Latest content added for UNT Digital Library Collection: UNT Theses and Dissertationshttp://digital.library.unt.edu/explore/collections/UNTETD/browse/?fq=untl_institution:UNT&fq=str_degree_discipline:Physics2016-03-20T10:34:12-05:00UNT LibrariesThis is a custom feed for browsing UNT Digital Library Collection: UNT Theses and DissertationsNonlinear and Quantum Optics Near Nanoparticles2016-03-20T10:34:12-05:00http://digital.library.unt.edu/ark:/67531/metadc822820/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc822820/"><img alt="Nonlinear and Quantum Optics Near Nanoparticles" title="Nonlinear and Quantum Optics Near Nanoparticles" src="http://digital.library.unt.edu/ark:/67531/metadc822820/thumbnail/"/></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>Fractional Calculus and Dynamic Approach to Complexity2016-03-20T10:34:12-05:00http://digital.library.unt.edu/ark:/67531/metadc822832/<p><a href="http://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="http://digital.library.unt.edu/ark:/67531/metadc822832/thumbnail/"/></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 Hydrogen2016-03-20T10:34:12-05:00http://digital.library.unt.edu/ark:/67531/metadc822803/<p><a href="http://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="http://digital.library.unt.edu/ark:/67531/metadc822803/thumbnail/"/></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>Complex Numbers in Quantum Theory2016-03-04T16:14:01-06:00http://digital.library.unt.edu/ark:/67531/metadc804988/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc804988/"><img alt="Complex Numbers in Quantum Theory" title="Complex Numbers in Quantum Theory" src="http://digital.library.unt.edu/ark:/67531/metadc804988/thumbnail/"/></a></p><p>In 1927, Nobel prize winning physicist, E. Schrodinger, in correspondence with Ehrenfest, wrote the following about the new theory: “What is unpleasant here, and indeed directly to be objected to, is the use of complex numbers. Psi is surely fundamentally a real function.” This seemingly simple issue remains unexplained almost ninety years later. In this dissertation I elucidate the physical and theoretical origins of the complex requirement. I identify a freedom/constraint situation encountered by vectors when, employed in accordance with adopted quantum representational methodology, and representing angular momentum states in particular. Complex vectors, quite simply, provide more available adjustable variables than do real vectors. The additional variables relax the constraint situation allowing the theory’s representational program to carry through. This complex number issue, which lies at the deepest foundations of the theory, has implications for important issues located higher in the theory. For example, any unification of the classical and quantum accounts of the settled order of nature, will rest squarely on our ability to account for the introduction of the imaginary unit.</p>A Precise Few-nucleon Size Difference by Isotope Shift Measurements of Helium2016-03-04T16:14:01-06:00http://digital.library.unt.edu/ark:/67531/metadc804828/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc804828/"><img alt="A Precise Few-nucleon Size Difference by Isotope Shift Measurements of Helium" title="A Precise Few-nucleon Size Difference by Isotope Shift Measurements of Helium" src="http://digital.library.unt.edu/ark:/67531/metadc804828/thumbnail/"/></a></p><p>We perform high precision measurements of an isotope shift between the two stable isotopes of helium. We use laser excitation of the 2^3 S_1-2^3 P_0 transition at 1083 nm in a metastable beam of 3He and 4He atoms. A newly developed tunable laser frequency selector along with our previous electro-optic frequency modulation technique provides extremely reliable, adaptable, and precise frequency and intensity control. The intensity control contributes negligibly to overall experimental uncertainty by stabilizing the intensity of the required sideband and eliminating the unwanted frequencies generated during the modulation of 1083 nm laser carrier frequency. The selection technique uses a MEMS based fiber switch and several temperature stabilized narrow band (~3 GHz) fiber gratings. A fiber based optical circulator and an inline fiber amplifier provide the desired isolation and the net gain for the selected frequency. Also rapid (~2 sec.) alternating measurements of the 2^3 S_1-2^3 P_0 interval for both species of helium is achieved with a custom fiber laser for simultaneous optical pumping. A servo-controlled retro-reflected laser beam eliminates residual Doppler effects during the isotope shift measurement. An improved detection design and software control makes negligible subtle potential biases in the data collection. With these advances, combined with new internal and external consistency checks, we are able to obtain results consistent with the best previous measurements, but with substantially improved precision. Our measurement of the 2^3 S_1-2^3 P_0 isotope shift between 3He and 4He is 31 097 535.2 (5) kHz. The most recent theoretic calculation combined with this measurement yields a new determination for nuclear size differences between 3He and 4He: ∆r_c=0.292 6 (1)_exp (8)_th (52)_exp fm, with a precision of less than a part in 〖10〗^4 coming from the experimental uncertainty (first parenthesis), and a part in 〖10〗^3 coming from theory. This value is consistent with electron scattering measurement, but a factor of 10 more precise. It is inconsistent (4 sigma) with a recent isotope shift measurement on another helium transition (2^1 S_0-2^3 S_1). Comparisons with ongoing muonic helium measurements may provide clues to the origin of what is currently called the proton puzzle: electronic and muonic measurements of the proton size do not agree. In the future, the experimental improvements described here can be used for higher precision tests of atomic theory and quantum electrodynamics, as well as an important atomic physics source of the fine structure constant.