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Electrical Conduction Mechanisms in the Disordered Material System P-type Hydrogenated Amorphous Silicon

Description: The electrical and optical properties of boron doped hydrogenated amorphous silicon thin films (a-Si) were investigated to determine the effect of boron and hydrogen incorporation on carrier transport. The a-Si thin films were grown by plasma enhanced chemical vapor deposition (PECVD) at various boron concentrations, hydrogen dilutions, and at differing growth temperatures. The temperature dependent conductivity generally follows the hopping conduction model. Above a critical temperature, the dominant conduction mechanism is Mott variable range hopping conductivity (M-VRH), where p = ¼, and the carrier hopping depends on energy. However, at lower temperatures, the coulomb interaction between charge carriers becomes important and Efros-Shklosvkii variable hopping (ES-VRH) conduction, where p=1/2, must be included to describe the total conductivity. To correlate changes in electrical conductivity to changes in the local crystalline order, the transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short- and mid-range order to the effects of growth temperature, boron, and hydrogen incorporation. With an increase of hydrogen and/or growth temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of short range order. It is seen that there is a direct correlation between the electrical conductivity and changes in the short and mid-range order resulting from the passivation of defects by hydrogen and the creation of trap states by boron. This work was done under the ARO grant W911NF-10-1-0410, William W. Clark Program Manager. The samples were provided by L-3 Communications.
Date: December 2014
Creator: Shrestha, Kiran (Engineer)

An Electro- Magneto-static Field for Confinement of Charged Particle Beams and Plasmas

Description: A system is presented that is capable of confining an ion beam or plasma within a region that is essentially free of applied fields. An Artificially Structured Boundary (ASB) produces a spatially periodic set of magnetic field cusps that provides charged particle confinement. Electrostatic plugging of the magnetic field cusps enhances confinement. An ASB that has a small spatial period, compared to the dimensions of a confined plasma, generates electro- magneto-static fields with a short range. An ASB-lined volume thus constructed creates an effectively field free region near its center. It is assumed that a non-neutral plasma confined within such a volume relaxes to a Maxwell-Boltzmann distribution. Space charge based confinement of a second species of charged particles is envisioned, where the second species is confined by the space charge of the first non-neutral plasma species. An electron plasma confined within an ASB-lined volume can potentially provide confinement of a positive ion beam or positive ion plasma. Experimental as well as computational results are presented in which a plasma or charged particle beam interact with the electro- magneto-static fields generated by an ASB. A theoretical model is analyzed and solved via self-consistent computational methods to determine the behavior and equilibrium conditions of a relaxed plasma. The equilibrium conditions of a relaxed two species plasma are also computed. In such a scenario, space charge based electrostatic confinement is predicted to occur where a second plasma species is confined by the space charge of the first plasma species. An experimental apparatus with cylindrical symmetry that has its interior surface lined with an ASB is presented. This system was developed by using a simulation of the electro- magneto-static fields present within the trap to guide mechanical design. The construction of the full experimental apparatus is discussed. Experimental results that show the characteristics of ...
Date: May 2014
Creator: Pacheco, Josè L.

Electromagnetically Modulated Sonic Structures

Description: 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.
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Date: May 2014
Creator: Walker, Ezekiel Lee

Electrostatic Effects in III-V Semiconductor Based Metal-optical Nanostructures

Description: The modification of the band edge or emission energy of semiconductor quantum well light emitters due to image charge induced phenomenon is an emerging field of study. This effect observed in quantum well light emitters is critical for all metal-optics based light emitters including plasmonics, or nanometallic electrode based light emitters. This dissertation presents, for the first time, a systematic study of the image charge effect on semiconductor–metal systems. the necessity of introducing the image charge interactions is demonstrated by experiments and mathematical methods for semiconductor-metal image charge interactions are introduced and developed.
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Date: May 2012
Creator: Gryczynski, Karol Grzegorz

Electrostatic Mechanism of Emission Enhancement in Hybrid Metal-semiconductor Light-emitting Heterostructures

Description: III-V nitrides have been put to use in a variety of applications including laser diodes for modern DVD devices and for solid-state white lighting. Plasmonics has come to the foreground over the past decade as a means for increasing the internal quantum efficiency (IQE) of devices through resonant interaction with surface plasmons which exist at metal/dielectric interfaces. Increases in emission intensity of an order of magnitude have been previously reported using silver thin-films on InGaN/GaN MQWs. the dependence on resonant interaction between the plasmons and the light emitter limits the applications of plasmonics for light emission. This dissertation presents a new non-resonant mechanism based on electrostatic interaction of carriers with induced image charges in a nearby metallic nanoparticle. Enhancement similar in strength to that of plasmonics is observed, without the restrictions imposed upon resonant interactions. in this work we demonstrate several key features of this new interaction, including intensity-dependent saturation, increase in the radiative recombination lifetime, and strongly inhomogeneous light emission. We also present a model for the interaction based on the aforementioned image charge interactions. Also discussed are results of work done in the course of this research resulting in the development of a novel technique for strain measurement in light-emitting structures. This technique makes use of a spectral fitting model to extract information about electron-phonon interactions in the sample which can then be related to strain using theoretical modeling.
Date: May 2012
Creator: Llopis, Antonio

