Alumni Dissertations and Theses

 
 

Alumni Dissertations and Theses

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  • NMR Studies of Ionic Liquids and Polymer Membrane Electrolytes for Batteries and Fuel Cells

    Author:
    SUFIA KHATUN
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Steve Greenbaum
    Abstract:

    In this thesis, NMR (Nuclear Magnetic Resonance) spectroscopic techniques are used to study ionic liquids (ILs) and polymer membranes for advanced Li-ion batteries and fuel cells. Besides structural measurements, dynamics and motional properties were also measured using pulsed field gradient spin echo and relaxation experiments. Five projects are described in this thesis. The first two projects involve 1H, 19F, 7Li and 13C NMR transport studies of two different ILs. The third project is a study of Nafion ionomer aggregations where diffusion and viscosity measurements are made. The fourth project involves the structural study of lithium triflate:PEO3 polymer membranes via 7Li static NMR. The fifth and final project described in the thesis involves 27Al NMR studies of a chloroaluminate (AlCl3) IL and the assessment of the tetrahedral Al species content.

  • PHASE LOCKING IN FIBER LASER ARRAYS

    Author:
    FANTING KONG
    Year of Dissertation:
    2010
    Program:
    Physics
    Advisor:
    Ying-Chih Chen
    Abstract:

    This dissertation reports a series of studies on phase locking in two-element laser arrays, with an emphasis on fiber laser arrays, and an application of Q-switched microchip laser to high-resolution photoacoustic imaging. Phase locking is achieved by coupling the lasing elements to a common Fourier-transform resonator, in which the lasing elements and the output mirror are positioned in the focal planes of a converging lens so that the far-field profiles of the laser elements are projected to the output mirror through Fourier transformation. Since the far-field profiles generally have simpler and more symmetric structures, the relative phase of the lasing elements can be selected by placing a simple spatial filter on the output mirror. We have studied phase locking in fiber lasers operating in the continuous-wave mode and in stimulated-Brillouin-scattering (SBS) Q-switched mode, and in Q-switched microchip laser arrays formed in a single crystal. These systems represent vastly different parameters which can affect the development of phase locking. We have found that the continuous-wave fiber lasers can always be phase locked and the relative phase remains stable despite random phase fluctuations in individual fibers. This is attributed to the combination of broad bandwidths of the fiber gain media and small frequency spacing of the longitudinal mode which allows resonance frequencies of the composite resonator of the laser array to be found under all circumstances. In short-pulse laser arrays, phase locking can be realized only when the fiber lengths are nearly equal in the SBS Q-switched fiber lasers, or the frequency mismatch is less than the bandwidth of the laser pulse in the microchip laser array. In the latter case, the boundary of phase-locked and unlocked states is characterized by partial coherence in the combined laser beam due to the pulses from the individual elements not perfectly overlapping in time. Photoacoustic images are constructed based on the ultrasound signals generated when a tissue undergoes thermal expansion after laser pulses are absorbed by chromophores in the tissue. The use of focused laser pulse and high-frequency ultrasound has led to much higher image resolution than obtainable with conventional pulse-echo ultrasound. The ability to identify chemical compositions in tissues based on their distinct wavelength-dependent optical absorption also leads to new capabilities in diagnostic imaging.

  • MODELING OF ULTRA-SHORT SOLITON PROPAGATION IN DETERMINISTIC AND STOCHASTIC NONLINEAR CUBIC MEDIA

    Author:
    Levent Kurt
    Year of Dissertation:
    2011
    Program:
    Physics
    Advisor:
    Sultan Catto
    Abstract:

    We study the short pulse dynamics in the deterministic and stochastic environment in this thesis. The integrable short pulse equation is a modelling equation for ultra-short pulse propagation in the infrared range in the optical fibers. We investigate the numerical proof for the exact solitary solution of the short pulse equation. Moreover, we demonstate that the short pulse solitons approximate the solution of the Maxwell equation numerically. Our numerical experiments prove the particle-like behaviour of the short pulse solitons. Furthermore, we derive a short pulse equation in the higher order. A stochastic counterpart of the short pulse equation is also derived through the use of the multiple scale expansion method for more realistic situations where stochastic perturbations in the dispersion are present. We numerically show that the short pulse solitary waves persist even in the presence of the randomness. The numerical schemes developed demonstrate that the statistics of the coarse-graining noise of the short pulse equation over the slow scale, and the microscopic noise of the nonlinear wave equation over the fast scale, agree to fairly good accuracy.

