Latest content added for Digital Library Collection: UNT Theses and Dissertationshttps://digital.library.unt.edu/explore/collections/UNTETD/browse/?fq=untl_institution:UNT&fq=str_degree_discipline:Physics2017-07-12T03:17:08-05:00UNT LibrariesThis is a custom feed for browsing Digital Library Collection: UNT Theses and DissertationsNonlinear Light Generation from Optical Cavities and Antennae2017-07-12T03:17:08-05:00https://digital.library.unt.edu/ark:/67531/metadc984232/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc984232/"><img alt="Nonlinear Light Generation from Optical Cavities and Antennae" title="Nonlinear Light Generation from Optical Cavities and Antennae" src="https://digital.library.unt.edu/ark:/67531/metadc984232/small/"/></a></p><p>Semiconductor based micro- and nano-structures grown in a systematic and controlled way using selective area growth are emerging as a promising route toward devices for integrated optical circuitry in optoelectronics and photonics field. This dissertation focuses on the experimental investigation of the nonlinear optical effects in selectively grown gallium nitride micro-pyramids that act as optical cavities, zinc oxide submicron rods and indium gallium nitride multiple quantum well core shell submicron tubes on the apex of GaN micro pyramids that act as optical antennae. Localized spatial excitation of these low dimensional semiconductor structures was optimized for nonlinear optical light (NLO) generation due to second harmonic generation (SHG) and multi-photon luminescence (MPL). The evolution of both processes are mapped along the symmetric axis of the individual structures for multiple fundamental input frequencies of light. Effects such as cavity formation of generated light, electron-hole plasma generation and coherent emission are observed. The efficiency and tunability of the frequency conversion that can be achieved in the individual structures of various geometries are estimated. By controlling the local excitation cross-section within the structures along with modulation of optical excitation intensity, the nonlinear optical process generated in these structures can be manipulated to generate coherent light in the UV-Blue region via SHG process or green emission via MPL process. The results show that these unique structures hold the potential to convert red input pulsed light into blue output pulsed light which is highly directional.</p>Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955104/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955104/"><img alt="Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films" title="Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films" src="https://digital.library.unt.edu/ark:/67531/metadc955104/small/"/></a></p><p>Among the well-known methods to form or modify the composition and physical properties of thin films, ion implantation has shown to be a very powerful technique. In particular, ion beam syntheses of binary iron silicide have been studied by several groups. Further, the interests in transition metal silicide systems are triggered by their potential use in advanced silicon based opto-electronic devices. In addition, ternary silicides have been by far less studied than their binary counterparts despite the fact that they have interesting magnetic and electronic properties. In this study, we investigate ion beam synthesis of Fe-Si binary structures and Fe-Co-Si ternary structures. This work involves fundamental investigation into development of a scalable synthesis process involving binary and ternary transitional metal silicide thin films and Nano-structures using low energy ion beams.
Binary structures were synthesized by implanting Fe- at 50 keV energy. Since ion implantation is a dynamic process, Dynamic simulation techniques were used in these studies to determine saturation fluences for ion implantation. Also, static and dynamic simulation results were compared with experimental results. The outcome of simulations and experimental results indicate, dynamic simulation codes are more suitable than static version of the TRIM to simulate high fluence, low energy and, heavy ion implantation processes. Furthermore, binary Fe-Si phase distribution was determined at different implantation fluences and annealing temperatures. A higher fluence implantation at 2.16×1017 atoms/cm2 and annealing at 500 oC showed three different Fe-Si phase formations (β-FeSi2, FeSi and Fe3Si) in substrate. Further, annealing the samples at 800 oC for 60 minutes converted the Fe3Si phase into FeSi2 and FeSi phases. As an extension, a second set of Fe- ion implantations was carried with the same parameters while the substrate was placed under an external magnetic field. External magnetic fields stimulate the formation of magnetic phase centers in the substrate. X-ray diffraction (XRD) results shows formation of ferromagnetic Fe3Si phase in the Si matrix after annealing at 500 oC for 60 minutes. In addition, X-ray photoelectron spectra (XPS) provide further evidence for ferromagnetic metallic behavior of Fe3Si in the substrate. Ternary Fe-Co-Si structures were synthesized by implanting Fe- & Co- into a Si (100) substrate at an energy of 50 keV at saturation fluences. Both Fe- & Co- co-implantation were performed under external magnetic fields to enhance magnetic phase formation. Fe(1-x)CoxSi B20-type cubic structure can be synthesized on Si(100) substrate with 0.4≤x≤0.55 concentration range using ion implantation under external magnetic field. Moreover, magnetic measurement indicates a possible magnetic phase transformation at ~50 K. Further, XPS results also provide evidence for metallic & ferromagnetic properties in the thin film