Latest content added for Digital Library Partner: UNT Librarieshttps://digital.library.unt.edu/explore/partners/UNT/browse/?fq=str_degree_discipline:Physics&start=40&fq=untl_collection:UNTETD2015-03-08T17:44:37-05:00UNT LibrariesThis is a custom feed for browsing Digital Library Partner: UNT LibrariesSynthesis Strategies and a Study of Properties of Narrow and Wide Band Gap Nanowires2015-03-08T17:44:37-05:00https://digital.library.unt.edu/ark:/67531/metadc499984/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc499984/"><img alt="Synthesis Strategies and a Study of Properties of Narrow and Wide Band Gap Nanowires" title="Synthesis Strategies and a Study of Properties of Narrow and Wide Band Gap Nanowires" src="https://digital.library.unt.edu/ark:/67531/metadc499984/small/"/></a></p><p>Various techniques to synthesize nanowires and nanotubes as a function of growth temperature and time were investigated. These include growth of nanowires by a chemical vapor deposition (CVD) system using vapor-liquid-solid (VLS) growth mechanism and electro-chemical synthesis of nanowires and nanotubes. Narrow band gap InSb Eg = 0.17 eV at room temp) nanowires were successively synthesized. Using a phase diagram, the transition of the nanowire from metallic- semiconducting- semi-metallic phase was investigated. A thermodynamic model is developed to show that the occurrence of native defects in InSb nanowires influenced by the nanowire growth kinetics and thermodynamics of defect formation. Wide band gap ZnO (Eg = 3.34 eV) and In2O3 (3.7 eV) were also synthesized. ZnO nanowires and nanotubes were successfully doped with a transition metal Fe, making it a Dilute Magnetic Semiconductor of great technological relevance. Structural and electronic characterizations of nanowires were studied for different semiconducting, metallic and semi-metallic nanowires. Electron transport measurements were used to estimate intrinsic material parameters like carrier concentration and mobility. An efficient gas sensing device using a single In2O3 nanowire was studied and which showed sensitivity to reducing gas like NH3 and oxidizing gas like O2 gas at room temperature. The efficiency of the gas sensing device was found to be sensitive to the nature of contacts as well as the presence of surface states on the nanowire.</p>Effects of Quantum Coherence and Interference2015-03-08T17:44:37-05:00https://digital.library.unt.edu/ark:/67531/metadc500094/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500094/"><img alt="Effects of Quantum Coherence and Interference" title="Effects of Quantum Coherence and Interference" src="https://digital.library.unt.edu/ark:/67531/metadc500094/small/"/></a></p><p>Quantum coherence and interference (QCI) is a phenomenon that takes place in all multi-level atomic systems interacting with multiple lasers. In this work QCI is used to create several interesting effects like lasing without inversion (LWI), controlling group velocity of light to extreme values, controlling the direction of propagation through non-linear phase matching condition and for controlling the correlations in field fluctuations. Controlling group velocity of light is very interesting because of many novel applications it can offer. One of the unsolved problems in this area is to achieve a slow and fast light which can be tuned continuously as a function of frequency. We describe a method for creation of tunable slow and fast light by controlling intensity of incident laser fields using QCI effects. Lasers are not new to the modern world but an extreme ultra-violet laser or a x-ray laser is definitely one of the most desirable technologies today. Using QCI, we describe a method to realize lasing at high frequencies by creating lasing without inversion. Role of QCI in creating correlations and anti-correlations, which are generated by vacuum fluctuations, in a three level lambda system coupled to two strong fields is discussed.</p>How Cooperative Systems Respond to External Forces2015-03-08T17:44:37-05:00https://digital.library.unt.edu/ark:/67531/metadc500014/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc500014/"><img alt="How Cooperative Systems Respond to External Forces" title="How Cooperative Systems Respond to External Forces" src="https://digital.library.unt.edu/ark:/67531/metadc500014/small/"/></a></p><p>Cooperative interactions permeate through nature, bringing about emergent behavior and complexity. Using a simple cooperative model, I illustrate the mean field dynamics that occur at the critical point of a second order phase transition in the framework of Langevin equations. Through this formalism I discuss the response, both linear and nonlinear, to external forces. Emphasis is placed on how information is transferred from one individual to another in order to facilitate the collective response of the cooperative network to a localized perturbation. The results are relevant to a wide variety of systems, ranging from nematic liquid crystals, to flocks and swarms, social groups, and neural networks.