Enhancements of Mechanical, Thermal Stability, and Tribological Properties by Addition of Functionalized Reduced Graphene Oxide in Epoxy

Description: The effects of octadecylamine-functionalized reduced graphene oxide (FRGO) on the frictional and wear properties of diglycidylether of bisphenol-A (DGEBA) epoxy are studied using a pin-on-disk tribometer. It was observed that the addition of FRGO significantly improves the tribological, mechanical, and thermal properties of epoxy matrix. Graphene oxide (GO) was functionalized with octadecylamine (ODA), and then reduction of oxygen-containing functional groups was carried out using hydrazine monohydrate. The Raman and x-ray photoelectron spectroscopy studies confirm significant reduction in oxygen-containing functional groups and formation of ODA functionalized reduced GO. The nanocomposites are prepared by adding 0.1, 0.2, 0.5 and 1.0 wt % of FRGO to the epoxy. The addition of FRGO increases by more than an order of magnitude the sliding distance during which the dynamic friction is ≤ 0.1. After this distance, the friction sharply increases to the range of 0.4 - 0.5. We explain the increase in sliding distance during which the friction is low by formation of a transfer film from the nanocomposite to the counterface. The wear rates in the low and high friction regimes are approximately 1.5 x 10-4 mm3/N·m and 5.5 x 10-4 mm3/N·m, respectively. The nanocomposites exhibit a 74 % increase in Young’s modulus with 0.5 wt. % of FRGO, and an increase in glass transition and thermal degradation temperatures.
Date: August 2014
Creator: Shah, Rakesh K.

Fractional Calculus and Dynamic Approach to Complexity

Description: 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.
Date: December 2015
Creator: Beig, Mirza Tanweer Ahmad

High Efficiency High Power Blue Laser by Resonant Doubling in PPKTP

Description: I developed a high power blue laser for use in scientific and technical applications (eg. precision spectroscopy, semiconductor inspection, flow cytometry, etc). It is linearly polarized, single longitudinal and single transverse mode, and a convenient fiber coupled continuous wave (cw) laser source. My technique employs external cavity frequency doubling and provides better power and beam quality than commercially available blue diode lasers. I use a fiber Bragg grating (FBG) stabilized infrared (IR) semiconductor laser source with a polarization maintaining (PM) fiber coupled output. Using a custom made optical and mechanical design this output is coupled with a mode matching efficiency of 96% into the doubling cavity. With this carefully designed and optimized cavity, measurements were carried out at various fundamental input powers. A net efficie ncy of 81 % with an output power of 680 mW at 486 nm was obtained using 840 mW of IR input. Also I report an 87.5 % net efficiency in coupling of blue light from servo locked cavity into a single mode PM fiber. Thus I have demonstrated a total fiber to fiber efficiency of 71% can be achieved in our approach using periodically poled potassium titanyl phosphate (PPKTP). To obtain these results, all losses in the system were carefully studied and minimized.
Date: August 2011
Creator: Danekar, Koustubh

Highly Efficient Single Frequency Blue Laser Generation by Second Harmonic Generation of Infrared Lasers Using Quasi Phase Matching in Periodically Poled Ferroelectric Crystals

Description: 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).
Date: August 2014
Creator: Khademian, Ali

How Cooperative Systems Respond to External Forces

Description: 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.
Date: May 2014
Creator: Svenkeson, Adam

Interaction of Plasmons and Excitons for Low-Dimension Semiconductors

Description: 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.
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Date: December 2014
Creator: Lin, Jie (physicist)

Ion Beam Synthesis of Binary and Ternary Transition Metal Silicide Thin Films

Description: 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 ...
Date: December 2016
Creator: Lakshantha, Wickramaarachchige Jayampath