structure</p>Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955084/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955084/"><img alt="Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates" title="Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates" src="https://digital.library.unt.edu/ark:/67531/metadc955084/small/"/></a></p><p>In this work, we study the use of a spatial light modulator (SLM) for local manipulation of phase in interfering laser beams to fabricate photonic crystal templates with embedded, engineered defects. A SLM displaying geometric phase patterns was used as a digitally programmable phase mask to fabricate 4-fold and 6-fold symmetric photonic crystal templates. Through pixel-by-pixel phase engineering, digital control of the phases of one or more of the interfering beams was demonstrated, thus allowing change in the interference pattern. The phases of the generated beams were programmed at specific locations, resulting in defect structures in the fabricated photonic lattices such as missing lattice line defects, and single-motif lattice defects in dual-motif lattice background. The diffraction efficiency from the phase pattern was used to locally modify the filling fraction in holographically fabricated structures, resulting in defects with a different fill fraction than the bulk lattice. Through two steps of phase engineering, a spatially variant lattice defect with a 90° bend in a periodic bulk lattice was fabricated. Finally, by reducing the relative phase shift of the defect line and utilizing the different diffraction efficiency between the defect line and the background phase pattern, desired and functional defect lattices can be registered into the background lattice through direct imaging of the designed phase patterns.</p>Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes2017-02-19T19:42:09-06:00https://digital.library.unt.edu/ark:/67531/metadc955114/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc955114/"><img alt="Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes" title="Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes" src="https://digital.library.unt.edu/ark:/67531/metadc955114/small/"/></a></p><p>We investigate the conditions under which electron irradiation of single-walled carbon nanotube (SWCNT) bundles with 2 keV electrons produces an increase in the Raman D peak. We find that an increase in the D peak does not occur when SWCNTs are preheated in situ at 600 C for 1 h in ultrahigh vacuum (UHV) before irradiation is performed. Exposing SWCNTs to air or other gases after preheating in UHV and before irradiation results in an increase in the D peak. Small diameter SWCNTs that are not preheated or preheated and exposed to air show a significant increase in the D and G bands after irradiation. X-ray photoelectron spectroscopy shows no chemical shifts in the C1s peak of SWCNTs that have been irradiated versus SWCNTs that have not been irradiated, suggesting that the increase in the D peak is not due to chemisorption of adsorbates on the nanotubes.</p>A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849601/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849601/"><img alt="A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation" title="A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation" src="https://digital.library.unt.edu/ark:/67531/metadc849601/small/"/></a></p><p>In this work over 200 temperature proxy data sets have been analyzed to determine if periodic and or quasi-periodic patterns exist in the data sets. References to the journal articles where data are recorded are provided. Chapter 1 serves an introduction to the problem of temperature determination in providing information on how various proxy data sources are derived. Examples are given of the techniques followed in producing proxy data that predict temperature for each method used. In chapter 2 temperature proxy data spanning the last 4000 years, from 2,000 BCE to 2,000 CE, are analyzed to determine if overarching patterns exist in proxy data sets. An average of over 100 proxy data sets was used to produce Figure 4. An overview of the data shows that several “peaks” can be identified. The data were then subjected to analysis using a series of frequency modulated cosine waves. This analysis led to a function that can be expressed by equation 3. The literature was examined to determine what mathematical models had been published to fit the experimental proxy data for temperature. A number of attempts have been made to fit data from limited data sets with some degree of success. Some other papers have used a sinusoidal function to best fit the changes in the temperature. After consideration of many published papers and reviewing long time streams of proxy data that appeared to have sine wave patterns, a new model was proposed for trial. As the patterns observed showed “almost” repeating sine cycles, a frequency modulated sine wave was chosen to obtain a best fit function. Although other papers have used a sinusoidal function to best fit the changes in the temperature, the “best fit” was limited. Thus, it was decided that a frequency modulated sine wave may be a better model that would provide a more precise fit. This proved to be the case and the more than 240 temperature proxy data sets were analyzed using Equation 3. In chapter 3 the time span for the proxy data was extended to cover the period of time 12,000 BCE to 2000 CE. The data were then tested by using the equation above to search for periodic/quasi-periodic patterns. These results are summarized under select conditions of time periods. In chapter 4 the interval of time is extended over 1,000,000 years of time to test for long period “periodic” changes in global temperature. These results are provided for overall analysis. The function f(x) as described above was used to test for periodic/quasi-periodic changes in the data. Chapter 5 provides an analysis of temperature proxy data for an interval of time of 3,000,000 years to establish how global temperature has varied during the last three million years. Some long-term quasi-periodic patterns are identified. Chapter 6 provides a summation of the model proposed for global temperature that can be expected if similar trends continue over future years as have prevailed for the past few million years. Data sets that were used in this work are tabulated in the appendices of this paper.