</p>Synthesis and Characterization of Ion Beam Assisted Silver Nanosystems in Silicon Based Materials for Enhanced Photocurrent Collection Efficiency2016-02-02T13:35:12-06:00http://digital.library.unt.edu/ark:/67531/metadc799502/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc799502/"><img alt="Synthesis and Characterization of Ion Beam Assisted Silver Nanosystems in Silicon Based Materials for Enhanced Photocurrent Collection Efficiency" title="Synthesis and Characterization of Ion Beam Assisted Silver Nanosystems in Silicon Based Materials for Enhanced Photocurrent Collection Efficiency" src="http://digital.library.unt.edu/ark:/67531/metadc799502/thumbnail/"/></a></p><p>In recent years a great deal of interest has been focused on the synthesis of transitional metal (e.g. Ag, Cu, Fe, Au) nanosystems at the surface to sub-surface regions of Si and SiO2 matrices for fundamental understanding of their structures as well as for development of technological applications with enhanced electronic and optical properties. The applications of the metal nanoparticle or nanocluster (NC) systems range from plasmonics, photovoltaic devices, medical, and biosensors. In all of these applications; the size, shape and distribution of the metallic NCs in the silicon matrix play a key role. Low energy ion implantation followed by thermal annealing (in vacuum or gas environment) is one of the most suitable methods for synthesis of NCs at near surfaces to buried layers below the surfaces of the substrates. This technique can provide control over depth and concentration of the implanted ions in the host matrix. The implanted low energy metal ions initially amorphizes the Si substrates while being distributed at a shallow depth near the substrate surface. When subject to thermal annealing, the implanted ions agglomerate to form clusters of different sizes at different depths depending upon the fluence. However, for the heavier ions implanted with high fluences (~1×1016 - 1×1017 atoms/cm2), there lies challenges for accurately predicting the distribution of the implanted ions due to sputtering of the surface as well as redistribution of the implants within the host matrix. In this dissertation, we report the investigation of the saturation of the concentration of the implanted ion species in the depth profiles with low energies (< 80 keV) metal ions (Ag and Au) in Si (100), while studying the dynamic changes during the ion implantation. Multiple low energies (30-80 keV) Ag ions with different fluences were sequentially implanted into commercially available Si wafers in order to facilitate the formation of Ag NCs with a wide ion distributions range. The light absorption profile according to different sizes of NCs at the near-surface layers in Si were investigated. We have investigated the formation of Ag NCs in the Si matrix as a function of implantation and thermal annealing parameters. The absorbance of light is increased in Ag implanted Si with a significant increase in the current collection in I-V (current-voltage) photo switching measurements. The experimental photovoltaic cells fabricated with the Ag implanted Si samples were optically characterized under AM (air mass) 1.5 solar radiation conditions (~1.0 kW/m2). An enhancement in the charge collection were measured in the annealed samples, where prominent Ag NCs were formed in the Si matrix compared to the as-implanted samples with the amorphous layer. The characterization techniques such as Rutherford Backscattering Spectroscopy, XPS-depth profiling, transmission electron microscopy, optical absorption, and I-V (current-voltage) photo switching measurements were employed to understand the underlying science in the observed properties. The results of these investigations are discussed in this research.