</p>Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization2015-01-27T06:52:49-06:00https://digital.library.unt.edu/ark:/67531/metadc490047/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc490047/"><img alt="Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization" title="Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization" src="https://digital.library.unt.edu/ark:/67531/metadc490047/small/"/></a></p><p>Semi-conductor industry relies heavily on silicon (Si). However, Si is not a direct-band gap semi-conductor. Consequently, Si does not possess great versatility for multi-functional applications in comparison with the direct band-gap III-V semi-conductors such as GaAs. To bridge this gap, what is ideally required is a semi-conductor material system that is based on silicon, but has significantly greater versatility. While sparsely studied, the semi-conducting silicides material systems offer great potential. Thus, I focused on the growth and structural characterization of ruthenium silicide and osmium silicide material systems. I also characterized iron silicon germanide films using extended x-ray absorption fine structure (EXAFS) to reveal phase, semi-conducting behavior, and to calculate nearest neighbor distances. The choice of these silicides material systems was due to their theoretically predicted and/or experimentally reported direct band gaps. However, the challenge was the existence of more than one stable phase/stoichiometric ratio of these materials. In order to possess the greatest control over the growth process, molecular beam epitaxy (MBE) has been employed. Structural and film quality comparisons of as-grown versus annealed films of ruthenium silicide are presented. Structural characterization and film quality of MBE grown ruthenium silicide and osmium silicide films via in situ and ex situ techniques have been done using reflection high energy electron diffraction, scanning tunneling microscopy, atomic force microscopy, cross-sectional scanning electron microscopy, x-ray photoelectron spectroscopy, and micro Raman spectroscopy. This is the first attempt, to the best of our knowledge, to grow osmium silicide thin films on Si(100) via the template method and compare it with the regular MBE growth method. The pros and cons of using the MBE template method for osmium silicide growth are discussed, as well as the structural differences of the as-grown versus annealed films. Future perspectives include further studies on other semi-conducting silicides material systems in terms of growth optimization and characterization.</p>Coherent Resonant Interaction and Harmonic Generation in Atomic Vapors2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc332243/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc332243/"><img alt="Coherent Resonant Interaction and Harmonic Generation in Atomic Vapors" title="Coherent Resonant Interaction and Harmonic Generation in Atomic Vapors" src="https://digital.library.unt.edu/ark:/67531/metadc332243/small/"/></a></p><p>This work examines the use of higher order multiphoton resonances in higher harmonic generation together with judicious exploitation of coherent interaction properties to achieve efficient harmonic generation. A detailed experimental study on third harmonic generation in two photon resonant coherent interaction and a theoretical study on four photon resonant coherent interaction have been conducted.
Two photon resonant coheren propagation in lithium vapor (2S-4S and 2S-3D interaction) has been studied in detail as a function of phase and delay of the interacting pulse sequence. Under coherent lossless propagation of 90 phase shifted pulse pair, third harmonic generation is enhanced. A maximum energy conversion efficiency of 1% was measured experimentally. This experiment shows that phase correlated pulse sequence can be used to control multiphoton coherent resonant effects.
A larger two photon resonant enhancement does not result in more efficient harmonic generation, in agreement with the theoretical prediction.
An accurate (to at least 0.5 A°) measurement of intensity dependent Stark shift has been done with the newly developed "interferometric wavemeter." Stark shifts as big as several pulse bandwidths (of picosecond pulses) result in a poor tuning of multiphoton resonance and become a limiting factor of resonant harmonic generation.