</p>Quantum Coherent Control and Propagation in Lambda System2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849750/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849750/"><img alt="Quantum Coherent Control and Propagation in Lambda System" title="Quantum Coherent Control and Propagation in Lambda System" src="https://digital.library.unt.edu/ark:/67531/metadc849750/small/"/></a></p><p>Strong coherence in quasi-resonant laser driven system interferes with effective relaxations, resulting in behaviors like, coherent population trapping and Electromagnetically induced transparency. The Raman system can optimize this utilizing excited coherence in the lambda system when exposed to counter- intuitive pump-stokes pulses. The phenomenon can result in complete population transfer between vibrational levels called Stimulated Raman adiabatic passage(STIRAP). STIRAP and CHIRAP have been studied with Gaussian and chirped pulses. The optical propagation effects in dense medium for these phenomenon is studied to calculate the limitations and induced coherences. Further, the effect of rotational levels has been investigated. The molecular vibrational coherence strongly depends on the effect of rotational levels. The change in coherence interaction for ro-vibrational levels are reported and explained. We have considered the effects on the phase of radiation related to rotational mechanical motion of quantum system by taking advantages in ultra strong dispersion medium provided by quantum coherence in lambda system. The enhanced Fizeau effect on a single atom is observed.</p>Charged Particle Transport and Confinement Along Null Magnetic Curves and in Various Other Nonuniform Field Configurations for Applications in Antihydrogen Production2016-06-28T16:28:55-05:00https://digital.library.unt.edu/ark:/67531/metadc849779/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc849779/"><img alt="Charged Particle Transport and Confinement Along Null Magnetic Curves and in Various Other Nonuniform Field Configurations for Applications in Antihydrogen Production" title="Charged Particle Transport and Confinement Along Null Magnetic Curves and in Various Other Nonuniform Field Configurations for Applications in Antihydrogen Production" src="https://digital.library.unt.edu/ark:/67531/metadc849779/small/"/></a></p><p>Comparisons between measurements of the ground-state hyperfine structure and gravitational acceleration of hydrogen and antihydrogen could provide a test of fundamental physical theories such as CPT (charge conjugation, parity, time-reversal) and gravitational symmetries. Currently, antihydrogen traps are based on Malmberg-Penning traps. The number of antiprotons in Malmberg-Penning traps with sufficiently low energy to be suitable for trappable antihydrogen production may be reduced by the electrostatic space charge of the positrons and/or collisions among antiprotons. Alternative trap designs may be needed for future antihydrogen experiments. A computational tool is developed to simulate charged particle motion in customizable magnetic fields generated by combinations of current loops and current lines. The tool is used to examine charged particle confinement in two systems consisting of dual, levitated current loops. The loops are coaxial and arranged to produce a magnetic null curve. Conditions leading to confinement in the system are quantified and confinement modes near the null curve and encircling one or both loops are identified. Furthermore, the tool is used to examine and quantify charged particle motion parallel to the null curve in the large radius limit of the dual, levitated current loops. An alternative to new trap designs is to identify the effects of the positron space in existing traps and to find modes of operation where the space charge is beneficial. Techniques are developed to apply the Boltzmann density relation along curved magnetic field lines. Equilibrium electrostatic potential profiles for a positron plasma are computed by solving Poisson's equation using a finite-difference method. Equilibria are computed in a model Penning trap with an axially varying magnetic field. Also, equilibria are computed for a positron plasma in a model of the ALPHA trap. Electric potential wells are found to form self-consistently. The technique is expanded to compute equilibria for a two-species plasma with an antiproton plasma confined by the positron space charge. The two-species equilibria are used to estimate timescales associated with three-body recombination, losses due to collisions between antiprotons, and temperature equilibration. An equilibrium where the three-body recombination rate is the smallest is identified.