Carbon nanotube based high resolution holograms.

Carbon nanotubes are used as the smallest possible scattering element for diffracting light in a highly controlled manner to produce a 2D image. An array of carbon nanotubes is elegantly patterned to produce a high resolution hologram. In response to incident light on the hologram, a high contrast and wide field of view CAMBRIDGE image is produced.

Computer generated holograms for carbon nanotube arrays.

Multiwalled carbon nanotubes are highly diffractive structures in the optical regime. Their metallic character and large scattering cross-section allow their usage as diffractive elements in Fraunhofer holograms. This work elaborates some important features of the far field diffraction patterns produced from periodic arrays of nanotubes. A theoretical approach for the interaction of arrays of nanotubes with light is presented and a computer generated hologram is calculated by means of periodical patterns. Based on the results, fabrication of carbon nanotube arrays (in holographic patterns) was performed. Experimentally measured diffraction patterns were in good agreement with the calculations.

Use of 2G coated conductors for efficient shielding of DC magnetic fields

This paper reports the results of an experimental investigation of the performance of two types of magnetic screens assembled from YBa2Cu3O7-d (YBCO) coated conductors. Since effective screening of the axial DC magnetic field requires the unimpeded flow of an azimuthal persistent current, we demonstrate a configuration of a screening shell made out of standard YBCO coated conductor capable to accomplish that. The screen allows the persistent current to flow in the predominantly azimuthal direction at a temperature of 77 K. The persistent screen, incorporating a single layer of superconducting film, can attenuate an external magnetic field of up to 5 mT by more than an order of magnitude. For comparison purposes, another type of screen which incorporates low critical temperature quasi-persistent joints was also built. The shielding technique we describe here appears to be especially promising for the realization of large scale high-Tc superconducting screens.

Magnetic shielding with YBCO coated conductors: Influence of the geometry on its performances

A superconducting magnetic shield can be built as a stack of several sections of milled 2G coated conductors. Each section consists of a closed loop where persistent currents can flow and provide a strong attenuation of external dc magnetic fields. The purpose of the present work is to study experimentally several geometries of such magnetic shields made out of YBa2Cu 3O7 (YBCO) coated conductors from SuperPower. Our aim is to investigate in detail the influence of the aspect ratio and the number of layers of the assembly on the magnetic shielding properties. In order to do so, the magnetic shield is subjected to an axial quasi-static ('dc') magnetic field ramped slowly at a fixed sweep rate. A Hall probe is used to measure the local magnetic induction inside the assembly as a function of the applied magnetic induction. Results show that the shielding factor, SF, (defined as the ratio between the applied magnetic induction and the magnetic induction measured inside the shield) is improved for increasing aspect ratios of the global coated conductor assembly and that the threshold magnetic induction (defined for SF = 10) increases with the number of layers. Using a double layer of 18 sections at T = 77K , dc magnetic fields up to 56 mT can be shielded by a factor larger than 10. Finally, the effect of an air gap of constant width between coated conductor sections is also characterized. © 2002-2011 IEEE.

Precursor flow rate manipulation for the controlled fabrication of twin-free GaAs nanowires on silicon substrates

Vertically oriented GaAs nanowires (NWs) are grown on Si(111) substrates using metal-organic chemical vapor deposition. Controlled epitaxial growth along the 111 direction is demonstrated following the deposition of thin GaAs buffer layers and the elimination of structural defects, such as twin defects and stacking faults, is found for high growth rates. By systematically manipulating the AsH 3 (group-V) and TMGa (group-III) precursor flow rates, it is found that the TMGa flow rate has the most significant effect on the nanowire quality. After capping the minimal tapering and twin-free GaAs NWs with an AlGaAs shell, long exciton lifetimes (over 700ps) are obtained for high TMGa flow rate samples. It is observed that the Ga adatom concentration significantly affects the growth of GaAs NWs, with a high concentration and rapid growth leading to desirable characteristics for optoelectronic nanowire device applications including improved morphology, crystal structure and optical performance. © 2012 IOP Publishing Ltd.

Phase separation induced by Au catalysts in ternary InGaAs nanowires.

We report a novel phase separation phenomenon observed in the growth of ternary In(x)Ga(1-x)As nanowires by metalorganic chemical vapor deposition. A spontaneous formation of core-shell nanowires is investigated by cross-sectional transmission electron microscopy, revealing the compositional complexity within the ternary nanowires. It has been found that for In(x)Ga(1-x)As nanowires high precursor flow rates generate ternary In(x)Ga(1-x)As cores with In-rich shells, while low precursor flow rates produce binary GaAs cores with ternary In(x)Ga(1-x)As shells. First-principle calculations combined with thermodynamic considerations suggest that this phenomenon is due to competitive alloying of different group-III elements with Au catalysts, and variations in elemental concentrations of group-III materials in the catalyst under different precursor flow rates. This study shows that precursor flow rates are critical factors for manipulating Au catalysts to produce nanowires of desired composition.

Twin-free uniform epitaxial GaAs nanowires grown by a two-temperature process.

We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallographic quality and optimal shape, grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. This is achieved by a two-temperature growth procedure, consisting of a brief initial high-temperature growth step followed by prolonged growth at a lower temperature. The initial high-temperature step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temperature employed for subsequent growth imparts superior nanowire morphology and crystallographic quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temperature grown nanowires. Two mechanisms are proposed to explain the success of this two-temperature growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temperature hysteresis of the Au-Ga nanoparticle alloy.

Influence of nanowire density on the shape and optical properties of ternary InGaAs nanowires.

