# Alumni Dissertations and Theses

• ### Phase locking of solid-state laser arrays

Author:
LIPING LIU
Year of Dissertation:
2010
Program:
Physics
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
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
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.

• ### Proton Pumping in Cytochrome c Oxidase

Author:
Jianxun Lu
Year of Dissertation:
2015
Program:
Physics
Marilyn Gunner
Abstract:

Cytochrome c oxidase (CcO) is a large trans-membrane protein, which is the final enzyme in the respiratory electron transport chain in mitochondria or aerobic bacte- ria. It implements proton pumping through the mitochondrial membrane against the electrochemical gradient, by utilizing the chemical energy released by reducing O2 to water. The active site of the chemical reaction is called the Binuclear Center (BNC) that is made up of heme a3, CuB, a Tyrosine residue and their ligands. The protein is reduced four times by electron from cytochromes c to reduce O2 and to generate four different BNC redox states step by step. In each reduction step a proton is delivered to the BNC and another proton is pumped across the protein to increase the trans-membrane proton gradient. In CcO, the pumped proton is firstly located in the proton loading site (PLS), and then is released out of the protein. In these processes, a high conserved Glutamate residue, plays an essential role on the proton translocation either to the BNC or the PLS. In this thesis, Multi-Conformational Continuum Electrostatics (MCCE) and Molecular Dynamics (MD) are combined to study the proton affinity (pKa) of the high conserved Glutamate residue and the identity of the PLS. This Glutamate residue is located in a hydrophobic cavity in the protein, and the simulations show that the hydration of the cavity is controlled by the protonation state of the propionic acid of heme a3, a group on the proton outlet pathway. The changes in hydration and elec- trostatic interactions lower the proton affinity by at least 5 kcal/mol. The identity of the residues in the PLS is another open question in CcO research, and various groups above the BNC have been considered as candidates. We designed a new model for the simulation via separating the catalytic cycle into smaller substates and monitoring the charge of all residues in the protein. The results demonstrates the PLS is a cluster rather than a single residue, and the proton affinity of the heme a3 propionic acids primarily determines the number of protons loaded into the PLS.

• ### Exploring Non-Equilibrium Dynamics in Time Dependent Density Functional Theory

Author:
Kai Luo
Year of Dissertation:
2015
Program:
Physics
Neepa Maitra
Abstract:

Time-dependent density functional theory(TDDFT) is a method of choice for calulations of excitation spectra and response properties in materials science and quantum chemistry. The many-body problem is mapped into a set of one-body Schr\"odinger equations, called the Kohn-Sham(KS) equations. In principle, the one-body potential can be chosen such that the density of the interacting system is exactly reproduced by the KS system. However, one component of the one-body potential has to be approximated and is typically adiabatic". Though in linear response regime adiabatic approximations give quite good spectra, it is important to explore their performances in non-equilibrium dynamics. \noindent In this thesis, I will present the results of the explorations on non-equilibrium dynamics in TDDFT. For the first study, a decomposition of exact exchange-correlation potential into kinetic and interaction components is derived. We compare the components with that of adiabatic" counterparts in non-perturbative dynamics and find that the interaction component is less poorly approximated adiabatically than the kinetic component. A salient feature is that step structures generically appear, of relevance in the second study. We prove that the step structures only appear in the non-linear response regime. We find an exact condition which is typically violated by the approximations in use today. Spuriously time-dependent spectra in TDDFT can be explained and we find that the more the condition is violated the worse the dynamics is. In last, we envision that orbital functionals are able to incorporate the memory effects and compensate the deficiencies of the adiabatic" approximations.

• ### Metasurfaces for photon sorting and selective absorption

Author:
Isroel Mandel
Year of Dissertation:
2015
Program:
Physics
David Crouse
Abstract:

Metamaterials are a recent discovery gaining much interest due to their promising applications to multiple devices in sensing, imaging, photovoltaics, nonlinear optics, heat conversion, sorters, and multitudes of other devices. These metamaterials are made of subunits called meta-atoms which take a role similar to that of atoms in bulk crystals. However, unlike their atom counterparts, these meta-atoms are macroscopic and can be engineered to respond to a driving field in a desired way. Metasurfaces, the 2-dimensional analog of metamaterials, have been shown to possess the ability to control light in novel ways. In this work, we investigate a particular type of metasurface namely a cavity array metasurface which consists of a metal film with an array of apertures which form the meta-atoms. We will discuss methods for using such metasurfaces to develop innovative forms of photon sorting and frequency selective absorption. The metasurface devices presented illustrate how, by designing the cavity meta-atoms, various desired global properties can be achieved. Among the devices we will demonstrate are a novel polarization sensing pixel implementing a 1-dimensional polarization sorting metasurface, a Stokes parameter sensor device implementing a novel 2-dimensional cavity array metasurface, a 2-dimensional perfect absorbing metasurface with subwavelength photon sorting in the microwave, a 2-dimensional transmitting metasurface with subwavelength photon sorting in the near-IR, and an actively tunable frequency selective perfect absorber using two 2-dimensional metasurface.

