Now showing items 15267-15286 of 20306

    • Quantum Control and Quantum Chaos in Atomic Spin Systems

      Jessen, Poul S.; Chaudhury, Souma; Jessen, Poul S.; Cronin, Alexander; Jones, Ronald J. (The University of Arizona., 2008)
      Laser-cooled atoms offer an excellent platform for testing new ideas of quantum control and measurement. I will discuss experiments where we use light and magnetic fields to drive and monitor non-trivial quantum dynamics of a large spin-angular momentum associated with an atomic hyperfine ground state. We can design Hamiltonians to generate arbitrary spin states and perform a full quantum state reconstruction of the results. We have implemented and verified time optimal controls to generate a broad variety of spin states, including spin-squeezed states useful for metrology. Yields achieved are of the range 0.8-0.9.We present a first experimental demonstration of the quantum kicked top, a popular paradigm for quantum and classical chaos. We make `movies' of the evolving quantum state which provides a direct observation of phase space dynamics of this system. The spin dynamics seen in the experiment includes dynamical tunneling between regular islands, rapid spreading of states throughout the chaotic sea, and surprisingly robust signatures of classical phase space structures. Our data show differences between regular and chaotic dynamics in the sensitivity to perturbations of the quantum kicked top Hamiltonian and in the average electron-nuclear spin entanglement during the first 40 kicks. The difference, while clear, is modest due to the small size of the spin.
    • Quantum Control and Quantum Tomography on Neutral Atom Qudits

      Jessen, Poul S.; Sosa Martinez, Hector; Jessen, Poul S.; Sandhu, Arvinder; Anderson, Brian P.; Deutsch, Ivan H. (The University of Arizona., 2016)
      Neutral atom systems are an appealing platform for the development and testing of quantum control and measurement techniques. This dissertation presents experimental investigations of control and measurement tools using as a testbed the 16-dimensional hyperfine manifold associated with the electronic ground state of cesium atoms. On the control side, we present an experimental realization of a protocol to implement robust unitary transformations in the presence of static and dynamic perturbations. We also present an experimental realization of inhomogeneous quantum control. Specifically, we demonstrate our ability to perform two different unitary transformations on atoms that see different light shifts from an optical addressing field. On the measurement side, we present experimental realizations of quantum state and process tomography. The state tomography project encompasses a comprehensive evaluation of several measurement strategies and state estimation algorithms. Our experimental results show that in the presence of experimental imperfections, there is a clear tradeoff between accuracy, efficiency and robustness in the reconstruction. The process tomography project involves an experimental demonstration of efficient reconstruction by using a set of intelligent probe states. Experimental results show that we are able to reconstruct unitary maps in Hilbert spaces with dimension ranging from d=4 to d=16. To the best of our knowledge, this is the first time that a unitary process in d=16 is successfully reconstructed in the laboratory.
    • Quantum Control and Squeezing of Collective Spins

