No description available.

No description available.

The generation and propagation of harmonics in an atomic gas are described for the case of an incident Bessel-Gauss beam. Theoretical expressions are derived for the far-field amplitude of the harmonic field by solving the propagation equation using an elaborate integral formalism. We establish simple rules which determine the optimum Bessel-Gauss beam with respect to phase-matching as a function of the medium properties, such as the dispersion and the gas density. Target depletion due to photoionization and refractive index variations originating from both free electrons and dressed linear atomic susceptibilities are taken into account. The intensity-dependent complex atomic dipole moment is calculated using nonpertur- bative methods. Numerical propagation calculations for hydrogen, xenon and argon are presented. For hydrogen we consider the third harmonic of a 355-nm, 15-ps pump beam up to 3 X 10(^13) W/cm(^2) intensity, similarly for xenon, but at lower intensities. For argon we consider the 17th and 19th harmonic of a 810-nm, 30-fs pump beam around 10(^14) W/cm(^2) intensity. We compare conversion efficiencies and both spatial and temporal far-field profiles for an optimized Bessel-Gauss beam with respect to a Gaussian beam of same power and/or peak focal intensity. For the case of hydrogen, we investigate the effect of an ac-Stark-shift induced atomic resonance. We find all results in good agreement with our theoretical predictions. We conclude from our studies that Bessel-Gauss beams can perform better in terms of conversion efficiency than a comparable Gaussian beam. We find this to originate essentially from the more flexible phase-matching conditions for Bessel-Gauss beams. Bessel-Gauss beams also allow for spatial separation of the harmonic and the incident field in the far-field region, owing to the conical shape of their spatial far-field profile. Both features make Bessel-Gauss beams an attractive alternative to Gaussian beams in a limited but substantial number of experimental conditions.

Design of the mechanical components greatly depends on their expected structural performances. In modern design applications these performances are quantified by computer-based analysis and occasionally confirmed by experimental measurements or theoretical calculations. The dependency of the mechanical product to the structural analysis process is more significant under the product’s multi-functionality aspect that requires analyses for a variety of Variable Input Parameters, to obtain various structural responses and against more than one failure or design criterion. Structural analysis is known as the expert field, which requires an upfront investment and facilitation to be implemented in commercial design environment. On the other hand, the product design process is a systematic and sequential activity that put the designer in the central role of decision making. Lack of mutual understanding between these two disciplines reduces the efficiency of the structural analysis for design. This research aims to develop an integrated methodology to embed the structural analysis in the design process. The proposed methodology in this research combines the benefits of state-of-the-art approaches, early simulation and Validation and Verification practice, towards the specified aim. Moreover the novelty of the proposed methodology is in creative implication of Quality Function Deployment method to include the product’s multi-functionality aspect. The QFD-Based Design Integrated Structural Analysis methodology produces a reliable platform to increase the efficiency of the structural analysis process for product design purpose. The application of this methodology is examined through an industrial case-study for the telescopic cantilever boom, as it appears in Access platforms, and Cranes products. Findings of the case-study create a reliable account for the structural performance in early stages of the design, and ensure the functionality of the proposed methodology.

Long range wireless power transmission (WPT) is a critical technology for the development of remote power systems for air and space vehicles as well as for point-to-point transmission on Earth. This can be achieved using either a laser for transmission in the infrared to optical frequency domain or by using microwaves. The objective of this research is to study the application of microwave power transmission (MPT) through the use of a so-called Bessel-beam whose unique propagation properties include a self-healing ability as well as non-diffractive properties. These two unique properties lead to an increase in the efficiency of microwave power transmission. In this research the propagation of a microwave Bessel-beam through a spatially inhomogeneous medium will be simulated in MATLAB using a plane wave spectrum representation of the electromagnetic beam field. The spatially inhomogeneous medium of interest here is the Earth's atmosphere whose electromagnetic properties (dielectric permittivity and electric conductivity) vary with altitude up through the ionosphere. The purpose of this research is to determine how efficiently a microwave Bessel beam can propagate in point-to-point transmission through the Earth's atmosphere as well as between satellites in Earth orbit.

