Mode control in vertical-cavity surface-emitting lasers by post-processing using focused ion-beaih etching

Single-mode emission is achieved in previously multimode gain-guided vertical-cavity surface-emitting lasers (VCSEL's) by localized modification of the mirror reflectivity using focused ion-beam etching. Reflectivity engineering is also demonstrated to suppress transverse mode emission in an oxide-confined device, reducing the spectral width from 1.2 nm to less than 0.5 nm.

Comment on 'processing and microstructure of single grain uranium-doped Y-Ba-Cu-O superconductor'

We comment on the paper by N Hari Babu et al. (2002 Supercond. Sci. Technol. 15 104-10) and point out misinterpretations of the chemical composition of U-bearing deposits observed in Y123. The observed small deposits are those of new compounds which do not contain Cu, rather than refined Y211 plus U, as stated by the authors. We further note that extensive literature, not quoted, is in disagreement by nearly an order of magnitude concerning the values of Pt and U doping at which the optimum value of Jc is obtained. Other related information, presently in the literature, which may be helpful to those working with this high temperature superconducting chemical system, is presented.

The unsteady development of a turbulent wake through a downstream low-pressure turbine blade passage

This paper presents two-dimensional LDA measurements of the convection of a wake through a low-pressure (LP) turbine cascade. Previous studies have shown the wake convection to be kinematic but have not provided details of the turbulent field. The spatial resolution of these measurements has facilitated the calculation of the production of turbulent kinetic energy and this has revealed a mechanism for turbulence production as the wake converts through the bladerow. The measured ensemble-averaged velocity field confirmed the previously reported kinematics of wake convection while the measurements of the turbulence quantities showed the wake fluid to be characterised by elevated levels of turbulent kinetic energy (TKE) and to have an anisotropic structure. Based on the measured mean and turbulence quantities, the production of turbulent kinetic energy was calculated. This highlighted a TKE production mechanism that resulted in increased levels of turbulence over the rear suction surface where boundary layer transition occurs. The turbulence production mechanism within the bladerow was also observed to produce more nearly isotropic turbulence. Production occurs when the principal stresses within the wake are aligned with the mean strains. This coincides with the maximum distortion of the wake within the blade passage and provides a mechanism for the production of turbulence outside of the boundary layer.

Millisecond processing beyond chip technology: From electronics to photonics

There is a clear and increasing interest in short time annealing processing far below one second, i.e. the lower limit of Rapid Thermal Processing (RTP) called spike annealing. This was driven by the need of suppressing the so-called Transient Enhanced Diffusion in advanced boronimplanted shallow pn-junctions in silicon technology. Meanwhile the interest in flash lamp annealing (FLA) in the millisecond range spread out into other fields related to silicon technology and beyond. This paper reports on recent experiments regarding shallow junction engineering in germanium, annealing of ITO layers on glass and plastic foil to form an conductive layer as well as investigations which we did during the last years in the field of wide band gap semiconductor materials (SiC, ZnO). A more common feature evolving from our work was related to the modeling of wafer stress during millisecond thermal processing with flash lamps. Finally recent achievements in the field of silicon-based light emission basing on Metal-Oxide-Semiconductor Light Emitting Devices will be reported. © 2007 IEEE.

Advanced thermal processing of semiconductor materials in the msec-range

This paper reviews the advances that flash lamp annealing brings to the processing of the most frequently used semiconductor materials, namely silicon and silicon carbide, thus enabling the fabrication of novel microelectronic structures and materials. The paper describes how such developments can translate into important practical applications leading to a wide range of technological benefits. Opportunities in ultra-shallow junction formation, heteroepitaxial growth of thin films of cubic silicon carbide on silicon, and crystallization of amorphous silicon films, along with the technical reasons for using flash lamp annealing are discussed in the context of state-of-the-art materials processing. © 2005 IEEE.

Bayesian signal processing

The application of Bayes' Theorem to signal processing provides a consistent framework for proceeding from prior knowledge to a posterior inference conditioned on both the prior knowledge and the observed signal data. The first part of the lecture will illustrate how the Bayesian methodology can be applied to a variety of signal processing problems. The second part of the lecture will introduce the concept of Markov Chain Monte-Carlo (MCMC) methods which is an effective approach to overcoming many of the analytical and computational problems inherent in statistical inference. Such techniques are at the centre of the rapidly developing area of Bayesian signal processing which, with the continual increase in available computational power, is likely to provide the underlying framework for most signal processing applications.

