Alumni Dissertations and Theses

 
 

Alumni Dissertations and Theses

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  • 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.

  • Control of Exciton Photon Coupling in Nano-structures

    Author:
    Xiaoze Liu
    Year of Dissertation:
    2014
    Program:
    Physics
    Advisor:
    Vinod Menon
    Abstract:

    In this thesis, we study the interaction of excitons with photons and plasmons and methods to control and enhance this interaction. This study is categorized in three parts: light-matter interaction in microcavity structures, direct dipole-dipole interactions, and plasmon-exciton interaction in metal-semiconductor systems. In the microcavity structures, the light-matter interactions become significant when the excitonic energy is in resonance with microcavity photons. New hybrid quantum states named polariton states will be formed if the strong coupling regime is achieved, where the interaction rate is faster than the average decay rate of the excitons and photons. Polaritons have been investigated in zinc oxide (ZnO) nanoparticles based microcavity at room temperature and stimulated emission of the polaritons has also been observed with a low optical pump threshold. Exictons in organic semiconductors (modeled as Frenkel excitons) are tightly bound to molecular sites, and differ considerably from loosely bound hydrogen atom-like inorganic excitons (modeled as Wannier-Mott excitons). This fundamental difference results in distinct optoelectronic properties. Not only strongly coupled to Wannier-Mott excitons in ZnO, the microcavity photons have also been observed to be simultaneously coupled to Frenkel excitons in 3,4,7,8-naphthalene tetracarboxylic dianhydride (NTCDA). The photons here act like a glue combining Wannier-Mott and Frenkel excitons into new hybrid polaritons taking the best from both constituents. Two-dimensional (2D) excitons in monolayer transition metal dichalcogenides (TMDs) have emerged as a new and fascinating type of Wannier-Mott-like excitons due to direct bandgap transition, huge oscillator strength and large binding energy. Monolayer molybdenum disulfide (MoS2) has been incorporated into the microcavity structure and 2D exciton-polaritons have been observed for the first time with directional emission in the strong coupling regime. Valley polarization has also been discussed in this MoS2 microcavity for the possible applications in spin switches and logic gates. The direct dipole-dipole type excitonic interactions have also been studied in inorganic-organic nanocomposites, where ZnO nanowire is taken as the inorganic constituent and NTCDA thin films as the organic constituent. The excitonic interactions can be classified into weak coupling regime and strong coupling regime. Forster Resonant Energy Transfer (FRET), which is in the weak coupling regime, has been observed in this hybrid system. The optimized optical nonlinearity has also been determined in the hybrid system via Z-scan measurements. Exciton-plasmon polariton, another example of strongly coupled state which results from the interaction between excitons and plasmons when they are in resonance, has also been investigated in this thesis. Two rhodamine dyes spincoated on the silver thin films have separately been observed to be strongly coupled to the surface plasmon modes. With observed new polariton states, energy transfer mechanism has been discussed for nonlinear optical applications.

  • 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.