      Jessen, Poul S.; Montaño, Enrique; Jessen, Poul S.; Anderson, Brian P.; Cronin, Alexander D.; Sandhu, Arvinder Singh; Wright, Ewan M. (The University of Arizona., 2015)
      Quantum control of many body atomic spins is often pursued in the context of an atom-light quantum interface, where a quantized light field acts as a "quantum bus" that can be used to entangle distant atoms. One key challenge is to improve the coherence of the atom-light interface and the amount of atom-light entanglement it can generate, given the constraints of working with multilevel atoms and optical fields in a 3D geometry. We have explored new ways to achieve this, through rigorous optimization of the spatial geometry, and through control of the internal atomic state. Our basic setup consists of a quantized probe beam passing through an atom cloud held in a dipole trap, first generating spin-probe entanglement through the Faraday interaction, and then using backaction from a measurement of the probe polarization to squeeze the collective atomic spin. The relevant figure of merit is the metrologically useful spin squeezing determined by the enhancement in the resolution of rotations of the collective spin, relative to the commonly used spin coherent state. With an optimized free-space geometry, and by using a 2-color probe scheme to suppress tensor light shifts, we achieve 3(2) dB of metrologically useful spin squeezing. We can further increase atom-light coupling by implementing internal state control to prepare spin states with larger initial projection noise relative to the spin coherent state. Under the right conditions this increase in projection noise can lead to stronger measurement backaction and increased atom-atom entanglement. With further internal state control the increased atom-atom entanglement can then be mapped to a basis where it corresponds to improved squeezing of, e.g., the physical spin-angular momentum or the collective atomic clock pseudospin. In practice, controlling the collective spin of N ~ 10⁶ atoms in this fashion is an extraordinarily difficult challenge because errors in the control of individual atoms tend to be highly correlated. By employing precise internal state control, we have prepared and detected projection noise limited "cat" states (which have initial projection noise that is larger by a factor of 2f = 8 for Cs relative to the spin coherent state) and estimate that we can generate up to 6.0(5) dB of metrologically useful spin squeezing, demonstrating the advantage of using the internal atomic structure as a resource for ensemble control.
    • Quantum Control in the Full Hyperfine Ground Manifold of Cesium

      Jessen, Poul S.; Smith, Aaron Coleman; Cronin, Alexander D.; Anderson, Brian P.; Leroy, Brian J.; Jessen, Poul S. (The University of Arizona., 2012)
      Cold atomic spins are a great platform for developing and testing control and measurement techniques. This thesis presents experimental investigations into quantum control and measurement using laser cooled cesium atoms. On the control side, we present an experimental realization of a protocol to achieve full controllability of the entire hyperfine ground manifold of cesium. In particular, we demonstrate the ability to map between arbitrary states with fidelity greater than 0.99, using a combination of static, radio frequency, and microwave magnetic fields. On the measurement side, we present an experimental realization of quantum state tomography. The tomography protocol begins by measuring expectation values of an informationally complete set of observables using a weak optical probe in combination with dynamical control. The measurement record is processed using two different state estimation algorithms, allowing us to estimate a quantum state with fidelity greater than 0.9.

      Meystre, Pierre; Chen, Wenzhou; Meystre, Pierre; Shupe, Michael; Jessen, Poul; Anderson, Brian P. (The University of Arizona., 2010)
      This dissertation contains a study of ultracold atoms in optical cavities. We particularly focus on two aspects of the coupled atom-cavity systems. In the first aspect, we implement the quantum nature of the light field to probe the quantum state of the atoms. This is interesting due to the nondestructive nature of the characterization of many-body atomic states. In the second aspect we study the cavity optomechanics that investigates the coupling of mechanical and optical degrees of freedom via radiation pressure. The optomechanical cavity provides an interesting nonlinear system to study the coupling between atoms and the intracavity field.In the context of cavity quantum electrodynamics we study the reflection of two counter-propagating modes of the light field in a high-Q ring cavity by ultracold atoms either in the Mott insulator state or in the superfluid state of an optical lattice. We find that the dynamics of the reflected light strongly depends on both the lattice spacing and the state of the matter-wave field. By using the Monte Carlo wave-function method to account for the cavity damping we also determine the two-time correlation function and the time-dependent physical spectrum of theretroreflected field. We find that the light field and the atoms become entangled if the latter are in a superfluid state. We also analyze quantitatively the entanglement between the atoms and the light.In cavity optomechanics the mechanical effect can either comes from a vibrating macroscopic oscillator or a collective density excitation of a Bose-Einstein condensate. First we use a Fabry-Perot-type cavity to study the opto-mechanically-induced bistable quantum phase transitions between superfluid and a Mott insulator states of an ultracold bosonic gases trapped inside the cavity. Secondly, we study the symmetricand antisymmetric collective density side modes of the BEC which results from the optomechanical effects of the light fields in a ring cavity.
    • Quantum Dot Applications for Detection of Bacteria in Water