  • Aperture Array Photonic Metamaterials: Theoretical approaches, numerical techniques and a novel application

    Author:
    Eli Lansey
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    David Crouse
    Abstract:

    Optical or photonic metamaterials that operate in the infrared and visible frequency regimes show tremendous promise for solving problems in renewable energy, infrared imaging, and telecommunications. However, many of the theoretical and simulation techniques used at lower frequencies are not applicable to this higher-frequency regime. Furthermore, technological and financial limitations of photonic metamaterial fabrication increases the importance of reliable theoretical models and computational techniques for predicting the optical response of photonic metamaterials. This thesis focuses on aperture array metamaterials. That is, a rectangular, circular, or other shaped cavity or hole embedded in, or penetrating through a metal film. The research in the first portion of this dissertation reflects our interest in developing a fundamental, theoretical understanding of the behavior of light's interaction with these aperture arrays, specifically regarding enhanced optical transmission. We develop an approximate boundary condition for metals at optical frequencies, and a comprehensive, analytical explanation of the physics underlying this effect. These theoretical analyses are augmented by computational techniques in the second portion of this thesis, used both for verification of the theoretical work, and solving more complicated structures. Finally, the last portion of this thesis discusses the results from designing, fabricating and characterizing a light-splitting metamaterial.

  • Usefulness of Nuclear Magnetic Resonance in the Study of a Variety of Battery Systems and Materials

    Author:
    Nicole Leifer
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Steve Greenbaum
    Abstract:

    The usefulness of solid state Nuclear Magnetic Resonance (NMR) spectroscopy in the analysis of lithium ion batteries is presented. Some background information on lithium batteries is given, in addition to a summary of current research areas. A comprehensive review of the use of NMR and Electron Paramagnetic Resonance (EPR) in lithium batteries research thus far is also presented. The electrodes studied were the standard LiCoO2 cathode cycled against mesocarbon microbead (MCMB) anodes, as well as Li2Ag2V4O11 and CFx cathodes cycled against metallic lithium anodes in primary batteries. The focus of half of the work concerns the elucidation of the Solid Electrolyte Interphase (SEI), an irreversibly formed side-product found on the electrode surfaces, composed mainly from the electrolyte components; one study provides a deeper insight into the inorganic components of the SEI, while the other SEI study focuses on the organic components via 13C MAS NMR studies of cycled electrodes. The other half is comprised of two additional studies in which atomic and electronic rearrangement are monitored in the electrodes at different stages of the battery cycling process.

  • A study of Optically Pumped Nuclear Magnetic Polarization on Gallium Arsenide

    Author:
    Yunpu Li
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Carlos Meriles
    Abstract:

    This thesis aims to study the dynamic process of optically pumped nuclear spin polarization on Gallium Arsenide. First of all, the time-resolved optical Faraday rotation is applied to observe the electron spin dynamics in the presence of the nuclear magnetic field. And then the optically-pumped NMR is measured on different parameters, including the dependences of light helicity, irradiation intensity, photon energy, illumination time and temperature. We report a new phenomenology at low irradiation intensity. A nuclear polarization model combining hyperfine and quadrupolar relaxation is developed, with experimental data supported. By exploiting the two competing mechanism on various photon energies, illumination intensity and NMR pulse sequences, we use high field stray-field NMR imaging to realize all-optical creation of three-dimensional patterning of positive and negative nuclear polarization on the micron length scale. Finally, the effect of nuclear spin diffusion effect is investigated. We demonstrate in a remarkable way the unambiguous evidence of diffusion, which results in a great enhancement of quadrupolar relaxation in the nanometer scale.

  • SYSTEMATIC CONTROL OF SCHOTTKY BARRIER HEIGHT BY PARTISAN INTERLAYERS

    Author:
    Yang Li
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Raymond Tung
    Abstract:

    The relationship between the starting surface structure and the Schottky barrier height (SBH) in metal-silicon systems has been investigated to explore the possibility of modifying the interface dipole through the insertion of an inorganic interface layer. A systematic and comprehensible way to perform this modification has been introduced as a "partisan interlayer" method and was extensively studied for a variety of interlayer elements, along with several choices for the metal. Employing elements with a larger electronegativity than that of silicon, a monolayer of As, S, or Cl was deposited on Si surfaces and processed to form stable surface structures. The electron affinities of these surfaces were measured by Kelvin probe and found to increase significantly from the clean surface, consistent with the expected charge transfer from Si to the adsorbates and also in agreement with results of ab initio density functional theory calculations. Subsequent deposition of metal on these adsorbate terminated semiconductor (ATS) surfaces led to the fabrication of metastable interface structures with the SBH successfully and significantly modified in a predictable manner from the clean Si results. The chemical stability of these surfaces that weakens the interaction with the deposited metal, likely leads to the preservation of electric dipole from such partisan interlayers. The partisan interlayer method was found to work particularly well with As-terminated Si(111) surface, on which a interface behavior parameter exceeding 0.50 was found. This exceeded the S-parameter usually observed for covalent semiconductors such as Si, ~ 0.1, and highlighted a major reason for the adjustability of the SBH by the PI method. The SBH of all interfaces studied in this work was inhomogeneous. Making use of the theory of electronic transport through inhomogeneous SBH and temperature-dependent measurements, the extent of the SBH nonuniformity was routinely characterized from the Schottky diodes. The largest adjustment in the SBH was observed for Au on S-terminated Si(100), where the n-type junction became nearly perfectly ohmic. It was demonstrated, for the first time, that quantitative information on the distribution of the SBH and the lateral size of conduction patches ("hot spots") could still be obtained from ohmic junctions. The physical basis for these analyses and the special experimental conditions which enabled these analyses were carefully explained. The implications of these results for SBH control of MS systems in general and the understanding of the formation of SBH in general are also discussed.