</p>Interaction of Plasmons and Excitons for Low-Dimension Semiconductors2016-02-02T13:35:12-06:00http://digital.library.unt.edu/ark:/67531/metadc799475/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc799475/"><img alt="Interaction of Plasmons and Excitons for Low-Dimension Semiconductors" title="Interaction of Plasmons and Excitons for Low-Dimension Semiconductors" src="http://digital.library.unt.edu/ark:/67531/metadc799475/thumbnail/"/></a></p><p>The effects of surface plasmon for InGaN/GaN multi-quantum wells and ZnO nanoparticles optical linear and nonlinear emission efficiency had been experimentally studied. Due to the critical design for InGaN MQWs with inverted hexagonal pits based on GaN, both contribution of surface plasmon effect and image charge effect at resonant and off resonant frequencies were experimentally and theoretically investigated. With off- resonant condition, the InGaN MQWs emission significantly enhanced by metal nanoparticles. This enhancement was caused by the image charge effect, due to the accumulation of carriers to NPs region. When InGaN emission resonated with metal particles SP modes, surface Plasmon effect dominated the emission process. We also studied the surface plasmon effect for ZnO nanoparticles nonlinear optical processes, SHG and TPE. Defect level emission had more contribution at high incident intensity. Emissions are different for pumping deep into the bulk and near surface. A new assumption to increase the TPE efficiency was studied. We thought by using Au nanorods localized surface plasmon mode to couple the ZnO virtual state, the virtual state’s life time would be longer and experimentally lead the emission enhancement. We studied the TPE phenomena at high and near band gap energy. Both emission intensity and decay time results support our assumption. Theoretically, the carriers dynamic mechanism need further studies.</p>Electromagnetically Modulated Sonic Structures2016-02-02T13:35:12-06:00http://digital.library.unt.edu/ark:/67531/metadc799496/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc799496/"><img alt="Electromagnetically Modulated Sonic Structures" title="Electromagnetically Modulated Sonic Structures" src="http://digital.library.unt.edu/ark:/67531/metadc799496/thumbnail/"/></a></p><p>Phononic crystals are structures composed of periodically arranged scatterers in a background medium that affect the transmission of elastic waves. They have garnered much interest in recent years for their macro-scale properties that can be modulated by the micro-scale components. The elastic properties of the composite materials, the contrast in the elastic properties of the composite materials, and the material arrangement all directly affect how an elastic wave will behave as it propagates through the sonic structure. The behavior of an elastic wave in a periodic structure is revealed in its transmission bandstructure, and modification of any the elastic parameters will result in tuning of the band structure. In this dissertation, a phononic crystal with properties that can be modulated using electromagnetic radiation, and more specifically, radio-frequency (RF) light will be presented.</p>Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals2016-02-02T13:35:12-06:00http://digital.library.unt.edu/ark:/67531/metadc799538/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc799538/"><img alt="Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals" title="Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals" src="http://digital.library.unt.edu/ark:/67531/metadc799538/thumbnail/"/></a></p><p>Performance and reliability of solid state laser diodes in the IR region exceeds those in the visible and UV part of the light spectrum. Single frequency visible and UV laser diodes with higher than 500 mW power are not available commercially. However we successfully stabilized a multi-longitudinal mode IR laser to 860 mW single frequency. This means high efficiency harmonic generation using this laser can produce visible and UV laser light not available otherwise. In this study we examined three major leading nonlinear crystals: PPMgO:SLN, PPKTP and PPMgO:SLT to generate blue light by second harmonic generation. We achieved record high net conversion efficiencies 81.3% using PPMgO:SLT (~500 mW out), and 81.1% using PPKTP (~700 mW out). In both these cases an external resonance buildup cavity was used. We also studied a less complicated single pass waveguide configuration (guided waist size of ~ 5 um compared to ~60 um) to generate blue. With PPMgO:SLN we obtained net 40.4% and using PPKT net 6.8% (110mW and 10.1 mW respectively).</p>Studies of Charged Particle Dynamics for Antihydrogen Synthesis2015-08-21T05:42:39-05:00http://digital.library.unt.edu/ark:/67531/metadc699934/<p><a href="http://digital.library.unt.edu/ark:/67531/metadc699934/"><img alt="Studies of Charged Particle Dynamics for Antihydrogen Synthesis" title="Studies of Charged Particle Dynamics for Antihydrogen Synthesis" src="http://digital.library.unt.edu/ark:/67531/metadc699934/thumbnail/"/></a></p><p>Synthesis and capture of antihydrogen in controlled laboratory conditions will enable precise studies of neutral antimatter. The work presented deals with some of the physics pertinent to manipulating charged antiparticles in order to create neutral antimatter, and may be applicable to other scenarios of plasma confinement and charged particle interaction. The topics covered include the electrostatic confinement of a reflecting ion beam and the transverse confinement of an ion beam in a purely electrostatic configuration; the charge sign effect on the Coulomb logarithm for a two component (e.g., antihydrogen) plasma in a Penning trap as well as the collisional scattering for binary Coulomb interactions that are cut off at a distance different than the Debye length; and the formation of magnetobound positronium and protonium.</p>