      Kuwahara, Sara Sadae; Cuello, Joel; Riley, Mark; Yoon, Jeong-Yeol; Gerba, Charles (The University of Arizona., 2009)
      Semiconductor nanocrystals, otherwise known as Quantum dots (Q dots), are a new type of fluorophore that demonstrates many advantages over conventional organic fluorophores. These advantages offer the opportunity to improve known immunofluorescent methods and immunofluorescent biosensors for rapid and portable bacterial detection in water. The detection of the micro organism Escherichia coli O157:H7 by attenuation of a fluorophore’s signal in water was evaluated alone and in the presence of another bacterial species. A sandwich immunoassay with antibodies adhered to SU-8 as a conventional comparison to our novel attenuation detection was also conducted. The assays were tested for concentration determination as well as investigation for cross reactivity and interference from other bacteria and from partial target cells. In order to immobilize the capture antibodies on SU-8, an existing immobilization self-assembly monolayer (SAM) for glass was modified. The SAM was composed of a silane ((3-Mercaptopropyl) trimethoxysilane (MTS)) and hetero-bifunctional cross linker (N-γ-maleimidobutyryloxy succinimide ester (GMBS)) was utilized in this procedure. The SU-8 surface was activated using various acids baths and oxygenated plasma, and it was determined that the oxygenated plasma yielded the best surface attachment of antibodies. The use of direct fluorophore signal attenuation for detection of the target E. coli resulted in the lowest detectable population of 1x10¹ cfu/mL. It was not specific enough for quantitative assessment of target concentration, but could accurately differentiate between targeted and non-targeted species. The sandwich immunofluorescent detection on SU-8 attained the lowest detectable population of 1x10⁴ cfu/ml. The presence of Klebsiella pneumoniae in solution caused some interference with detection either from cross reactivity or binding site blocking. Partial target cells also caused interference with the detection of the target species, mainly by blocking binding sites so that differences in concentration were not discernable. The signal attenuation not only had a better lowest detectable population but also had less measurement error than the sandwich immunoassay on SU-8 which suffered from non-uniformed surface coverage by the antibodies.
    • Quantum Dots Targeted to VEGFR2 for Molecular Imaging of Colorectal Cancer

      Utzinger, Urs; Carbary, Jordan Leslie; Utzinger, Urs; Barton, Jennifer K.; Romanowski, Marek; Lynch, Ronald (The University of Arizona., 2015)
      Advances in optical imaging have provided methods for visualizing molecular expression in tumors in vivo, allowing the opportunity to study the complexity of the tumor microenvironment. The development of fluorescent contrast agents targeted to molecules expressed in cancer cells is critical for in vivo imaging of the tumors. Contrast agents emitting in the near infrared (NIR) allow for an increased depth of penetration in tissue due to decreased absorption and scattering. There is also significantly less autofluorescence from tissue in the NIR. Quantum dots are nanoscopic particles of semiconductors whose fluorescent emission wavelength is tunable by the size of the particle with desirable fluorescent qualities such as a wide range of excitation wavelengths, a narrow emission band, high quantum efficiency, high photostablility, and they can be produced to emit throughout the NIR imaging window. It has been shown that vascular endothelial growth factor receptor 2 (VEGFR2) is upregulated in many cancers, including colorectal, as it is important in tumor angiogenesis and is considered a predictor for clinical outcome and, in some instances, is used for targeted therapy with anti-angiogenic drugs. For these reasons, quantum dots bioconjugated to VEGFR2 antibodies have the potential to provide contrast between normal tissue and cancer, as well as a mechanism for evaluating the molecular changes associated with cancer in vivo. In this dissertation, we present on the design of two contrast agents using quantum dots targeted to VEGFR2 for use in the molecular imaging of colon cancer, both ex vivo and in vivo. First, as a preliminary ex vivo investigation into their efficacy, Qdot655® (655nm emission) were bioconjugated to anti-VEGFR2 antibodies through streptavidin/biotin linking. The resulting QD655-VEGFR2 contrast agent was used to label colon adenoma in vivo and imaged ex vivo with significant increase in contrast between diseased and undiseased tissue, allowing for fluorescence based visualization of the VEGFR2 expressing diseased areas of the colon with high sensitivity and specificity. Then, QD655-VEGFR2 was used in a longitudinal in vivo study to investigate ability to correlate fluorescence signal to tumor development over time using optical coherence tomography and laser induced fluorescence spectroscopy (OCT/LIF) dual-modality imaging. The contrast agent was able to target VGEFR2 expressing diseased areas of colon; however, challenges in fully flushing the unbound contrast agent from the colon before imaging arise when moving from ex vivo imaging to in vivo image. Lastly, lead sulfide (PbS) quantum dots were made by colloidal synthesis to emit at a 940 nm (QD940) and conjugated to anti-VEGFR2 primary antibodies through streptavidin/biotin linking. The resulting QD940-VEGFR2 contrast agent was then used to label cells in vitro. The QD940-VEGFR2 molecules were able to positively label VEGFR2 expressing cells and did not label VEGFR2 negative cells. Very low photoluminescence and large amounts of aggregation after conjugation of the quantum dot to streptavidin was detected. Improvements to the quantum dot stability through synthesis, capping and conjugation techniques must be made for this contrast agent to be effective as a contrast agent for cancer imaging.
    • Quantum induction and Higgs mass

