InAs/GaAs quantum-dot superluminescent light-emitting diode monolithically grown on a Si substrate

Building optoelectronic devices on a Si platform has been the engine behind the development of Si photonics. In particular, the integration of optical interconnects onto Si substrates allows the fabrication of complex optoelectronic circuits, potentially enabling chip-to-chip and system-to-system optical communications at greatly reduced cost and size relative to hybrid solutions. Although significant effort has been devoted to Si light generation and modulation technologies, efficient and electrically pumped Si light emitters have yet to be demonstrated. In contrast, III–V semiconductor devices offer high efficiency as optical sources. Monolithic integration of III–V on the Si platform would thus be an effective approach for realizing Si-based light sources. Here, we describe the first superluminescent light-emitting diode (SLD) monolithically grown on Si substrates. The fabricated two-section InAs/GaAs quantum-dot (QD) SLD produces a close-to-Gaussian emission spectrum of 114 nm centered at 1255 nm wavelength, with a maximum output power of 2.6 mW at room temperature. This work complements our previous demonstration of an InAs/GaAs QD laser directly grown on a Si platform and paves the way for future monolithic integration of III–V light sources required for Si photonics

Relaxation of surface charge on rotating dielectric spheres: Implications on dynamic electrorheological effects

We have examined the effect of an oscillatory rotation of a polarized dielectric particle. The rotational motion leads to a re-distribution of the polarization charge on the surface of the particle. We show that the time averaged steady-state dipole moment is along the field direction, but its magnitude is reduced by a factor which depends on the angular velocity of rotation. As a result, the rotational motion of the particle reduces the electrorheological effect. We further assume that the relaxation of polarized charge is arised from a finite conductivity of the particle or host medium. We calculate the relaxation time based on the Maxwell-Wagner theory, suitably generalized to include the rotational motion. Analytic expressions for the reduction factor and the relaxation time are given and their dependence on the angular velocity of rotation will be discussed.Comment: Accepted for publications by Phys. Rev.

Al0.2Ga0.8As solar cells monolithically grown on Si and GaAs by MBE for III-V/Si tandem dual-junction applications

Al0.2Ga0.8As photovoltaic solar cells have been monolithically grown on silicon substrates by Molecular Beam Epitaxy. Due to the 4% lattice mismatch between AlGaAs and Si, Threading Dislocations (TDs) nucleate at the III-V/Si interface and propagate to the active region of the cells where they act as recombination centers, reducing the performances of the devices. In order to reduce the Threading Dislocation Density (TDD) in the active layers of the cells, InAlAs Strained Layer Superlattice (SLS) Dislocation Filter Layers (DFLs) have been used. For one of the samples, in-situ Thermal Cycle Annealing (TCA) steps have additionally been performed during growth. For comparison purposes, reference Al0.2Ga0.8As solar cells have been grown lattice-matched on GaAs. For the sample grown on Si without TCA, the TDD has been reduced from over 7×109cm-2 at the III-V/Si interface to 3×107cm-2 in the base of the cells. With TCA, the TDD has been reduced throughout the sample from over 3×109cm-2 in the initial epilayers to 8(±2)×106cm-2 in the base of the cells. For the best devices, the Voc improves from 833mV on Si without TCA to 895mV using TCA, compared with 1070mV for the reference sample grown lattice-matched on GaAs. Similarly the fill factor improves from 73.7% on Si without TCA to 74.8% using TCA, compared with 78.4% on GaAs. The high bandgap-voltage offset obtained both on Si and GaAs indicates a non-optimal bulk AlGaAs material quality due to non-ideal growth conditions

Eficiência agronômica da mistura de uréia com zeolita natural aplicada na cultura da roseira (Rosa spp.), Nova Friburgo, RJ.

Com objetivo de avaliar a eficiência agronômica da mistura de uréia com zeolita natural (arenito zeolítico), realizou-se experimento em área de produção comercial de flores de corte em Nova Friburgo, RJ, Brasil. Foram realizadas duas adubações de cobertura na cultura da roseira com duas doses de uréia (60 e 120 kg N. ha-1 ) misturadas ou não com arenito zeolítico, na proporção de 20% p/p. As colheitas foram avaliadas quantitativamente, através do total de hastes (HT), massa fresca (MFT) e massa seca (MST) produzidas, e qualitativamente, através da divisão em hastes curtas (HC), médias (HM) e longas (HL). Determinou-se também a extração total de N pelas hastes comerciais colhidas (EXTNH) e a eficiência agronômica dos fertilizantes (EAF). A eficiência agronômica dos fertilizantes (EAF) sobre as produções de HT, MFT, MST e EXTNH é elevada pelo efeito do arenito zeolítico, e a maior influência sobre HT provém do aumento nas produções de HL, elevando a qualidade das hastes produzidas. A mistura de arenito zeolítico com uréia mantém maior quantidade de N-uréia no sistema solo-planta, liberando o nutriente lentamente para a solução do solo, em sincronia com as demandas nutricionais das plantas, resultando no aumento da eficiência agronômica dos fertilizantes avaliados. O arenito zeolítico (CETEM) apresenta características favoráveis para uso em desenvolvimento de fertilizantes nitrogenados de liberação lenta, principalmente com objetivo de aumentar a eficiência agronômica dos fertilizantes