A complete theory has been developed for harmonic generation in a four photon resonant coherent interaction. A numerical application of the theory to the Hg atom successfully interprets the experimental observations in terms of the phase dependent stimulated Raman scattering. With the intensity required for four photon resonant transition, the calculation predicts a dramatic Stark shift effect which completely destroys the resonance condition. This model provides a basis for the development of future schemes for efficient higher order coherent upconversion.</p>Two Photon Resonant Picosecond Pulse Propagation in Lithium Vapor2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc331546/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc331546/"><img alt="Two Photon Resonant Picosecond Pulse Propagation in Lithium Vapor" title="Two Photon Resonant Picosecond Pulse Propagation in Lithium Vapor" src="https://digital.library.unt.edu/ark:/67531/metadc331546/small/"/></a></p><p>The work of this dissertation has been to prove that the coherence of multiphoton excitation can be studied by an appropriately phased and time delayed sequence of pulses. An application of this fundamental study of coherence has been made for the enhancement of third harmonic generation. The coherent recovery of the energy lost to the two photon absorption process enalled a larger propagation distance for the fundamental than in an interaction which is incoherent or coherent, but not using a 90 degree phase shifted pulse pair. Phase matching over this longer propagation distance gave an enhancement of third harmonic generation.</p>Dispersion of the Nonlinear Refractive Index of CS₂ in the Spectral Range of 9-11 μm2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc332448/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc332448/"><img alt="Dispersion of the Nonlinear Refractive Index of CS₂ in the Spectral Range of 9-11 μm" title="Dispersion of the Nonlinear Refractive Index of CS₂ in the Spectral Range of 9-11 μm" src="https://digital.library.unt.edu/ark:/67531/metadc332448/small/"/></a></p><p>The nonlinear refractive index (n2) of room temperature liquid CS2 in the wavelength range of 9 to 11 micrometers is measured. A line tunable hybrid C02 TEA laser and amplifier system is used for the experiments. In these measurements the well known photoacoustic method is utilized to observe the onset of whole beam self-focusing. The photoacoustic signal in a CS2 cell, much longer than the confocal parameter, is monitored. The departure of the acoustic signal from linear growth marks the critical power for the onset of nonlinearity. It is experimentally verified that the phenomenon is power dependent as expected from self-focusing theory. The value of n2 is then calculated from the theoretical model of self focusing. Measurements of the on-axis irradiance transmitted through the nonlinear material as well as the measurements of beam distortion are used to verify the validity of the photoacoustic method. In all the measurements the on-axis intensity was smaller than the calculated threshold intensity for stimulated Brillouin scattering. The back reflection was monitored to make sure that stimulated Brillouin scattering was not playing a role in the phenomenon.</p>L- and M-Shell X-Ray Production Cross Sections of Neodymium Gadolinium, Holmium, Ytterbium, Gold and Lead by 25-MeV Carbon and 32-MeV Oxygen Ions2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc330892/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc330892/"><img alt="L- and M-Shell X-Ray Production Cross Sections of Neodymium Gadolinium, Holmium, Ytterbium, Gold and Lead by 25-MeV Carbon and 32-MeV Oxygen Ions" title="L- and M-Shell X-Ray Production Cross Sections of Neodymium Gadolinium, Holmium, Ytterbium, Gold and Lead by 25-MeV Carbon and 32-MeV Oxygen Ions" src="https://digital.library.unt.edu/ark:/67531/metadc330892/small/"/></a></p><p>L- and M-shell x-ray production cross sections have been measured for thin solid targets of neodymium, gadolinium, holmium, ytterbium, gold, and lead by 25 MeV 12/6C^q+ (q=4,5,6) and by 32 MeV 16/8O^q+ (q=5,7,8). The cross sections were determined from measurements made with thin targets (< 2.5 μg/cm2). For projectiles with one or two K-shell vacancies, the target x-ray production cross sections were found to be enhanced over those for projectiles without a K-shell vacancy. The sum of direct ionization to the continuum (DI) plus electron capture (EC) to the L, M, N... shells and EC to the K-shell of the projectile have been extracted from the data. The results are compared to the predictions of first Born theories, i.e., plane wave Born approximation for DI and Oppenheimer-Brinkman-Kramers formula of Nikolaev for EC and to the ECPSSR approach that accounts for Energy loss and Coulomb deflection of the projectile as well as for Relativistic and Perturbed Stationary States of inner shell electrons.