Hot streak and vane coolant migration in a downstream rotor

This paper describes a method of improving the cooling of the hub region of high-pressure turbine (HPT) rotor by making better use of the unsteady coolant flows originating from the upstream vane. The study was performed computationally on an engine HPT stage with representative inlet hot streak and vane coolant conditions. An experimental validation study of hot streak migration was undertaken on two low-speed test facilities. The unsteady mechanisms that transport hot and cold fluid within the rotor hub region are first examined. It was found that vortex-blade interaction dominated the unsteady transport of hot and cold fluid in the rotor hub region. This resulted in the transport of hot fluid onto the rotor hub and pressure surface, causing a peak in the surface gas temperatures. The vane film coolant was found to have only a limited effect in cooling this region. A new cooling configuration was thus examined which exploits the unsteadiness in rotor hub to aid transport of coolant towards regions of high rotor surface temperatures. The new coolant was introduced from a slot upstream of the vane. This resulted in the feed of slot coolant at a different phase and location relative to the vane film coolant within the rotor. The slot coolant was entrained into the unsteady rotor secondary flows and transported towards the rotor hub-pressure surface region. The slot coolant reduced the peak time-averaged rotor temperatures by a similar amount as the vane film coolant despite having only a sixth of the coolant mass flow. Copyright © 2008 by ASME.

VGs for a normal SBLI with a downstream diffuser

New types of vortex generators for boundary layer control were investigated experimentally in a flow field which contains a Mach 1.4 normal Shockwave followed by a subsonic diffuser. A parametric study of device height and distance upstream of the normal shock was undertaken with two novel devices: ramped-vanes and split-ramps. Flowfield diagnostics included high-speed Schlieren, oil flow visualization, and pitot-static pressure measurements. A number of flowfield parameters including flow separation, pressure recovery, centerline incompressible boundary layer shape factor, and shock stability were analyzed and compared to the baseline. All configurations tested yielded an elimination of centerline flow separation with the presence of the vortex generators. However, the devices also tended to increase the three-dimensionality of the flow with increased side-wall interaction. When located 25δo upstream of the normal shock, the largest ramped-vane device (whose height was about 0.75 the incoming uncontrolled boundary layer thickness, δo) yielded the smallest centerline incompressible shape factor and the least streamwise oscillations of the normal shock. However, additional studies are needed to better understand the corner interaction effects, which are substantial. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.

A transonic vane for use downstream of a pressure gain combustor

Replacing a conventional combustor in a gas turbine with one that produces a pressure gain could significantly raise cycle efficiency. For this efficiency gain to be achieved the exit flow from the combustor must be coupled to the downstream turbine such that the pressure gain produced by the combustor is retained and such that the turbine efficiency is maintained. The exit flow from a pressure gain combustor will often contain a high velocity unsteady jet. It has previously been proposed that ejectors should be used to harness the energy in the unsteady jet, this paper proposes combining an ejector with the first stage vane, producing a single compact component that preserves the combustion driven pressure gain and delivers a suitable flow to the turbine so that its efficiency is not compromised. This novel component has been experimentally tested for the first time. The performance of this first prototype design is found to be low due to high levels of loss generated by secondary flows. However possible mitigation strategies are discussed. It is shown that the unsteadiness at exit form the ejector-vane is reduced compared to the inlet flow. If a pulse combustor were incorporated into a gas turbine, it is unlikely that the level of unsteadiness experienced in a downstream rotor will be significantly larger that that due to the periodic passing of upstream wakes. Copyright © 2010 by Jonathan Heffer.

RNA processing and Arabidopsis flowering time control.

Plants control their flowering time in order to ensure that they reproduce under favourable conditions. The components involved in this complex process have been identified using a molecular genetic approach in Arabidopsis and classified into genetically separable pathways. The autonomous pathway controls the level of mRNA encoding a floral repressor, FLC, and comprises three RNA-binding proteins, FCA, FPA and FLK. FCA interacts with the 3'-end RNA-processing factor FY to autoregulate its own expression post-transcriptionally and to control FLC. Other components of the autonomous pathway, FVE and FLD, regulate FLC epigenetically. This combination of epigenetic and post-transcriptional control gives precision to the control of FLC expression and flowering time.

Dissecting Arabidopsis thaliana DICER function in small RNA processing, gene silencing and DNA methylation patterning.

