Mechanism of enhanced light output in InGaN-based microlight emitting diodes

Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output

Systematic characterization of thermodynamic and dynamical phase behavior in systems with short-ranged attraction

In this paper we demonstrate the feasibility and utility of an augmented version of the Gibbs ensemble Monte Carlo method for computing the phase behavior of systems with strong, extremely short-ranged attractions. For generic potential shapes, this approach allows for the investigation of narrower attractive widths than those previously reported. Direct comparison to previous self-consistent Ornstein-Zernike approximation calculations are made. A preliminary investigation of out-of-equilibrium behavior is also performed. Our results suggest that the recent observations of stable cluster phases in systems without long-ranged repulsions are intimately related to gas-crystal and metastable gas-liquid phase separation.Comment: 10 pages, 8 figure

InGaN nano-ring structures for high-efficiency light emitting diodes

A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated

Fluctuation limits of strongly degenerate branching systems

Functional limit theorems for scaled fluctuations of occupation time processes of a sequence of critical branching particle systems in $\R^d$ with anisotropic space motions and strongly degenerated splitting abilities are proved in the cases of critical and intermediate dimensions. The results show that the limit processes are constant measure-valued Wienner processes with degenerated temporal and simple spatial structures.Comment: 15 page

Fault-tolerant quantum computation with high threshold in two dimensions

We present a scheme of fault-tolerant quantum computation for a local architecture in two spatial dimensions. The error threshold is 0.75% for each source in an error model with preparation, gate, storage and measurement errors.Comment: 4 pages, 4 figures; v2: A single 2D layer of qubits (simple square lattice) with nearest-neighbor translation-invariant Ising interaction suffices. Slightly improved threshol

TTCPR: a PMC receiver for TTC

Abstract The TTCPR receiver is a mezzanine card intended for use in distributing TTC information to Data Acquisition and Trigger Crates in the ATLAS Prototype Integration activities. An original prototype run of these ~cards was built for testbeam and integration studies, implemented in both the PMC and PCI form factors, using the TTCrx chips from the previous manufacture. When the new TTCrx chips became available, the TTCPR was redesigned to take advantage of the availability and enhanced features of the new TTCRX(1), and a run of 20 PMC cards was manufactured, and has since been used in integration studies and the testbeam. The TTCPR uses the AMCC 5933(2) to manage the PCI port, an Altera 10K30A(3) to provide all the logic so that the functionality may be easily altered, and provides a 4K deep FIFO to retain TTC data for subsequent DMA through the PCI port. In addition to DMA's which are mastered by the Add On logic, communication through PCI is accomplished via mailboxes, interrupts, and the pass-through feature of the 5933. An interface to the I2C bus of the TTCRX is provided so that internal registers may be accessed, and the card supports reinitialization of the TTCRX from PCI. Software has been developed to support operation of the TTCPR under both LynxOS and Linux. I. History of the TTCPR The TTCPR was developed in response to a need for TTC(4) information in the Data Acquisition from TileCal Modules in the ATLAS Test Beam. Specifically, it was desired to have EventID, Bunch Counter, and Trigger Type available from TTC in the data records. It was useful to have the TTC information available to processors in the Data Acquisition crates through PCI ports, and to have the data transferred to the processor's address space via an externally mastered DMA. Accordingly, the TTCPR was designed as a mezzanine card in the PMC form factor. The original cards utilized the older nonradhard version of the TTCRX, because the new radhard version was not available at that time. When it became clear that the new TTCRX would be available soon and also that it would not be possible to obtain any more of the older TTCRX chips, the TTCPR was redesigned, and enhancements were added to take advantage of the features of the new TTCRX. This new TTCPR was produced and has been used successfully in data acquisition at the ATLAS Test Beam. The card has also been implemented in the PCI form factor. The TTCPR in the PMC version is shown in II. Architecture of the TTCPR A block diagram of the TTCPR is shown i

Performance of AAOmega: the AAT multi-purpose fibre-fed spectrograph

AAOmega is the new spectrograph for the 2dF fibre-positioning system on the Anglo-Australian Telescope. It is a bench-mounted, double-beamed design, using volume phase holographic (VPH) gratings and articulating cameras. It is fed by 392 fibres from either of the two 2dF field plates, or by the 512 fibre SPIRAL integral field unit (IFU) at Cassegrain focus. Wavelength coverage is 370 to 950nm and spectral resolution 1,000-8,000 in multi-Object mode, or 1,500-10,000 in IFU mode. Multi-object mode was commissioned in January 2006 and the IFU system will be commissioned in June 2006. The spectrograph is located off the telescope in a thermally isolated room and the 2dF fibres have been replaced by new 38m broadband fibres. Despite the increased fibre length, we have achieved a large increase in throughput by use of VPH gratings, more efficient coatings and new detectors - amounting to a factor of at least 2 in the red. The number of spectral resolution elements and the maximum resolution are both more than doubled, and the stability is an order of magnitude better. The spectrograph comprises: an f/3.15 Schmidt collimator, incorporating a dichroic beam-splitter; interchangeable VPH gratings; and articulating red and blue f/1.3 Schmidt cameras. Pupil size is 190mm, determined by the competing demands of cost, obstruction losses, and maximum resolution. A full suite of VPH gratings has been provided to cover resolutions 1,000 to 7,500, and up to 10,000 at particular wavelengths.Comment: 13 pages, 4 figures; presented at SPIE, Astronomical Telescopes and Instrumentation, 24 - 31 May 2006, Orlando, Florida US

Electronic structure of the muonium center as a shallow donor in ZnO

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a [0001] axis, indicating that they are located at the AB_{O,//} and BC_{//} sites. It is inferred from their small ionization energy (~6 meV and 50 meV) and hyperfine parameters (~10^{-4} times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.Comment: 4 pages, 4 figures, submitted to PR