  • Strongly-correlated 2D Electron Systems in Si-MOSFETs

    Author:
    Shiqi Li
    Year of Dissertation:
    2015
    Program:
    Physics
    Advisor:
    Myriam Sarachik
    Abstract:

    Si-MOSFETs are basic building blocks of present-day integrated circuits. Above a threshold gate voltage, a layer of two-dimensional electrons is induced near the silicon-silicon dioxide interface of a Si-MOSFET. According to theory for noninteracting and weakly interacting electrons, no metallic state can exist in two dimensions in zero magnetic field in the limit of zero temperature. However, in strongly interacting electron systems the observation of a resistivity that changes from metallic to insulating temperature dependence has fueled a debate over whether this signals a quantum phase transition to a metallic phase in two dimensions. In this thesis I will present the results of two detailed experimental studies performed on high mobility Si-MOSFET samples. In the first study, we find the thermopower of this low-disorder, strongly interacting 2D electron system in silicon diverges at a finite disorder-independent density, providing evidence that this IS a transition to a new phase at low densities. For the second study, we conducted measurements on I-V characteristics as well as the AC voltage generated by the sample in the insulating phase. Nonlinear I-V characteristics observed in the insulating phase have been attributed to the presence of an additional conduction channel due to a sliding electron solid (Wigner crystal). We seek to provide evidence for the presence of a zero-field Wigner solid by detecting the noise generated by the sliding crystallites.

  • Spontaneous Time-Reversal Symmetry Breaking in Two Dimensional Electronic Systems

    Author:
    Wei Liu
    Year of Dissertation:
    2014
    Program:
    Physics
    Advisor:
    Alexander Punnoose
    Abstract:

    The discovery of high temperature superconductivity inspired a number of novel proposals, one of which, put forward by C.M.Varma, involves the breaking of time-reversal symmetry to explain the physics of the underdoped pseudogap phase. It was proposed that time-reversal symmetry is spontaneously broken as a result of strong repulsion between the Cu-O electrons to form loop-currents in the system. In this work, we developed a general theory to study the quantum phase transitions in the 2 dimensional strongly interacting electronic systems in which time-reversal symmetry is spontaneously broken in the ground state. We first applied the theory of magnetic groups to identify electronic current-loop patterns in two physically relevant systems: (i) 2-band model involving spinless electrons on a honeycomb lattice with next-nearest-neighbor interactions; (ii) 3-band $CuO_{2}$ model with and without lattice distortions. Next, by examining the correlation function within the standard ring and ladder Dyson series approximations, we identify the effective Hamiltonian with the relevant interactions responsible for creating low-energy fluctuations near the quantum critical point. The mean-field analysis of this effective Hamiltonian elucidated the fact that time-reversal symmetry breaking in a 2-band model is in the same universality class as the interband particle-hole pair condensation instability which occurs in the semi-conductors under large enough particle-hole attraction. Using Hubbard-Stratonovich transformation and functional integral method, we are able to investigate this instability and the static susceptibility in the condensed phase both in half-filling and doped case. Away from half-filling, because the condensates are metallic and couple to the gapless collective intraband particle-hole excitations, we find that the static susceptibility is generically negative as a result of this coupling, which implies that the condensates are unstable.

  • Phase locking of solid-state laser arrays

    Author:
    LIPING LIU
    Year of Dissertation:
    2010
    Program:
    Physics
    Advisor:
    Ying-Chih Chen
    Abstract:

    This thesis reports a study of phase locking in solid-state laser arrays of a variety of configurations, including a 2x2 Nd:YVO4 continuous-wave laser array, a two-element passively Q-switched Nd,Cr:YAG laser array, a two-element continuous-wave ytterbium fiber laser array, and a two-element ytterbium fiber lasers passively Q-switched by stimulated Brillouin scattering. Phase locking is accomplished by coupling the lasing elements into a common Fourier-transform resonator, with the lasing elements placed at one focal plane of a converging lens and the output mirror placed at the other focal plane. The control of the relative phase among the elements is done by placing a spatial filter in front of the output mirror to introduce different modal losses. We have succeeded in achieving highly stable phase-locked operation in the in-phase mode in continuous-wave laser arrays. The fringe visibility of the phase-locked beam is nearly 1. As the coupling strength decreases, the transition from phase locked to unlocked mode is abrupt. Phase locking of nano-second pulsed laser arrays requires a tight control of the resonance frequencies and path lengths of the individual lasing elements to ensure the pulses generated by the individual elements to occur simultaneously. As the difference in path lengths increase or the coupling strengths decrease, the transition from the phase locked to the unlocked states is characterized by a gradual loss of coincidence of the pulses from the individual elements and a reduction in the fringe contrast in the combined laser beam. The current approach of phase locking has high efficiency and is applicable to two-dimensional laser arrays containing a large number of elements.