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Analysis of waveguiding properties of VCSEL structures

Description: In this paper, the authors explore the feasibility of using the distributed Bragg reflector, grown on the substrate for a VCSEL (Vertical Cavity Surface Emitting Laser), to provide waveguiding within the substrate. This waveguiding could serve as an interconnection among VCSELs in an array. Before determining the feasibility of waveguide interconnected VCSELs, two analysis methods are presented and evaluated for their applicability to this problem. The implementations in Mathematica of both these methods are included. Results of the analysis show that waveguiding in VCSEL structures is feasible. Some of the many possible uses of waveguide interconnected VCSELs are also briefly discussed. The tools and analysis presented in this report can be used to evaluate such system concepts and to do detailed design calculations.
Date: September 1996
Creator: Erteza, I. A.
Partner: UNT Libraries Government Documents Department

Simulational studies of epitaxial semiconductor superlattices: Quantum dynamical phenomena in ac and dc electric fields

Description: Using high-accuracy numerical methods the author investigates the dynamics of independent electrons in both ideal and realistic superlattices subject to arbitrary ac and/or dc electric fields. For a variety of superlattice potentials, optically excited initial wave packets, and combinations of ac and dc electric fields, he numerically solves the time-dependent Schroedinger equation. In the case of ideal periodic superlattice potentials, he investigates a long list of dynamical phenomena involving multiple miniband transitions and time-dependent electric fields. These include acceleration effects associated with interminiband transitions in strong fields, Zener resonances between minibands, dynamic localization with ac fields, increased single-miniband transport with an auxiliary resonant ac field, and enhanced or suppressed interminiband probability exchange using an auxiliary ac field. For all of the cases studied, the resulting time-dependent wave function is analyzed by projecting the data onto convenient orthonormal bases. This allows a detailed comparison with approximately analytic treatments. In an effort to explain the rapid decay of experimentally measured Bloch oscillation (BO) signals the author incorporates a one-dimensional representation of interface roughness (IR) into their superlattice potential. He shows that as a result of IR, the electron dynamics can be characterized in terms of many discrete, incommensurate frequencies near the Block frequency. Chapters 2, 3, 4 and 5 have been removed from this report and will be processed separately.
Date: October 8, 1997
Creator: Reynolds, J.
Partner: UNT Libraries Government Documents Department

A New Lattice Formulation of the Continuum

Description: It is well known that the Dirac equation on a discrete lattice in D dimension has 2D degenerate solutions. The usual method of removing these spurious solutions encounters difficulties with chiral symmetry when the lattice spacing l not equal 0, as demonstrated by the persistent problem of pion and kaon masses. On the other hand, we recall that in any crystal in nature, all the electrons do move in a lattice and satisfy the Dirac equation; yet there is not a single physical result that has ever been entangled with a spurious fermion solutions. Therefore it should not be difficult to eliminate these unphysical elements. On a discrete lattice, particles hop from point to point, whereas in a real crystal the lattice structure is embedded in a continuum and electrons move continuously from lattice cell to lattice cell. In a discrete system, the lattice function are defined only on individual points (or links, as in the case of gauge fields). However, in a crystal the electron state vector is represented by the Bloch wave functions which are continuous functions in tau(right arrow) and herein lies one of the essential differences.
Date: December 31, 1997
Creator: Pang, Yang & Ren, Haicang
Partner: UNT Libraries Government Documents Department

Electronic bandstructure of an incommensurate crystal

Description: The consequences of an incommensurate lattice modulation on the electronic energy levels have been studied by optical transmission experiments on Rb/sub 2/ZnBr/sub 4/. The results are analyzed with a simple tight-binding model in which the superspace symmetry of the crystal is taken into account. The lattice translational symmetry of crystalline matter leads to the well known concepts of the Brillouin zones, Bloch electrons, phonons and the like. In a crystal where the lattice is periodically distorted with a period that is incommensurate with the underlying lattice, this translational symmetry is broken. Nonetheless, incommensurate crystals are perfectly ordered and can be described by higher dimensional so-called superspace groups. In this paper we will show how this superspace approach provides a natural framework to understand their electronic bandstructure as well. 5 references, 3 figures.
Date: June 1, 1984
Creator: Rasing, T.
Partner: UNT Libraries Government Documents Department