We have synthesized ternary InGaAs nanowires on (111)B GaAs surfaces by metal-organic chemical vapor deposition. Au colloidal nanoparticles were employed to catalyze nanowire growth. We observed the strong influence of nanowire density on nanowire height, tapering, and base shape specific to the nanowires with high In composition. This dependency was attributed to the large difference of diffusion length on (111)B surfaces between In and Ga reaction species, with In being the more mobile species. Energy dispersive X-ray spectroscopy analysis together with high-resolution electron microscopy study of individual InGaAs nanowires shows large In/Ga compositional variation along the nanowire supporting the present diffusion model. Photoluminescence spectra exhibit a red shift with decreasing nanowire density due to the higher degree of In incorporation in more sparsely distributed InGaAs nanowires.

III-V nanowires for optoelectronics

The optical and structural properties of binary and ternary III-V nanowires including GaAs, InP, In(Ga)As, Al(Ga)As, and GaAs(Sb) nanowires by metal-organic chemical vapour deposition are investigated, Au colloidal nanoparticles are employed to catalyze nanowire growth. Zinc blende or wurtzite crystal structures with some stacking faults are observed for these nanowires by high resolution transmission electron microscope. In addition, the properties of heterostructure nanowires including GaAs-AlGaAs core-shell nanowires, GaAs-InAs nanowires, and GaAs-GaSb nanowires are reported. Single nanowire luminescence properties from optically bright InP nanowires are reported. Interesting phenomena such as two-temperature procedure, nanowire height enhancement of isolated ternary InGaAs nanowires, kinking effect of InAs-GaAs heterostructure nanowires, and unusual growth property of GaAs-GaSb heterostructure nanowires are investigated. These nanowires will play an essential role in future optoelectronic devices.

Quantum dots and nanowires for optoelectronic device applications

InGaAs quantum dots (QDs) and nanowires have been grown on GaAs by metal-organic chemical vapour deposition on GaAs (100) and (111)B substrates, respectively. InGaAs QD lasers were fabricated and characterised. Results show ground-state lasing at about 1150 nm in devices with lengths greater than 2.5 mm. We also observed a strong influence of nanowire density on nanowire height specific to nanowires with high indium composition. This dependency was attributed to the large difference of diffusion length on (111)B surfaces between In and Ga reaction species, with In being the more mobile species. Selective area epitaxy for applications in quantum-dot optoelectronic device integration is also discussed in this paper. ©2006 IEEE.

Predictive control using an FPGA with application to aircraft control

Alternative and more efficient computational methods can extend the applicability of model predictive control (MPC) to systems with tight real-time requirements. This paper presents a system-on-a-chip MPC system, implemented on a field-programmable gate array (FPGA), consisting of a sparse structure-exploiting primal dual interior point (PDIP) quadratic program (QP) solver for MPC reference tracking and a fast gradient QP solver for steady-state target calculation. A parallel reduced precision iterative solver is used to accelerate the solution of the set of linear equations forming the computational bottleneck of the PDIP algorithm. A numerical study of the effect of reducing the number of iterations highlights the effectiveness of the approach. The system is demonstrated with an FPGA-in-the-loop testbench controlling a nonlinear simulation of a large airliner. This paper considers many more manipulated inputs than any previous FPGA-based MPC implementation to date, yet the implementation comfortably fits into a midrange FPGA, and the controller compares well in terms of solution quality and latency to state-of-the-art QP solvers running on a standard PC. © 1993-2012 IEEE.

User target intention recognition from cursor position using kalman filter

This paper discusses user target intention recognition algorithms for pointing - clicking tasks to reduce users' pointing time and difficulty. Predicting targets by comparing the bearing angles to targets proposed as one of the first algorithms [1] is compared with a Kalman Filter prediction algorithm. Accuracy and sensitivity of prediction are used as performance criteria. The outcomes of a standard point and click experiment are used for performance comparison, collected from both able-bodied and impaired users. © 2013 Springer-Verlag Berlin Heidelberg.

Catalyst composition and impurity-dependent kinetics of nanowire heteroepitaxy.

The mechanisms and kinetics of axial Ge-Si nanowire heteroepitaxial growth based on the tailoring of the Au catalyst composition via Ga alloying are studied by environmental transmission electron microscopy combined with systematic ex situ CVD calibrations. The morphology of the Ge-Si heterojunction, in particular, the extent of a local, asymmetric increase in nanowire diameter, is found to depend on the Ga composition of the catalyst, on the TMGa precursor exposure temperature, and on the presence of dopants. To rationalize the findings, a general nucleation-based model for nanowire heteroepitaxy is established which is anticipated to be relevant to a wide range of material systems and device-enabling heterostructures.

A self-adaptive genetic algorithm applied to multi-objective optimization of an airfoil

Genetic algorithms (GAs) have been used to tackle non-linear multi-objective optimization (MOO) problems successfully, but their success is governed by key parameters which have been shown to be sensitive to the nature of the particular problem, incorporating concerns such as the numbers of objectives and variables, and the size and topology of the search space, making it hard to determine the best settings in advance. This work describes a real-encoded multi-objective optimizing GA (MOGA) that uses self-adaptive mutation and crossover, and which is applied to optimization of an airfoil, for minimization of drag and maximization of lift coefficients. The MOGA is integrated with a Free-Form Deformation tool to manage the section geometry, and XFoil which evaluates each airfoil in terms of its aerodynamic efficiency. The performance is compared with those of the heuristic MOO algorithms, the Multi-Objective Tabu Search (MOTS) and NSGA-II, showing that this GA achieves better convergence.