      Just, Kurt W.; Kwong, Kam-Yuen (The University of Arizona., 2001)
      With our newly proposed dynamical Higgs mechanism and Quantum Induction programme, Higgs mass is predicted at M(H) ≈ 190 GeV by using our modified renormalization group equations. The same procedure also explains the top quark mass correctly.
    • Quantum Information Science with Neutral Atoms

      Rakreungdet, Worawarong; Jessen, Poul S.; Jessen, Poul S.; Anderson, Brian P.; Cronin, Alexander; Wright, Ewan (The University of Arizona., 2008)
      We study a system of neutral atoms trapped in a three-dimensional optical lattice suitable for the encoding, initialization and manipulation of atomic qubits. The qubits are manipulated by applied electromagnetic fields interacting with dipole moments of the atoms via light shifts, Raman transitions, Zeeman shifts, and microwave transitions. Our lattice is formed by three orthogonal one-dimensional lattices, which have different frequencies so that interference terms average to zero. This geometry allows considerable freedom in designing the component one-dimensional lattices, so that they provide not only confinement but also independent control in each dimension. Our atomic qubits are initialized from a laser-cooled atomic sample by Raman sideband cooling in individual lattice potential wells. We have demonstrated accurate and robust one-qubit manipulation using resonant microwave fields. In practice such control operations are always subject to errors, in our case spatial inhomogeneities in the microwave Rabi frequency and the light shifted qubit transition frequency. Observation of qubit dynamics in near real time allows us to minimize these inhomogeneities, and therefore optimize qubit logic gates. For qubits in the lattice, we infer a fidelity of 0.990(3) for a single pi-pulse. We have also explored the use of NMR-type pulse techniques in order to further reduce the effect of errors and thus improve gate robustness in the atom/lattice system. Our schemes for two-qubit quantum logic operations are based on controlled collisional interactions. We have experimented with two schemes in order to probe these collisions. The first involves manipulation of the center-of-mass wavepackets of two qubits in a geometry corresponding to two partially overlapping Mach-Zender interferometers. Unfortunately, this scheme has proven extremely sensitive to phase errors, as the wavepackets are moved by the optical lattice. The other scheme starts with two qubits in spatially separated traps, and utilizes microwaves to drive one or both qubits into a third trap in-between the two qubits. Once the wavepackets overlap, the collisions create a large energy shift which can be probed spectroscopically.
    • Quantum state preparation in an optical lattice