A simple theorem to generate exact black hole solutions

Under certain conditions imposed on the energy-momentum tensor, a theorem that characterizes a two-parameter family of static and spherically symmetric solutions to Einstein's field equations (black holes), is proved. A discussion on the asymptotics, regularity, and the energy conditions is provided. Examples that include the best known exact solutions within these symmetries are considered. A trivial extension of the theorem includes the cosmological constant {\it ab-initio}, providing then a three-parameter family of solutions.Comment: 14 pages; RevTex; no figures; typos corrected; references adde

Water Dynamics at Protein Interfaces: Ultrafast Optical Kerr Effect Study

The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at 80 cm–1, which is assigned to a bending of the protein amide chain

Solar interacting protons versus interplanetary protons in the core plus halo model of diffusive shock acceleration and stochastic re-acceleration

With the first observations of solar γ-rays from the decay of pions, the relationship of protons producing ground level enhancements (GLEs) on the Earth to those of similar energies producing the γ-rays on the Sun has been debated. These two populations may be either independent and simply coincident in large flares, or they may be, in fact, the same population stemming from a single accelerating agent and jointly distributed at the Sun and also in space. Assuming the latter, we model a scenario in which particles are accelerated near the Sun in a shock wave with a fraction transported back to the solar surface to radiate, while the remainder is detected at Earth in the form of a GLE. Interplanetary ions versus ions interacting at the Sun are studied for a spherical shock wave propagating in a radial magnetic field through a highly turbulent radial ray (the acceleration core) and surrounding weakly turbulent sector in which the accelerated particles can propagate toward or away from the Sun. The model presented here accounts for both the first-order Fermi acceleration at the shock front and the second-order, stochastic re-acceleration by the turbulence enhanced behind the shock. We find that the re-acceleration is important in generating the γ-radiation and we also find that up to 10% of the particle population can find its way to the Sun as compared to particles escaping to the interplanetary space

Multilayers of InGaAs Nanostructures Grown on GaAs(210) Substrates

Multilayers of InGaAs nanostructures are grown on GaAs(210) by molecular beam epitaxy. With reducing the thickness of GaAs interlayer spacer, a transition from InGaAs quantum dashes to arrow-like nanostructures is observed by atomic force microscopy. Photoluminescence measurements reveal all the samples of different spacers with good optical properties. By adjusting the InGaAs coverage, both one-dimensional and two-dimensional lateral ordering of InGaAs/GaAs(210) nanostructures are achieved

Automatic Prediction of Facial Trait Judgments: Appearance vs. Structural Models

Evaluating other individuals with respect to personality characteristics plays a crucial role in human relations and it is the focus of attention for research in diverse fields such as psychology and interactive computer systems. In psychology, face perception has been recognized as a key component of this evaluation system. Multiple studies suggest that observers use face information to infer personality characteristics. Interactive computer systems are trying to take advantage of these findings and apply them to increase the natural aspect of interaction and to improve the performance of interactive computer systems. Here, we experimentally test whether the automatic prediction of facial trait judgments (e.g. dominance) can be made by using the full appearance information of the face and whether a reduced representation of its structure is sufficient. We evaluate two separate approaches: a holistic representation model using the facial appearance information and a structural model constructed from the relations among facial salient points. State of the art machine learning methods are applied to a) derive a facial trait judgment model from training data and b) predict a facial trait value for any face. Furthermore, we address the issue of whether there are specific structural relations among facial points that predict perception of facial traits. Experimental results over a set of labeled data (9 different trait evaluations) and classification rules (4 rules) suggest that a) prediction of perception of facial traits is learnable by both holistic and structural approaches; b) the most reliable prediction of facial trait judgments is obtained by certain type of holistic descriptions of the face appearance; and c) for some traits such as attractiveness and extroversion, there are relationships between specific structural features and social perceptions

Lattice Boltzmann simulations of soft matter systems

This article concerns numerical simulations of the dynamics of particles immersed in a continuum solvent. As prototypical systems, we consider colloidal dispersions of spherical particles and solutions of uncharged polymers. After a brief explanation of the concept of hydrodynamic interactions, we give a general overview over the various simulation methods that have been developed to cope with the resulting computational problems. We then focus on the approach we have developed, which couples a system of particles to a lattice Boltzmann model representing the solvent degrees of freedom. The standard D3Q19 lattice Boltzmann model is derived and explained in depth, followed by a detailed discussion of complementary methods for the coupling of solvent and solute. Colloidal dispersions are best described in terms of extended particles with appropriate boundary conditions at the surfaces, while particles with internal degrees of freedom are easier to simulate as an arrangement of mass points with frictional coupling to the solvent. In both cases, particular care has been taken to simulate thermal fluctuations in a consistent way. The usefulness of this methodology is illustrated by studies from our own research, where the dynamics of colloidal and polymeric systems has been investigated in both equilibrium and nonequilibrium situations.Comment: Review article, submitted to Advances in Polymer Science. 16 figures, 76 page