</p>Fractional Calculus and Dynamic Approach to Complexity2016-03-20T10:34:12-05:00https://digital.library.unt.edu/ark:/67531/metadc822832/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc822832/"><img alt="Fractional Calculus and Dynamic Approach to Complexity" title="Fractional Calculus and Dynamic Approach to Complexity" src="https://digital.library.unt.edu/ark:/67531/metadc822832/small/"/></a></p><p>Fractional calculus enables the possibility of using real number powers or complex number powers of the differentiation operator. The fundamental connection between fractional calculus and subordination processes is explored and affords a physical interpretation for a fractional trajectory, that being an average over an ensemble of stochastic trajectories. With an ensemble average perspective, the explanation of the behavior of fractional chaotic systems changes dramatically. Before now what has been interpreted as intrinsic friction is actually a form of non-Markovian dissipation that automatically arises from adopting the fractional calculus, is shown to be a manifestation of decorrelations between trajectories. Nonlinear Langevin equation describes the mean field of a finite size complex network at criticality. Critical phenomena and temporal complexity are two very important issues of modern nonlinear dynamics and the link between them found by the author can significantly improve the understanding behavior of dynamical systems at criticality. The subject of temporal complexity addresses the challenging and especially helpful in addressing fundamental physical science issues beyond the limits of reductionism.</p>Variational Calculations of Positronium Scattering with Hydrogen2016-03-20T10:34:12-05:00https://digital.library.unt.edu/ark:/67531/metadc822803/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc822803/"><img alt="Variational Calculations of Positronium Scattering with Hydrogen" title="Variational Calculations of Positronium Scattering with Hydrogen" src="https://digital.library.unt.edu/ark:/67531/metadc822803/small/"/></a></p><p>Positronium-hydrogen (Ps-H) scattering is of interest, as it is a fundamental four-body Coulomb problem. We have investigated low-energy Ps-H scattering below the Ps(n=2) excitation threshold using the Kohn variational method and variants of the method with a trial wavefunction that includes highly correlated Hylleraas-type short-range terms. We give an elegant formalism that combines all Kohn-type variational methods into a single form. Along with this, we have also developed a general formalism for Kohn-type matrix elements that allows us to evaluate arbitrary partial waves with a single codebase. Computational strategies we have developed and use in this work will also be discussed.With these methods, we have computed phase shifts for the first six partial waves for both the singlet and triplet states. The 1S and 1P phase shifts are highly accurate results and could potentially be viewed as benchmark results. Resonance positions and widths for the 1S-, 1P-, 1D-, and 1F-waves have been calculated.We present elastic integrated, elastic differential, and momentum transfer cross sections using all six partial waves and note interesting features of each. We use multiple effective range theories, including several that explicitly take into account the long-range van der Waals interaction, to investigate scattering lengths for the 1,3S and 1,3P partial waves and effective ranges for the 1,3S-wave.</p>Nonlinear and Quantum Optics Near Nanoparticles2016-03-20T10:34:12-05:00https://digital.library.unt.edu/ark:/67531/metadc822820/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc822820/"><img alt="Nonlinear and Quantum Optics Near Nanoparticles" title="Nonlinear and Quantum Optics Near Nanoparticles" src="https://digital.library.unt.edu/ark:/67531/metadc822820/small/"/></a></p><p>We study the behavior of electric fields in and around dielectric and metal nanoparticles, and prepare the ground for their applications to a variety of systems viz. photovoltaics, imaging and detection techniques, and molecular spectroscopy. We exploit the property of nanoparticles being able to focus the radiation field into small regions and study some of the interesting nonlinear, and quantum coherence and interference phenomena near them. The traditional approach to study the nonlinear light-matter interactions involves the use of the slowly varying amplitude approximation (SVAA) as it simplifies the theoretical analysis. However, SVVA cannot be used for systems which are of the order of the wavelength of the light. We use the exact solutions of the Maxwell's equations to obtain the fields created due to metal and dielectric nanoparticles, and study nonlinear and quantum optical phenomena near these nanoparticles. We begin with the theoretical description of the electromagnetic fields created due to the nonlinear wavemixing process, namely, second-order nonlinearity in an nonlinear sphere. The phase-matching condition has been revisited in such particles and we found that it is not satisfied in the sphere. We have suggested a way to obtain optimal conditions for any type and size of material medium. We have also studied the modifications of the electromagnetic fields in a collection of nanoparticles due to strong near field nonlinear interactions using the generalized Mie theory for the case of many particles applicable in photovoltaics (PV). We also consider quantum coherence phenomena such as modification of dark states, stimulated Raman adiabatic passage (STIRAP), optical pumping in $4$-level atoms near nanoparticles by using rotating wave approximation to describe the Hamiltonian of the atomic system. We also considered the behavior of atomic and the averaged atomic polarization in $7$-level atoms near nanoparticles. This could be used as a prototype to study any $n-$level atomic system experimentally in the presence of ensembles of quantum emitters. In the last chapter, we suggested a variant of a pulse-shaping technique applicable in stimulated Raman spectroscopy (SRS) for detection of atoms and molecules in multiscattering media. We used discrete-dipole approximation to obtain the fields created by the nanoparticles.</p>