Piezoelectric energy harvester has become a new powering choice for small electronic device. Due to its piezoelectric effect, electric energy can be obtained from ambient vibrations. This thesis is intending to build a frequency-adjustable piezoelectric energy harvester system. The system is structured with two piezoelectric bimorph beams, which are connected to each other by a spring. The feasibility of the frequency-adjustable piezoelectric energy harvester has been proved by investigating effects of the spring, loading mass and impedance on the operation frequencies.

Ion beam erosion of solid surfaces is known to produce a variety of surface morpholo- gies, such as pits, mounds or crests. Very often self-organized patterns composed of highly correlated arrays of dots or ripples at sub-micrometer and nanometer length scale could be obtained. Ion beam erosion patterning have demonstrated the poten- tial to tailor related surface properties for optoelectronic and spintronic applications, such as modulated photoemission induced by quantum con¯nement of nanodots and magnetic anisotropy induced by nanoripples. On the other hand, one considerable practical importance and e®ect of ion beam erosion is that of surface smoothing of nanometer features, during etching or ¯lm deposition coincident with energetic species. In my dissertation, systematic investigations of ripple formation and smooth- ing during low energy Ar+ ion erosion of sapphire surfaces using synchrotron grazing incidence small angle x-ray scattering and atomic force microscopy are performed. It is found in the pattern formation that the wavelength of ripples can be varied over a remarkably wide range by changing the ion incidence angle. The ion induced viscous °ow smoothing mechanism explains the general trends of the ripple wavelength at low temperature and incidence angles larger than 30±. The behavior at high temper- atures suggests relaxation by surface di®usion. However, strong smoothing is inferred from the observed ripple wavelength near normal incidence, which is not consistent with either surface di®usion or viscous °ow relaxation. Furthermore, a real-time x- ray scattering experiment is presented showing that ion smoothing of a pre-patterned surface near normal incidence is consistent with the e®ect of a collision-induced lat- eral current. Quantitative agreement is obtained using ion-collision simulations to compute the magnitude of the surface current. The results lead to predictions for the surface morphology phase diagram as a function of ion beam energy and incidence angle that substantially agree with experimental observations. The ion-induced lat- eral current smoothing model is applicable to many surfaces that become amorphous but maintain the stoichiometry of bulk materials during ion bombardment.

This thesis attempted to examine the role of various task factors on children's group and individual learning on the balance beam: task presentation, task difficulty initial knowledge and physical feedback. Initially the literature is reviewed, there then follows a series of four empirical studies. The first empirical study examined if the mixed ability groups produced more learning then similar ability groups on Wilkening and Anderson's (1982) balance beam task. The study did not find the effect but it is suggested that this was due to the unsuitability of the task to the methodology used. The second study attempted to examine the effect on two levels of task difficulty on learning in groups on Sielger's (1976, 198) balance beam. It was found with higher difficulty task items the individual conditions, where children worked on their own, revealed less learning than the group work conditions, an effect that was found with the easier items. Also it was found that the children's pre-test classifications affected whether they advanced or not; the children with higher pre-test classifications showing less advancement. The third study attempted and succeeded in replicating this result. It also examined the verbal interactions between the children within the groupwork conditions. It was found that there was little relation between the dialogue within the interaction and the outcome in the post-test. The fourth study analysed the role of feedback in learning in the Sielger's balance beam, the children either worked on their own or in groups on the balance beam. They were given feedback on either all items or just the items that they performed poorly on the pre-test. It was found that the manipulation had only a slight effect on learning, but it was found that the pattern of learning across the three phases of the experiment was different for the group work conditions and the individual conditions. These findings are discussed in relation in other models and the role of the social situation is suggested.