</p>Photon Exchange Between a Pair of Nonidentical Atoms with Two Forms of Interactions2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc331447/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc331447/"><img alt="Photon Exchange Between a Pair of Nonidentical Atoms with Two Forms of Interactions" title="Photon Exchange Between a Pair of Nonidentical Atoms with Two Forms of Interactions" src="https://digital.library.unt.edu/ark:/67531/metadc331447/small/"/></a></p><p>A pair of nonidentical two-level atoms, separated by a fixed distance R, interact through photon exchange. The system is described by a state vector which is assumed to be a superposition of four "essential states": (1) the first atom is excited, the second one is in the ground state, and no photon is present, (2) the first atom is in its ground state, the second one is excited, and no photon is present, (3) both atoms are in their ground states and a photon is present, and (4) both atoms are excited and a photon is also present. The system is initially in state (1). The probabilities of each atom being excited are calculated for both the minimally-coupled interaction and the multipolar interaction in the electric dipole approximation. For the minimally-coupled interaction Hamiltonian, the second atom has a probability of being instantaneously excited, so the interaction is not retarded. For the multipolar interaction Hamiltonian, the second atom is not excited before the retardation time, which agrees with special relativity. For the minimally-coupled interaction the nonphysical result occurs because the unperturbed Hamiltonian is not the energy operator in the Coulomb gauge. For the multipolar Hamiltonian in the electric dipole approximation the unperturbed Hamiltonian is the energy operator. An active view of unitary transformations in nonrelativistic quantum electrodynamics is used to derive transformation laws for the potentials of the electromagnetic field and the static Coulomb potential. For a specific choice of unitary transformation the transformation laws for the potentials are used in the minimally-coupled second-quantized Hamiltonian to obtain the multipolar Hamiltonian, which is expressed in terms of the quantized electric and magnetic fields.</p>Anomalous Behavior in the Rotational Spectra of the v₈=2 and the v₈=3 Vibrations for the ¹³C and ¹⁵N Tagged Isotopes of the CH₃CN Molecule in the Frequency Range 17-95 GHz2014-08-22T18:00:56-05:00https://digital.library.unt.edu/ark:/67531/metadc330976/<p><a href="https://digital.library.unt.edu/ark:/67531/metadc330976/"><img alt="Anomalous Behavior in the Rotational Spectra of the v₈=2 and the v₈=3 Vibrations for the ¹³C and ¹⁵N Tagged Isotopes of the CH₃CN Molecule in the Frequency Range 17-95 GHz" title="Anomalous Behavior in the Rotational Spectra of the v₈=2 and the v₈=3 Vibrations for the ¹³C and ¹⁵N Tagged Isotopes of the CH₃CN Molecule in the Frequency Range 17-95 GHz" src="https://digital.library.unt.edu/ark:/67531/metadc330976/small/"/></a></p><p>The rotational microwave spectra of the three isotopes (^13CH_3^12C^15N, ^12CH_3^13C^15N, and ^13CH_3^13C^15N) of the methyl cyanide molecule in the v_8=3, v_8=2, v_7=1 and v_4=1 vibrational energy levels for the rotational components 1£J£5 (for a range of frequency 17-95 GHz.) were experimentally and theoretically examined. Rotational components in each vibration were measured to determine the mutual interactions in each vibration between any of the vibrational levels investigated.
The method of isotopic substitution was employed for internal tuning of each vibrational level by single and double substitution of ^13C in the two sites of the molecule. It was found that relative frequencies within each vibration with respect to another vibration were shifted in a systematic way. The results given in this work were interpreted on the basis of these energy shifts. Large departure between experimentally measured and theoretically predicted frequency for the quantum sets (J, K=±l, ϑ=±1), Kϑ-l in the v_8=3 vibrational states for the ^13c and ^15N tagged isotopes of CH_3CN showed anomalous behavior which was explained as being due to Fermi resonance. Accidently strong resonances (ASR) were introduced to account for some departures which were not explained by Fermi resonance.</p>