Basic studies of atomic dynamics. Progress report, October 1, 1974-- September 30, 1975

Description: Topics covered include: fitting many-body theories into the theory of atomic spectra and collisions; variation of atomic properties along the periodic system; angular distribution of collision products (electron-atom collisions); application of atomic points of view to crystal lattice phenomena; and double photoionization of neon. (GHT)
Date: October 1, 1975
Creator: Fano, U.
Partner: UNT Libraries Government Documents Department

Excited state electron spin coherence (ESESC) studies of triplet states in molecular solids

Description: The field of coherent spectroscopy of two-level systems is applied to the lowest triplet state of organic molecules. By neglecting the triplet sublevel not coupled by the field, it is possible to describe the remaining two levels with Feynman-Vernon-Hellwarth geometrical representation of a general two-level system. The equations of motion of the pseudomagnetization are derived after transformation to the rotating frame, as are Bloch-type equations which include phenomenological relaxation times. The loss of coherence due to exchange between triplet states with different Larmor frequencies but identical zero-field dipolar tensor axes is then discussed. By writing two sets of coupled Bloch equations, expressions for the effective decay rate and frequency shift of the experimentally monitored triplet system are derived and discussed in the limits of slow and rapid exchange. This analysis is applied to intramolecular tunneling between different configurations of cyclopentanone. It is shown by both spin locking and CW spectra that the tunneling rate is considerably slower than the phosphorescence decay rate of the lowest triplet state. Rotary echoes are considered, both on- and off-resonance, with Average Hamiltonian theory. It is shown that relaxation fields perpendicular to the driving field are averaged while those parallel to it are not. The inhomogeneity in the broadening mechanism is completely removed by on-resonance rotary echoes but only partially eliminated by off-resonance rotary echoes. Calculations for off-resonance rotary echo intensities are presented and extended to include triplet sublevel population kinetics and inhomogeneous broadening. Finally, experimental observation of rotary echoes in several 1,2,4,5-Tetrachlorobenzene systems is reported and compared with the theoretical predictions made.
Date: February 1, 1978
Creator: Tarrasch, M.E.
Partner: UNT Libraries Government Documents Department

Positron-annihilation study of the equilibrium vacancy ensemble in aluminum

Description: A preliminary report is presented of a positron-annihilation study of the equilibrium vacancy ensemble in aluminum using one- and two-dimensional angular correlation of annihilation radiation (ACAR) measurements versus temperature. The annihilation characteristics of a positron from the Bloch state, and the monovancy- and divacancy-trapped states have been calculated self-consistently within a supercell, including many-body enhancement effects, and are compared with experiment. 4 figures.
Date: June 1, 1982
Creator: Fluss, M.J.; Berko, S.; Chakraborty, B.; Hoffmann, K.; Lippel, P. & Siegel, R.W.
Partner: UNT Libraries Government Documents Department

The electronic structure and the state of compositional order in metallic alloys

Description: Many two-component (A,B) systems crystallize into a random solid solution. In such a state the atoms occupy a more or less regular array of lattice sites but each site can be A or B in a random fashion. Then, on lowering the temperature, the system will either phase separate or order, starting at some transition temperature T/sub c/. The aim of these lectures is to present a microscopic approach to the understanding of these scientifically interesting and technologically important processes. 64 refs., 19 figs.
Date: January 1, 1987
Creator: Gyorffy, B.L.; Johnson, D.D.; Pinski, F.J.; Nicholson, D.M. & Stocks, G.M.
Partner: UNT Libraries Government Documents Department