      Jessen, Poul S.; Meystre, Pierre; Hamann, Steven Eugene (The University of Arizona., 1998)
      This dissertation reports on quantum state preparation of cesium atoms in a two-dimensional optical lattice, by resolved-sideband Raman cooling. An optical lattice is a periodic potential produced by the light shift interaction between an atom and light field. Laser cooled atoms can become strongly localized about the bottom of potential wells in an optical lattice, where they occupy a discrete spectrum of bound vibrational energy levels. The distribution over vibrational levels of atoms in the lattice is characterized by the mean vibrational excitation, n . In an optical lattice, absorption and emission of photons from lattice beams causes n to increase in time. This source of heating is always present, but its rate can be greatly reduced in a lattice detuned far from the atomic resonance. Sideband cooling is an efficient means of transferring atoms from higher into lower-lying vibrational levels and, thus, it reduces n for the ensemble. If the sideband cooling rate is much greater than the heating rate, then n approaches zero and virtually all atoms are in the lowest vibrational level in their potential wells. Our sideband cooling scheme involves stimulated Raman transitions between bound states in the potential wells of a pair of magnetic sublevels, followed by optical pumping, for a net loss of one quantum of vibration per cooling cycle. The process accumulates 98% of atoms in the ground vibrational level of a potential well associated with a single Zeeman substate. Each atom in the lattice is then very close to a pure state. For two-dimensional lattice with sideband cooling we find nx≈ny≈0.008 &parl0;16&parr0; . Various issues related to state preparation and sideband cooling are also discussed in the context of a one dimensional lin ⊥ lin optical lattice. These include improvement of laser cooling in a near resonance lattice by application of weak magnetic fields, transfer of atoms from near into far off-resonance lattices, and heating rates in far off-resonance lattices.
    • Quantum Systems in Bernoulli Potentials

      Wehr, Jan; Bishop, Michael Anthony; Friedlander, Leonid; McLaughlin, Kenneth; Sims, Robert; Wehr, Jan (The University of Arizona., 2013)
      Quantum mechanics is a theory developed to explain both particle and wave-like properties of small matter such as light and electrons. The consequences of the theory can be counter-intuitive but lead to mathematical and physical theory rich in fascinating phenomena and challenging questions. This dissertation investigates the nature of quantum systems in Bernoulli distributed random potentials for systems on the one dimensional lattice {0, 1, ..., L, L+1} ⊂ Z in the large system limit L → ∞. For single particle systems, the behavior of the low energy states is shown to be approximated by systems where positive potential is replaced by infinite potential. The approximate shape of these states is described, the asymptotics of their eigenvalues are calculated in the large system limit L → ∞, and a Lifschitz tail estimate on the sparsity of low energy states is proven. For interacting multi-particle systems, a Lieb-Liniger model with Bernoulli distributed potential is studied in the Gross-Pitaevskii approximation. First, to investigate localization in these settings, a general inequality is proven to bound from below the support of the mean-field. The bound depends on the per particle energy, number of particles, and interaction strength. Then, the ground state for the one-dimensional lattice with Bernoulli potential is studied in the large system limit. Specifically, the case where the product of interaction strength and particle density is near zero is considered to investigate whether localization can be recovered.
    • Quantum theories of self-localization

      Scott, Alwyn C.; Bernstein, Lisa Joan (The University of Arizona., 1991)
      In the classical dynamics of coupled oscillator systems, nonlinearity leads to the existence of stable solutions in which energy remains localized for all time. Here the quantum-mechanical counterpart of classical self-localization is investigated in the context of two model systems. For these quantum models, the terms corresponding to classical nonlinearities modify a subset of the stationary quantum states to be particularly suited to the creation of nonstationary wavepackets that localize energy for long times. The first model considered here is the Quantized Discrete Self-Trapping model (QDST), a system of anharmonic oscillators with linear dispersive coupling used to model local modes of vibration in polyatomic molecules. A simple formula is derived for a particular symmetry class of QDST systems which gives an analytic connection between quantum self-localization and classical local modes. This formula is also shown to be useful in the interpretation of the vibrational spectra of some molecules. The second model studied is the Frohlich/Einstein Dimer (FED), a two-site system of anharmonically coupled oscillators based on the Frohlich Hamiltonian and motivated by the theory of Davydov solitons in biological protein. The Born-Oppenheimer perturbation method is used to obtain approximate stationary state wavefunctions with error estimates for the FED at the first excited level. A second approach is used to reduce the first excited level FED eigenvalue problem to a system of ordinary differential equations. A simple theory of low-energy self-localization in the FED is discussed. The quantum theories of self-localization in the intrinsic QDST model and the extrinsic FED model are compared.