• ### Magnetic deflagration in the molecular magnet Mn12-ac

Author:
Sean McHugh
Year of Dissertation:
2009
Program:
Physics
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.

• ### Solid State NMR Studies of Materials for Energy Technology

Author:
Chandana Nambukara Kodiweera Arachchilage
Year of Dissertation:
2010
Program:
Physics
Steve Greenbaum
Abstract:

Abstract SOLID STATE NMR STUDIES OF MATERIALS FOR ENERGY TECHNOLOGY by Chandana K. Nambukara Kodiweera Arachchilage Adviser: Professor Steve Greenbaum Presented in this thesis are NMR investigations of the dynamical and structural properties of materials for energy conversion and storage devices. 1H and 2H NMR was used to study water and methanol transportation in sulfonated poly(arylene ether ketone) based membranes for direct methanol fuel cells (DMFC). These results are presented in chapter 3. The amount of liquid in the membrane and ion exchange capacity (IEC) are two main factors that govern the dynamics in these membranes. Water and methanol diffusion coefficients also are comparable. Chapters 4 and 5 are concerned with 31P and 1H NMR in phosphoric acid doped PBI membranes (para-PBI and 2OH-PBI) as well as PBI membranes containing ionic liquids (H3PO4/PMIH2PO4/PBI). These membranes are designed for higher-temperature fuel cell operation. In general, stronger short and long range interactions were observed in the 2OH-PBI matrix, yielding reduced proton transport compared to that of para-PBI. In the case of H3PO4/PMIH2PO4/PBI, both conductivity and diffusion are higher for the sample with molar ratio 2/4/1. Finally, chapter 6 is devoted to the 31P NMR MAS study of phosphorus-containing structural groups on the surfaces of micro/mesoporous activated carbons. Two spectral features were observed and the narrow feature identifies surface phosphates while the broad component identifies heterogeneous subsurface phosphorus environments including phosphate and more complex structure multiple P-C, P-N and P=N bonds.

• ### Nuclear Magnetic Resonance Studies on Lithium and Sodium Electrode Materials for Rechargeable Batteries

Author:
Tetiana Nosach
Year of Dissertation:
2014
Program:
Physics
Steve Greenbaum
Abstract:

In this thesis, Nuclear Magnetic Resonance (NMR) spectroscopic techniques are used to study lithium and sodium electrode materials for advanced rechargeable batteries. Three projects are described in this thesis. The first two projects involve 6Li, 7Li and 31P NMR studies of two cathode materials for advanced rechargeable batteries. The third project is a study of sodium titanate cathode materials for Na-ion batteries, where 1H, 7Li, and 23Na static and magic angle spinning NMR were used in order to obtain detailed information on the chemical environments.

• ### Quantum Rotational Effects in Nanomagnetic Systems

Author:
Michael O'Keeffe
Year of Dissertation:
2013
Program:
Physics
Eugene Chudnovsky
Abstract:

Quantum tunneling of the magnetic moment in a nanomagnet must conserve the total angular momentum. For a nanomagnet embedded in a rigid body, reversal of the magnetic moment will cause the body to rotate as a whole. When embedded in an elastic environment, tunneling of the magnetic moment will cause local elastic twists of the crystal structure. In this thesis, I will present a theoretical study of the interplay between magnetization and rotations in a variety of nanomagnetic systems which have some degree of rotational freedom. We investigate the effect of rotational freedom on the tunnel splitting of a nanomagnet which is free to rotate about its easy axis. Calculating the exact instanton of the coupled equations of motion shows that mechanical freedom of the particle renormalizes the easy axis anisotropy, increasing the tunnel splitting. To understand magnetization dynamics in free particles, we study a quantum mechanical model of a tunneling spin embedded in a rigid rotor. The exact energy levels for a symmetric rotor exhibit first and second order quantum phase transitions between states with different values the magnetic moment. A quantum phase diagram is obtained in which the magnetic moment depends strongly on the moments of inertia. An intrinsic contribution to decoherence of current oscillations of a flux qubit must come from the angular momentum it transfers to the surrounding body. Within exactly solvable models of a qubit embedded in a rigid body and an elastic medium, we show that slow decoherence is permitted if the solid is macroscopically large. The spin-boson model is one of the simplest representations of a two-level system interacting with a quantum harmonic oscillator, yet has eluded a closed-form solution. I investigate some possible approaches to understanding its spectrum. The Landau-Zener dynamics of a tunneling spin coupled to a torsional resonator show that for certain parameter ranges the system exhibits multiple Landau-Zener transitions. These transitions coincide in time with changes in the oscillator dynamics. A large number of spins on a single oscillator coupled only through the in-phase oscillations behaves as a single large spin, greatly enhancing the spin-phonon coupling.