Ion Beam Synthesis of Carbon Assisted Nanosystems in Silicon Based Substrates

Description: The systematic study of the formation of β-SiC formed by low energy carbon ion (C-)implantation into Si followed by high temperature annealing is presented. The research is performed to explore the optimal annealing conditions. The formation of crystalline β-SiC is clearly observed in the sample annealed at 1100 °C for a period of 1 hr. Quantitative analysis is performed in the formation of β-SiC by the process of implantation of different carbon ion fluences of 1×1017, 2×1017, 5×1017, and 8×1017 atoms /cm2 at an ion energy of 65 keV into Si. It is observed that the average size of β-SiC crystals decreased and the amount of β-SiC crystals increased with the increase in the implanted fluences when the samples were annealed at 1100°C for 1 hr. However, it is observed that the amount of β-SiC linearly increased with the implanted fluences up to 5×1017 atoms /cm2. Above this fluence the amount of β-SiC appears to saturate. The stability of graphitic C-C bonds at 1100°C limits the growth of SiC precipitates in the sample implanted at a fluence of 8×1017 atoms /cm2 which results in the saturation behavior of SiC formation in the present study. Secondly, the carbon cluster formation process in silica and the characterization of formed clusters is presented. Silicon dioxide layers ~500 nm thick are thermally grown on a Si (100) wafer. The SiO2 layers are then implanted with 70 keV carbon ions at a fluence of 5×1017 atoms/cm2. The implanted samples are annealed 1100 °C for different time periods of 10 min., 30 min., 60 min., 90 min., and 120 min., in the mixture of argon and hydrogen gas (96 % Ar + 4% hydrogen). Photoluminescence spectroscopy reveals UV to visible emission from the samples. A detail mechanism of the photoluminescence and its possible origin is discussed ...
Date: May 2011
Creator: Poudel, Prakash Raj

Local Phase Manipulation for Multi-Beam Interference Lithography for the Fabrication of Two and Three Dimensional Photonic Crystal Templates

Description: 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.
Date: December 2016
Creator: Lutkenhaus, Jeffrey Ryan

Low-Energy Electron Irradiation of Preheated and Gas-Exposed Single-Wall Carbon Nanotubes

Description: 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.
Date: December 2016
Creator: Ecton, Philip

Modification of Graphene Properties: Electron Induced Reversible Hydrogenation, Oxidative Etching and Layer-by-layer Thinning

Description: In this dissertation, I present the mechanism of graphene hydrogenation via three different electron sources: scanning electron microscopy, e-beam irradiation and H2 and He plasma irradiation. in each case, hydrogenation occurs due to electron impact fragmentation of adsorbed water vapor from the sample preparation process. in the proposed model, secondary and backscattered electrons generated from incident electron interactions with the underlying silicon substrate are responsible for the dissociation of water vapor. Chemisorbed H species from the dissociation are responsible for converting graphene into hydrogenated graphene, graphane. These results may lead to higher quality graphane films having a larger band gap than currently reported. in addition, the dissertation presents a novel and scalable method of controllably removing single atomic planes from multi-layer graphene using electron irradiation from an intense He plasma under a positive sample bias. As the electronic properties or multi-layer graphene are highly dependent on the number of layers, n, reducing n in certain regions has many benefits. for example, a mask in conjunction with this thinning method could be used for device applications.
Date: May 2012
Creator: Jones, Jason David

Nanoscale Materials Applications: Thermoelectrical, Biological, and Optical Applications with Nanomanipulation Technology

Description: In a sub-wavelength scale, even approaching to the atomic scale, nanoscale physics shows various novel phenomena. Since it has been named, nanoscience and nanotechnology has been employed to explore and exploit this small scale world. For example, with various functionalized features, nanowire (NW) has been making its leading position in the researches of physics, chemistry, biology, and engineering as a miniaturized building block. Its individual characteristic shows superior and unique features compared with its bulk counterpart. As one part of these research efforts and progresses, and with a part of the fulfillment of degree study, novel methodologies and device structures in nanoscale were devised and developed to show the abilities of high performing thermoelectrical, biological, and optical applications. A single β-SiC NW was characterized for its thermoelectric properties (thermal conductivity, Seebeck coefficient, and figure of merit) to compare with its bulk counterpart. The combined structure of Ag NW and ND was made to exhibit its ability of clear imaging of a fluorescent cell. And a plasmonic nanosture of silver (Ag) nanodot array and a β-SiC NW was fabricated to show a high efficient light harvesting device that allows us to make a better efficient solar cell. Novel nanomanipulation techniques were developed and employed in order to fabricate all of these measurement platforms. Additionally, one of these methodological approaches was used to successfully isolate a few layer graphene.
Date: August 2011
Creator: Lee, Kyung-Min

A New Approach for Transition Metal Free Magnetic Sic: Defect Induced Magnetism After Self-ion Implantation