      Sargent, Murray; HOLM, DAVID ALLEN. (The University of Arizona., 1985)
      This dissertation formulates and applies a theory describing how one or two strong classical waves and one or two weak quantum mechanical waves interact in a two-level medium. The theory unifies many topics in quantum optics, such as resonance fluorescence, saturation spectroscopy, modulation spectroscopy, the build up of laser and optical bistability instabilities, and phase conjugation. The theory is based on a quantum population pulsation approach that resembles the semiclassical theories, but is substantially more detailed. Calculations are performed to include the effects of inhomogeneous broadening, spatial hole burning, and Gaussian transverse variations. The resonance fluorescence spectrum in a high finesse optical cavity is analyzed in detail, demonstrating how stimulated emission and multiwave processes alter the spectrum from the usual three peaks. The effects of quantum noise during the propagation of weak signal and conjugate fields in phase conjugation and modulation spectroscopy are studied. Our analysis demonstrates that quantum noise affects not only the intensities of the signal and conjugate, but also their relative phase, and in particular we determine a quantum limit to the semiclassical theory of FM modulation spectroscopy. Finally, we derive the corresponding theory for the two-photon, two-level medium. This yields the first calculation of the two-photon resonance fluorescence spectrum. Because of the greater number of possible interactions in the two-photon two-level model, the theoretical formalism is considerably more complex, and many effects arise that are absent in the one-photon problem. We discuss the role of the Stark shifts on the emission spectrum and show how the Rayleigh scattering is markedly different.
    • Quantum transport theory.

      Shin, Ghi Ryang.; Rafelski, Johann; Thews, Robert L.; Kohler, Sigurd; Shupe, Michael A.; McIntyre, Laurence C. (The University of Arizona., 1993)
      Within the framework of the quantum transport theory based on the Wigner transform of the density matrix I study first in non-relativistic and subsequently in relativistic formulation a number of applications. I also develop further the recently proposed relativistic theory: the classical limit is carefully derived and the integral equations of the relativistic Wigner function derived explicitly. I show how it is possible to obtain the Schwinger like particle production rate from relativistic quantum transport equations. Noteworthy numerical results address the shape of the relativistic Wigner function of a given quantum state. Other numerical studies are primarily oriented towards the time evolution of the Wigner function--I can presently solve only the nonrelativistic case in which there is no mixing between particle production and flow phenomena: I consider numerically the fate of the muon after muon catalyzed fusion.
    • Quantum tunneling and coherent wavepacket dynamics in an optical lattice

      Jessen, Poul S.; Wright, Ewan; Haycock, David Lamoreaux (The University of Arizona., 2000)
      This dissertation reports on the experimental study of coherent wavepacket dynamics of cesium atoms in the double-well potentials of a one-dimensional, far-off-resonance optical lattice. An optical lattice is the periodic potential produced by the light shift interaction of an atom with the light field of interfering laser beams. With the proper choice of laser parameters and external magnetic fields, an array of double-well potentials is created. Using the techniques of laser cooling, atoms are trapped in the lattice and are prepared in a pure state through a combination of enhanced laser cooling in a near-resonance lattice and state selection in an accelerated far-off-resonance lattice. The atoms are prepared on one side of the double-well potential, and the atomic wavepackets will then oscillate between the left and right localized states of the double-well potential. Entanglement between the internal and external degrees of freedom makes it possible to follow the center-of-mass motion of the atoms by measuring the ground state magnetic populations via Stern-Gerlach analysis. The coherent dynamics of these wavepackets was studied under various combinations of lattice parameters such as lattice depth, applied transverse and longitudinal magnetic fields. There is excellent agreement between the experimentally measured oscillation frequencies and those predicted from a numerical analysis of the bandstructure of the lattice. For certain lattice parameters the total energy of the atom is below the potential barrier and the coherent motion corresponds to tunneling through a classically forbidden barrier. At specific times during the oscillation the atomic wavepacket corresponds to a coherent superposition of the mesoscopically distinct left and right localized states.
    • Quantum weak turbulence with applications to semiconductor lasers

