i>CP</i> violation in the <i>B</i> meson system. A central part of the B<sub>A</sub>B<sub>AR</sub> detector is CsI(TI) electromagnetic calorimeter. To make precision measurements with a calorimeter in a high luminosity environment requires that the crystals are well calibrated and continually monitored for radiation damage. However, this should not impact the total integrated luminosity. To achieve this goal a fibre-optic light pulser system was designed. The light sources chosen were Xenon flash lamps. A novel light distribution method was developed using an array of graded index microlenses. Initial results from performance studies are presented.
6

Optical correlation using pixellated spatial light modulators

Heddle, Steven B. January 1993 (has links)
Optical correlation by filtering in the Fourier plane of an optical processor has recently received much attention, due to the advent of rapidly reconfigurable Fourier plane filters - spatial light modulators (SLMs). Many algorithms have been developed to generate filters, mostly binary in nature in line with the capabilities of current SLMs. This thesis reviews optical correlation techniques and SLM technology, before turning attention to the consequences of the pixellated structure of the Fourier plane filters generally necessary to enable arbitrary filter patterns to be written to an SLM. Conventionally the pixels are identical and regularly spaced on a rectangular grid. This is shown to lead to replication in the output plane of the correlator, and aliasing if the input exceeds dimensions related to the pixel spacing in the Fourier plane flter. If the input is also provided by an SLM, this requires the number of pixels across the filter SLM (its space-bandwidth product) to be greater than or equal to that of the input, even though the target itself may occupy only a small area of the total input scene, in order to prevent aliasing and misleading correlation results. If the replication and aliasing could be prevented, the space bandwidth product of the filter need only be matched to that of the target to be detected, rather than the entire input scene. The replicas arise through convolution with distinct spectral orders in the SLM's Fourier spectrum. A means of eliminating the spectral orders other than the zero order through randomisation of the pixel positions is presented and analysed, and implemented on transmissive matrix addressed SLMs custom designed and built for the work of this thesis.
7

The optical characterisation and kinetics of Ag photodissolution in amorphous As-S films

Zekak, Abdelkarim January 1994 (has links)
The present work is concerned with a fundamental study of the metal photodissolution effect in chalcogenide glasses and in particular of those aspects which are relevant to one of the main application areas, the fabrication of diffractive optical elements for operation in the infrared. This study has dealt exclusively with glasses in the As-S system and has used mainly Ag as the source for photodoping. The optical constants of undoped and photodoped As-S films of various compositions have been measured over a wide wavelength range. Pre-annealing the undoped As-S films produced an overall increase of about 3% in the refractive index, n, from the visible to the far-infrared region. The magnitude of the pre-annealing effect was found to increase with increasing As content, being most pronounced at As<SUB>40</SUB>S<SUB>60</SUB>. The maximum overall increase in n produced by photodoping was 20%. The single oscillator and dispersion energies, E<SUB>o</SUB> and E<SUB>d</SUB>, were obtained by fitting the measured dispersion relations using the Wemple-Didomenico model. The optical gap and E<SUB>o</SUB> were found to decrease and E<SUB>d</SUB> and the refractive index to increase with increasing Ag content. Regarding the dependence on As-S composition, for the most heavily Ag-doped films, n, E<SUB>o</SUB> and E<SUB>d</SUB> all exhibited a peak at ˜30 at.% As but the change in the optical gap was similar to that of the undoped films. The time dependence of the process was examined and found to be definitely linear for the conditions used. It was also found that the photodissolution process does not stop when the Ag is exhausted.
8

Study of interference between modulated light beams

Wykes, Catherine January 1971 (has links)
No description available.
9

Micro-scale processing of diamond structures and devices

Lee, Chee-Leong January 2008 (has links)
In this project, Inductively Coupled Plasma (ICP) etching of natural and synthetic diamond using oxygen-based (Ar/0₂) and chlorine-based (Ar/Cl₂) plasmas was investigated. The effects of ICP platen and coil powers and gas pressures on the diamond etching were also studied. It was demonstrated that the diamond etch rate and etch selectivity using Ar/Cl₂ plasma etching, with high etch controllability (on the nanometre scale) and capabilities of removing the sub-surface damage and improving the surface smoothness, is only half of those by using Ar/0₂ plasma. Based on these studies, various diamond micro-optics including diamond microspherical positive and negative lenses, micro-cylindrical and micro-ring lenses, complex micro-lenses and optical gratings were designed and successfully fabricated. Structural and optical characterisations revealed the high optical quality and functionality of the fabricated diamond micro-optics. Applications of diamond micro-optics, such as micro-lensed micro-VECSEL photonic devices were also demonstrated. Utilising ICP Ar/Ch plasma etching and other processes, prototype diamond electronic devices such as diamond MISFETs and MOSFETs have been fabricated. To realize such diamond FET devices, ohmic contacts with low specific contact resistances, which can be achieved by applying aqua regia surface treatment and laser ablation techniques on the p-doped diamond, are imperative. However, it was found that the gate leakage currents of the diamond MISFET devices were quite large due to leakage through the diamond intrinsic layer under the gate. To overcome this problem, diamond MOSFET devices employing Si0₂ as gate insulation material were developed, which were able to sustain large gate biases with very low leakage currents. However, these diamond MOSFET devices could not be fully pinched-off which may be due to the high boron concentration and relatively thick delta layer, which leads to excessive charge in the channel of the device. By fabricating a recess gate diamond p-MOSFET, gate field control was much enhanced.
10

The effect of radiation damage on the TL properties of Zircon

Amin, Y. Mohd January 1985 (has links)
Theoretical models of thermoluminescence (TL) and some physical properties of zircons are presented. Descriptions of the TL equipment and the experimental methods used, including the etching techniques, are given. Several methods of analysing the TL glow curves are described and some values of kinetics parameters are obtained. Radiation damage, caused by the decay of uranium (and thorium) present as impurities in the crystals, is estimated by measuring the spontaneous-fission track density in them (rather than a-tracks, which are not revealed by etching the zircons). The distribution of fission tracks in a crystal also gives a representation of the uniformity of radiation damage in it. It is found that TL sensitivity as well as the amount of natural TL are adversely affected by the radiation damage. By annealing the crystals, the damage is "healed-up" as can be seen from the reduction in fission-track density, and the TL sensitivity, recovers, reaching a maximum when all the tracks vanished. Induced radiation damage, caused by bombarding the crystals with heavy ions (e.g. U-23β,α , etc.) is also found to adversely affect the TL properties. Annealing can, once again, heal the induced damage. The etching time (to fully develop the tracks) and the x-ray diffraction angle of the (112) plane are show~ to decrease with increasing radiation damage. The TL spectra (between 400-600 nm) of less damaged and synthetic zircons have two peaks at _480 nm and ",580 nm , However in highly damaged zircons (natural track density is greater than ",1 X 108 cm-2), broad TL spectra with a possible peak at -600 nm are obtained. TL in zircons is found to fade anomalously and the rate of fading increases with radiation damage. In highly damaged zircons, the long wavelengths (yellow-orange) fade faster than the shorter wavelengths (blue-green). A possible mechanism for the anomalous TL fading is also discussed. Phototransfer-thenmolwninescence (PTTL) is used to study the high temperature peak ( '"500oC). Optical bleaching using UV and white light on some of the TL peaks is investigated. It is concluded that zircon is not suitable for conventional TL dating because of anomalous fading of the "normal" (-500oC) TL peaks. However, it is possible,. by means of PTTL, to use the "high" temperature peak (between _600-650oC) for TL dating.

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