Alumni Dissertations

 

Alumni Dissertations

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  • 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.

  • Infrared and Raman Spectroscopy Study of Layered Systems

    Author:
    Jian Li
    Year of Dissertation:
    2013
    Program:
    Physics
    Advisor:
    Jiufeng Tu
    Abstract:

    Optical spectroscopy studies the interaction between light (photon) and matter. During such interaction, different processes such as reflection, transmission, scattering, absorption or fluorescence can occur. Among all the optical spectroscopic techniques, infrared (IR) and Raman spectroscopy are most commonly used. In an Infrared process, photons are absorbed. The required light source emits polychromatic Infrared light and when it passes through or being reflected by the sample the light is partially absorbed. The frequency dependent absorption allows one to study the electronic and vibrational structure of the sample. On the other hand, the Raman spectroscopy is second order in nature where the photon is scattered instead of being absorbed. A monochromatic light source is used instead of a continuous spectrum. Generally, the dominate effect in an optical process is absorption and transmission but a (small) portion of photons are scattered. A small fraction of photons change their energy/wavelength during the scattering. Depending on the scale of the change in energy, those inelastic scatterings can be categorized into Brillouin scattering and Raman scattering. Although sharing the same mechanism, different energy scale require completely different experimental setups for Brillouin scattering and Raman scattering. In the study of infrared and Raman spectroscopy, group theory is a very helpful tool. The calculation of absolute intensity of an optical transition is rather difficult and sometimes infeasible, especially for crystal vibrations. Group theory is the mathematical language that describes the symmetry property of the physical system. Selection rules based on symmetry consideration had been predicted. Group theory, especially representation theory, is an important branch of condensed matter physics. Both theoretical and experimental results of my PhD research are presented. The topics being covered are: infrared study of iron based superconductor BaFe$_{1.85}$Co$_{0.15}$As$_{2}$; the study of Raman scattering with Laguerre-Gaussian (LG) beam.

  • 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.

  • Systematic Tuning of Silicon Schottky Barrier Height by Atomic Interlayers with Low Electronegativities

    Author:
    Wei Long
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Raymond Tung
    Abstract:

    The Schottky barrier height (SBH) is of great importance to the functionality of semiconductor devices, as it governs the carrier transport across the metal-semiconductor (MS) interface. The presence of the Fermi level (FL) pinning phenomena makes tuning the SBH a difficult goal to achieve. The technique of "partisan interlayer" (PI) was proposed recently to modify the SBH, where stable adsorbate-terminated semiconductor (ATS) surfaces were used to form SBs with subsequently applied metal. When elements with large electronegativities were used to form the ATS, the PI technique was effective in reducing the n-type SBH and increasing the p-type SBH, driven by the expected transfer of charge from the semiconductor to the adsorbates. In this thesis work, elements with electronegativities smaller than that of the semiconductor are used as surface termination. SBHs for Ag, Au and In on Si surfaces are found to increase for the n-type and decrease for the p-type interfaces, by as much as 0.25eV, when Ga, Mg and K are used to terminate the Si surfaces. The present results are thus in agreement with the expected charge transfers from elements with smaller electronegativities to silicon and illustrate the general validity of the PI technique. The chemical stability of these surfaces likely weakens the MS interaction and leads to the (partial) preservation of the surface dipole at the MS interface. However, large degrees of SBH inhomogeneity are observed for diodes on these surfaces, likely due to insufficient stability of these surfaces to completely withstand metal interaction. These results are discussed within the basic models of SBH formation and the implications of these results for SBH control of MS systems are also addressed.

  • Magnetic deflagration in the molecular magnet Mn12-ac

    Author:
    Sean McHugh
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Myriam Sarachik
    Abstract:

    In 1995, Paulsen and Park observed abrupt spontaneous reversals of the magnetization in crystals of the molecular magnet Mn12-ac, which they dubbed ``magnetic avalanches". They suggested that the magnetic avalanches were a thermal runaway process where the reversing spins release heat stimulating further relaxation. Various exotic phenomena were proposed as an alternative explanations. In 2005, Suzuki et al. established that this spontaneous magnetic relaxation occurs as a ``front" separating regions of opposing magnetization that propagates at a constant speed through the crystal. They suggested that this propagating front is analogous to a flame in chemical deflagration and introduced the thermal relaxation process, magnetic deflagration. The analysis presented there was limited by lack of data. A more thorough comparison with the theory would require the ability to trigger avalanches in a more controlled way rather than wait for their spontaneous occurrence. The work presented in this thesis is a continuation of the program initiated by Suzuki. Significant progress experimental progress has been made allowing us to trigger avalanches over a wide range of conditions. The magnetization dynamics and the ignition temperatures are studied in detail using an array of micro-sized Hall sensors and Germanium thermometers. In addition, we report the existence of a new species of avalanches consisting only of the fast-relaxing isomers of Mn12-ac, the so-called ``minor species". We explore avalanches of both species, as well as the interaction between them. Finally, a detailed analysis is performed to compare the experiment with the theory of magnetic deflagration. We find the theory of magnetic deflagration to be consistent with the data and extract values for the key physical quantities: the thermal diffusivity and avalanche front temperatures. Agreement between our predicted values and an independent measurement of these quantities would provide compelling verification of the theory.