      Newell, Alan C.; Lvov, Yuri Victorovich, 1969- (The University of Arizona., 1998)
      Based on a model Hamiltonian appropriate for the description of fermionic systems such as semiconductor lasers, we describe a natural asymptotic closure of the BBGKY hierarchy in complete analogy with that derived for classical weak turbulence. The main features of the interaction Hamiltonian are the inclusion of full Fermi statistics containing Pauli blocking and a simple, phenomenological, uniformly weak two particle interaction potential equivalent to the static screening approximation. The resulting asymytotic closure and quantum kinetic Boltzmann equation are derived in a self consistent manner without resorting to a priori statistical hypotheses or cumulant discard assumptions. We find a new class of solutions to the quantum kinetic equation which are analogous to the Kolmogorov spectra of hydrodynamics and classical weak turbulence. They involve finite fluxes of particles and energy across momentum space and are particularly relevant for describing the behavior of systems containing sources and sinks. We explore these solutions by using differential approximation to collision integral. We make a prima facie case that these finite flux solutions can be important in the context of semiconductor lasers. We show that semiconductor laser output efficiency can be improved by exciting these finite flux solutions. Numerical simulations of the semiconductor Maxwell Bloch equations support the claim.
    • Quantum Well Intermixing For Photonic Integrated Circuits

      Kost, Alan R; Sun, Xiaolan; Kost, Alan R; Kost, Alan R.; Honkanen, Seppo; Kueppers, Franko (The University of Arizona., 2007)
      In this thesis, several aspects of GaAsSb/AlSb multiple quantum well (MQW) heterostructures have been studied. First, it was shown that the GaAsSb MQWs with a direct band gap near 1.5 μm at room temperature could be monolithically integrated with AlGaSb/AlSb or AlGaAsSb/AlAsSb Bragg mirrors, which can be applied to Vertical Cavity Surface Emitting Lasers (VCSELs). Secondly, an enhanced photoluminescence from GaAsSb MQWs was reported. The photoluminescence strength increased dramatically with arsenic fraction as conjectured. The peak photoluminescence from GaAs(0.31)Sb(0.69) was 208 times larger than that from GaSb. Thirdly, the strong photoluminescence from GaAsSb MQWs and the direct nature of the band gap near 1.5 μm at room temperature make the material favorable for intermixing studies. The samples were treated with ion implantation followed by rapid thermal annealing (RTA). A band gap blueshift as large as 198 nm was achieved with a modest ion dose and moderate annealing temperature. Photoluminescence strength for implanted samples generally increased with the annealing temperature. The energy blueshift was attributed to the interdiffusion of both the group III and group V sublattices. Finally, based on the interesting properties of GaAsSb MQWs, including the direct band gap near 1.5 μm, strong photoluminescence, a wide range of wavelength (1300 – 1500 nm) due to ion implantation-induced quantum well intermixing (QWI), and subpicosecond spin relaxation reported by Hall et al, we proposed to explore the possibilities for ultra-fast optical switching by investigating spin dynamics in semiconductor optical amplifiers (SOAs) containing InGaAs and GaSb MQWs. For circularly polarized pump and probe waves, the numerical simulation on the modal indices showed that the difference between the effective refractive index of the TE and TM modes was quite large, on the order of 0.03, resulting in a significant phase mismatch in a traveling length larger than 28 μm. Thus the FWM conversion efficiency was exceedingly small and the FWM mechanism in SOAs used for investigation of all-optical polarization switching was strongly limited.