<div>Achievements in the growth of ultra-pure III-V semiconductor materials using state of the art molecular beam epitaxy (MBE) machine has led to the discovery of new physics and technological innovations. High mobility two-dimensional electron gas (2DEG) embedded in GaAs/Al_xGa_1−xAs heterostructures provides an unparalleled platform for many-body physics including fractional quantum Hall effect. On the other hand, single electron devices fabricated on modulation doped GaAs/Al_xGa_1−xAs heterostructures have been extensively used for fabrication of quantum devices such as spin qubit with application in quantum computing. Furthermore, epitaxial hybrid superconductor-semiconductor heterostructures with ultra clean superconductor-semiconductor interface have been grown using MBE technique to explore rare physical quantum state of the matter namely Majorana zero modes with non-abelian exchange statistics.</div><div><br></div><div><div>Chapter 1 in the manuscript starts with description of GaAs MBE system at Purdue University and continues with the modifications have been made to MBE hardware and growth conditions for growing heterostrcutures with 2DEG mobility exceeding 35 × 10⁶ cm⁻²/V s. Utilizing an ultra-high pure Ga source material and its further purification by thermal evaporation in the vacuum are determined to have major impact on growth of high mobility GaAs/Al_xGa_1−xAs heterostructures.</div></div><div><br></div><div>Chapter 2 reports a systematic study on the effect of silicon doping density on low frequency charge noise and conductance drift in laterally gated nanostructures fabricated on modulation doped GaAs/Al_xGa_1−xAs heterostructures grown by Molecular Beam Epitaxy (MBE). The primary result of this study is that both charge noise and conductance drift are strongly impacted by the silicon doping used to create the two-dimensional electron gas. These findings shed light on the physical origin of the defect states responsible for charge noise and conductance drift. This is especially significant for spin qubit devices, which require minimization of conductance drift and charge noise for stable operation and good coherence. <br></div><div><br></div><div>Chapter 3 demonstrates measurements of the induced superconducting gap in 2D hybrid Al/Al_0.15In_0.85As/InAs heterostructures which is a promising platform for scaling topological qubits based on Majorana zero modes. The 2DEG lies in an InAs quantum well and is separated from the epitaxial Al layer by a barrier of Al_0.15In_0.85As with thickness d. Due to hybridization between the wave functions of 2DEG and superconductor, the strength of induced gap in the 2DEG largely depends on the barrier thickness. This chapter presents a systematic study of the strength of the induced gap in hybrid Al/Al_0.15In_0.85As/InAs superconductor/semiconductor heterostructures as a function of barrier thickness.<br></div><div><br></div><div><br></div>

The South African national calibration facility is currently not equipped for mammography, dose-measuring equipment. A therapy X-ray machine was used as a calibration unit at the national secondary standards dosimetry laboratory (SSDL) for medium and low energy X-ray, therapy calibrations. It is not necessarily intuitive that the latter calibrations are applicable to diagnostic X-ray beams generated by high frequency generators. The response of measuring equipment calibrated in a therapy X-ray beam, compared to its response in a diagnostic or clinical mammography unit, is unknown. The aim of the research was to investigate whether there was a measurable difference between the X-ray beam qualities available for low energy diagnostic radiology and radiation therapy, i.e. up to 100 kV. The beam qualities studied included both mammography and conventional diagnostic radiography, i.e. nominally 20 kV to 100 kV. The diagnostic and therapy X-ray tubes under investigation had different target-filter combinations, inherent filtration and theoretically, different X-ray spectra. Practically, spectrometry of X-ray beams is not possible because of the sophistication of the instrumentation, comprehensive analyses being very time consuming and not practically applicable to the clinical environment (Kharatti and Zarrad, 2003). Furthermore, not all SSDL’s or Hospitals have access to spectral analysers. Clinical beam quality is instead specified in terms of both the tube peak voltage and the half-value layer (HVL), the thickness of material that will reduce the maximum output of the X-ray beam to 50%. The goal was to compare measured HVLl’s to the ones recommended by the International Electro technical Commission (IEC-61267, 2005) for available mammography beam qualities. The method was validated using attenuation curves. The attenuation curves were then used to derive the suitability of the X-ray spectra for calibration of mammography ionisation chambers (Waggener and Blough, 1999). One of the low energy therapy units was found to be suitable for introducing a regional calibration service for mammography.