Description: SiC has become an attractive wide bandgap semiconductor due to its unique physical and electronic properties and is widely used in high temperature, high frequency, high power and radiation resistant applications. SiC has been used as an alternative to Si in harsh environments such as in the oil industry, nuclear power systems, aeronautical, and space applications. SiC is also known for its polytypism and among them 3C-SiC, 4H-SiC and 6H-SiC are the most common polytypes used for research purposes. Among these polytypes 4H-SiC is gaining importance due to its easy commercial availability with a large bandgap of 3.26 eV at room temperature. Controlled creation of defects in materials is an approach to modify the electronic properties in a way that new functionality may result. SiC is a promising candidate for defect-induced magnetism on which spintronic devices could be developed. The defects considered are of room temperature stable vacancy types, eliminating the need for magnetic impurities, which easily diffuse at room temperature. Impurity free vacancy type defects can be created by implanting the host atoms of silicon or carbon. The implantation fluence determines the defect density, which is a critical parameter for defect induced magnetism. Therefore, we have studied the influence of low fluence low energy silicon and carbon implantation on the creation of defects in n-type 4H-SiC. The characterization of the defects in these implanted samples was performed using the techniques, RBS-channeling and Raman spectroscopy. We have also utilized these characterization techniques to analyze defects created in much deeper layers of the SiC due to implantation of high energy nitrogen ions. The experimentally determined depths of the Si damage peaks due to low energy (60 keV) Si and C ions with low fluences (< 1015 cm-2) are consistent with the SRIM-2011 simulations. From RBS-C Si sub-lattice measurements for different fluences ...
Date: May 2013
Creator: Kummari, Venkata Chandra Sekhar

A Non-equilibrium Approach to Scale Free Networks

Description: Many processes and systems in nature and society can be characterized as large numbers of discrete elements that are (usually non-uniformly) interrelated. These networks were long thought to be random, but in the late 1990s, Barabási and Albert found that an underlying structure did in fact exist in many natural and technological networks that are now referred to as scale free. Since then, researchers have gained a much deeper understanding of this particular form of complexity, largely by combining graph theory, statistical physics, and advances in computing technology. This dissertation focuses on out-of-equilibrium dynamic processes as they unfold on these complex networks. Diffusion in networks of non-interacting nodes is shown to be temporally complex, while equilibrium is represented by a stable state with Poissonian fluctuations. Scale free networks achieve equilibrium very quickly compared to regular networks, and the most efficient are those with the lowest inverse power law exponent. Temporally complex diffusion also occurs in networks with interacting nodes under a cooperative decision-making model. At a critical value of the cooperation parameter, the most efficient scale free network achieves consensus almost as quickly as the equivalent all-to-all network. This finding suggests that the ubiquity of scale free networks in nature is due to Zipf's principle of least effort. It also suggests that an efficient scale free network structure may be optimal for real networks that require high connectivity but are hampered by high link costs.
Date: August 2012
Creator: Hollingshad, Nicholas W.

Nonlinear and Quantum Optics Near Nanoparticles

Description: 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 ...
Date: December 2015
Creator: Dhayal, Suman

Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization

Description: 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 ...
Date: December 2013
Creator: Elmarhoumi, Nader M.

Quantum Coherent Control and Propagation in Lambda System

Description: 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.
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Date: May 2016
Creator: Singh, Pooja

A Search for Periodic and Quasi-Periodic Patterns in Select Proxy Data with a Goal to Understanding Temperature Variation

Description: 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 ...
Date: May 2016
Creator: Otto, James

Sputtering of Bi and Preferential Sputtering of an Inhomogeneous Alloy

Description: Angular distributions and total yields of atoms sputtered from bismuth targets by normally incident 10 keV -50 keV Ne+ and Ar+ ions have been measured both experimentally and by computer simulation. Polycrystalline Bi targets were used for experimental measurements. The sputtered atoms were collected on high purity aluminum foils under ultra-high vacuum conditions, and were subsequently analyzed using Rutherford backscattering spectroscopy. The Monte-Carlo based SRIM code was employed to simulate angular distributions of sputtered Bi atoms and total sputtering yields of Bi to compare with experiment. The measured sputtering yields were found to increase with increasing projectile energy for normally incident 10 keV - 50 keV Ne+ and Ar+ ions. The shapes of the angular distributions of sputtered Bi atoms demonstrated good agreement between experiment and simulation in the present study. The measured and simulated angular distributions of sputtered Bi exhibited an over-cosine tendency. The measured value of the degree of this over-cosine nature was observed to increase with increasing incident Ne+ ion energy, but was not strongly dependent on incident Ar+ ion energy. The differential angular sputtering yield and partial sputtering yields due to Ar ion bombardment of an inhomogeneous liquid Bi:Ga alloy have been investigated, both experimentally and by computer simulation. Normally incident 25 keV and 50 keV beams of Ar+ were used to sputter a target of 99.8 at% Ga and 0.2 at% Bi held at 40° C in ultra-high vacuum (UHV), under which conditions the alloy is known to exhibit extreme Gibbsian surface segregation that produces essentially a monolayer of Bi atop the bulk liquid. Angular distributions of sputtered neutrals and partial sputtering yields obtained from the conversion of areal densities of Bi and Ga atoms on collector foils were determined. The Monte-Carlo based SRIM code was employed to simulate the experiment and obtain the angular ...
Date: December 2014
Creator: Deoli, Naresh T.