Graph bisection algorithms

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1986.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Bibliography: leaves 64-66.by Thang Nguyen Bui.Ph.D

Optical engineering of iii-nitride nanowire light-emitting diodes and applications

Applications of III-nitride nanowires are intensively explored in different emerging technologies including light-emitting diodes (LEDs), laser diodes, photodiodes, biosensors, and solar cells. The synthesis of the III-nitride nanowires by molecular beam epitaxy (MBE) is investigated with significant achievements. III-nitride nanowires can be grown on dissimilar substrates i.e., silicon with nearly dislocation free due to the effective strain relaxation. III-nitride nanowires, therefore, are perfectly suited for high performance light emitters for cost-effective fabrication of the advanced photonic-electronic integrated platforms. This dissertation addresses the design, fabrication, and characterization of III-nitride nanowire full-color micro-LED (µLED) on silicon substrates for µLED display technologies, high-efficient ultraviolet (UV) LEDs, and spectral engineering for narrow band LEDs. In this dissertation, InGaN/AlGaN nanowire µLEDs were demonstrated with highly stable emission which can be varied from the blue to red spectrum. Additionally, by integrating full-color emissions in a single nanowire, phosphor-free white-color µLEDs are achieved with an unprecedentedly high color rendering index of ~ 94. Such high-performance µLEDs are perfectly suitable for the next generation high-resolution micro-display applications. Moreover, the first demonstration of two-step surface passivation using Potassium Hydroxide (KOH) and Ammonium Sulfide (NH4)2Sx is reported. The photoluminescence, electroluminescence, and optical power of the 335 nm AlGaN nanowire UV LEDs show improvements by 49%, 83%, and 65%, respectively. Such enhanced performance is attributed to the mitigation of the surface nonradiative recombination on the nanowire surfaces. A combination of KOH and (NH4)2Sx treatment shows a promising approach for high efficiency and high power AlGaN nanowire UV LEDs. The LEDs with narrow spectra are highly desirable light sources for precisely controlled applications such as phototherapy. In this regard, we have further demonstrated narrow spectral nanowire LEDs using on-chip integrated bandpass filters. To achieve narrow band spectra, the bandpass filters are designed and fabricated using all-dielectric and metal-dielectric multilayers for visible and UV regions, respectively. They are fabricated onto LED devices as a single photonic platform to achieve the narrow band LEDs for innovative applications like phototherapy for wound healing

Folding model study of the charge-exchange scattering to the isobaric analog state and implication for the nuclear symmetry energy

The Fermi transition (\Delta L=\Delta S=0 and \Delta T=1) between the nuclear isobaric analog states (IAS), induced by the charge-exchange (p,n) or (3He,t) reaction, can be considered as "elastic" scattering of proton or 3He by the isovector term of the optical potential (OP) that flips the projectile isospin. The accurately measured (p,n) or (3He,t) scattering cross-section to the IAS can be used, therefore, to probe the isospin dependence of the proton or 3He optical potential. Within the folding model, the isovector part of the OP is determined exclusively by the neutron-proton difference in the nuclear densities and the isospin dependence of the effective nucleon-nucleon (NN) interaction. Because the isovector coupling explicitly links the isovector part of the proton or 3He optical potential to the cross section of the charge-exchange (p,n) or (3He,t) scattering to the IAS, the isospin dependence of the effective (in-medium) NN interaction can be well tested in the folding model analysis of these charge-exchange reactions. On the other hand, the same isospin- and density dependent NN interaction can also be used in a Hartree-Fock calculation of asymmetric nuclear matter, to estimate the nuclear matter energy and its asymmetry part (the nuclear symmetry energy). As a result, the fine-tuning of the isospin dependence of the effective NN interaction against the measured (p,n) or (3He,t) cross sections should allow us to make some realistic prediction of the nuclear symmetry energy and its density dependence.Comment: Accepted for publication in European Physical Journal A - "Hadrons and Nuclei

A Unifying Framework for Gaussian Process Pseudo-Point Approximations using Power Expectation Propagation

Gaussian processes (GPs) are flexible distributions over functions that enable high-level assumptions about unknown functions to be encoded in a parsimonious, flexible and general way. Although elegant, the application of GPs is limited by computational and analytical intractabilities that arise when data are sufficiently numerous or when employing non-Gaussian models. Consequently, a wealth of GP approximation schemes have been developed over the last 15 years to address these key limitations. Many of these schemes employ a small set of pseudo data points to summarise the actual data. In this paper we develop a new pseudo-point approximation framework using Power Expectation Propagation (Power EP) that unifies a large number of these pseudo-point approximations. Unlike much of the previous venerable work in this area, the new framework is built on standard methods for approximate inference (variational free- energy, EP and Power EP methods) rather than employing approximations to the probabilistic generative model itself. In this way all of the approximation is performed at `inference time' rather than at `modelling time', resolving awkward philosophical and empirical questions that trouble previous approaches. Crucially, we demonstrate that the new framework includes new pseudo-point approximation methods that outperform current approaches on regression and classification tasks