Small RNAs have several important biological functions. MicroRNAs (miRNAs) and trans-acting small interfering RNAs (tasiRNAs) regulate mRNA stability and translation, and siRNAs cause post-transcriptional gene silencing of transposons, viruses and transgenes and are important in both the establishment and maintenance of cytosine DNA methylation. Here, we study the role of the four Arabidopsis thaliana DICER-LIKE genes (DCL1-DCL4) in these processes. Sequencing of small RNAs from a dcl2 dcl3 dcl4 triple mutant showed markedly reduced tasiRNA and siRNA production and indicated that DCL1, in addition to its role as the major enzyme for processing miRNAs, has a previously unknown role in the production of small RNAs from endogenous inverted repeats. DCL2, DCL3 and DCL4 showed functional redundancy in siRNA and tasiRNA production and in the establishment and maintenance of DNA methylation. Our studies also suggest that asymmetric DNA methylation can be maintained by pathways that do not require siRNAs.

An ARGONAUTE4-containing nuclear processing center colocalized with Cajal bodies in Arabidopsis thaliana.

ARGONAUTE4 (AGO4) and RNA polymerase IV (Pol IV) are required for DNA methylation guided by 24 nucleotide small interfering RNAs (siRNAs) in Arabidopsis thaliana. Here we show that AGO4 localizes to nucleolus-associated bodies along with the Pol IV subunit NRPD1b; the small nuclear RNA (snRNA) binding protein SmD3; and two markers of Cajal bodies, trimethylguanosine-capped snRNAs and the U2 snRNA binding protein U2B''. AGO4 interacts with the C-terminal domain of NRPD1b, and AGO4 protein stability depends on upstream factors that synthesize siRNAs. AGO4 is also found, along with the DNA methyltransferase DRM2, throughout the nucleus at presumed DNA methylation target sites. Cajal bodies are conserved sites for the maturation of ribonucleoprotein complexes. Our results suggest a function for Cajal bodies as a center for the assembly of an AGO4/NRPD1b/siRNA complex, facilitating its function in RNA-directed gene silencing at target loci.

Role of RNA polymerase IV in plant small RNA metabolism.

In addition to the three RNA polymerases (RNAP I-III) shared by all eukaryotic organisms, plant genomes encode a fourth RNAP (RNAP IV) that appears to be specialized in the production of siRNAs. Available data support a model in which dsRNAs are generated by RNAP IV and RNA-dependent RNAP 2 (RDR2) and processed by DICER (DCL) enzymes into 21- to 24-nt siRNAs, which are associated with different ARGONAUTE (AGO) proteins for transcriptional or posttranscriptional gene silencing. However, it is not yet clear what fraction of genomic siRNA production is RNAP IV-dependent, and to what extent these siRNAs are preferentially processed by certain DCL(s) or associated with specific AGOs for distinct downstream functions. To address these questions on a genome-wide scale, we sequenced approximately 335,000 siRNAs from wild-type and RNAP IV mutant Arabidopsis plants by using 454 technology. The results show that RNAP IV is required for the production of >90% of all siRNAs, which are faithfully produced from a discrete set of genomic loci. Comparisons of these siRNAs with those accumulated in rdr2 and dcl2 dcl3 dcl4 and those associated with AGO1 and AGO4 provide important information regarding the processing, channeling, and functions of plant siRNAs. We also describe a class of RNAP IV-independent siRNAs produced from endogenous single-stranded hairpin RNA precursors.

FY is an RNA 3' end-processing factor that interacts with FCA to control the Arabidopsis floral transition

The nuclear RNA binding protein, FCA, promotes Arabidopsis reproductive development. FCA contains a WW protein interaction domain that is essential for FCA function. We have identified FY as a protein partner for this domain. FY belongs to a highly conserved group of eukaryotic proteins represented in Saccharomyces cerevisiae by the RNA 3' end-processing factor, Pfs2p. FY regulates RNA 3' end processing in Arabidopsis as evidenced through its role in FCA regulation. FCA expression is autoregulated through the use of different polyadenylation sites within the FCA pre-mRNA, and the FCA/FY interaction is required for efficient selection of the promoter-proximal polyadenylation site. The FCA/FY interaction is also required for the downregulation of the floral repressor FLC. We propose that FCA controls 3' end formation of specific transcripts and that in higher eukaryotes, proteins homologous to FY may have evolved as sites of association for regulators of RNA 3' end processing.