      Hunt, John Henry, 1940- (The University of Arizona., 1969)
    • Quasars in galaxy cluster environments.

      Green, Richard; Ellingson, Erica. (The University of Arizona., 1989)
      The evolution of radio loud quasars is found to be strongly dependent upon their galaxy cluster environment. Previous studies (Yee and Green 1987) have shown that bright quasars at z ∼ 0.6 are found in clusters as rich as Abell richness class 1, while high luminosity quasars at lower redshifts are found only in poorer environments. An observational study of the environments of 66 low luminosity quasars with 0.3 < z < 0.6 yields several objects in rich clusters of galaxies. This result implies that radio loud quasars in these environments have faded approximately 3 magnitudes in the interval between redshifts 0.6 and 0.4, corresponding to a luminosity e-folding fading time of 900 million years, similar to the dynamical timescale of these environments. The analysis of low luminosity radio quiet quasars indicate that they are never found in rich environments, suggesting that they are a physically different class of objects. Properties of the quasar environment are investigated to determine constraints on the physical mechanisms of quasar formation and evolution. The optical cluster morphology indicates that the cluster cores have smaller radii and higher galaxy densities than are typical for low redshift clusters of similar richness. Radio morphologies may indicate that the formation of a dense intra-cluster medium is associated with the quasars' fading at these epochs. Galaxy colors appear to be normal, but there may be a tendency for clusters associated with high luminosity quasars to contain a higher fraction of gas-rich galaxies than those associated with low luminosity quasars, a result consistent with the formation of an ICM. Multislit spectroscopic observations of galaxies associated with high luminosity quasars indicate that quasars are preferentially located in regions of low relative velocity dispersion, either in rich clusters of abnormally low velocity dispersion, or in poor groups which are dynamically normal. This suggests that galaxy-galaxy interactions may play a role in quasar formation and sustenance. Virialization of rich clusters and the subsequent increase in galaxy velocities may therefore be responsible for the fading of quasars in rich environments.
    • Quasi-four-level laser design and analysis of Nd:YAG operating at the 946 nm transition

      Powell, Richard C.; Koehler, Elka Ertur (The University of Arizona., 2000)
      Nd:YAG, well known for its operation at 1064nm, has a weaker transition at 946nm, whose lower level is thermally populated. This dissertation describes the design and development of a diode pumped, room temperature, quasi-four-level laser operating at the 946nm transition of Nd:YAG. The design addresses two primary issues in obtaining an efficient, high energy oscillator at 946nm. These are the ground state reabsorption losses due to the thermally occupied lower laser level, and the population inversion losses incurred at the much stronger 1064nm, transition. With 55 mJ in the normal mode, and 25 mJ in the q-switched mode, the output energies obtained are the highest energies per pulse reported to date for a diode pumped, 946mn Nd:YAG laser. A quasi-four-level laser theory is developed and used to optimize oscillator parameters affected by the thermally occupied lower laser level. The laser material length and the folded V shaped cavity are selected to maximize the gain per round trip in the cavity. The availability of stacked and microlensed diode array bars, along with an efficient pump coupling technique, allows the use of an end pumped configuration which provides the high pump density required to reach threshold in quasi-four-level lasers. The oscillator design was further refined to eliminate possible parasitic lasing paths and minimize amplified spontaneous emission losses at the 1064nm, transition. A large diameter laser disk with a Samarium doped cladding, which absorbs the 1064nm, radiation, reduces the number of 1064nm, ASE paths which deplete the inversion density in the pumped volume. The cladding significantly improves the storage efficiency, and hence the q-switched efficiency, of the oscillator. Although the oscillator was developed specifically for remote sensing of atmospheric water vapor, other applications can also benefit from the development of an efficient 946nm laser source. When frequency doubled, this wavelength allows access to the blue, which is highly desirable for high density data storage, displays, biological applications, and underwater communications.