4,526 research outputs found
Optical bistability in Er-Yb co-doped phosphate glass microspheres at room temperature
We experimentally demonstrate optical bistability in Er3+-Yb3+ phosphate
glass microspheres at 295 K. Bistability is associated with both Er3+
fluorescence and lasing behavior, and chromatic switching. The chromatic
switching results from an intrinsic mechanism exploiting the thermal coupling
of closely-spaced energy levels, and occurs simultaneously with the intensity
switching. A contrast ratio of 3.2 has been obtained for chromatic switching,
and the intensity switching shows ratios of 2.4 for 550 nm and, 1.8 for the 660
nm fluorescence emissions, and 11 for the IR lasing at 1.5 um. Concurrent with
these observations, we investigate a temperature dependent absorption of pump
power which exhibits bistable behavior. The influences of the host matrix on
lasing and fluorescence mechanisms are highlighted.Comment: 22 pages, 6 figure
Bayesian Analysis of Solar Oscillations
A Bayesian probability based approach is applied to the problem of detecting
and parameterizing oscillations in the upper solar atmosphere for the first
time. Due to its statistical origin, this method provides a mechanism for
determining the number of oscillations present, gives precise estimates of the
oscillation parameters with a self-consistent statistical error analysis, and
allows the oscillatory model signals to be reconstructed within these errors.
A highly desirable feature of the Bayesian approach is the ability to resolve
oscillations with extremely small frequency separations. The code is applied to
SOHO/CDS (Solar and Heliospheric Observatory/Coronal Diagnostic Spectrometer) O
V 629A observations and resolves four distinct P4, P5, P6 and P7 p-modes within
the same sunspot transition region. This suggests that a spectrum of
photospheric p-modes is able to propagate into the upper atmosphere of the Sun
and Sun-like stars, and places precise observational constraints on models of
umbral eigen modes.Comment: Accepted for publication in The Astrophysical Journal. It is
scheduled for the ApJ July 20, 2008, v682n 1 issu
Disruption of host-seeking behaviour by the salmon louse,Lepeophtheirus salmonis,using botanically derived repellents
The potential for developing botanically derived natural products as novel feed-through repellents for disrupting settlement of the salmon louse, Lepeophtheirus salmonis (Caligidae) upon farmed Atlantic salmon, Salmo salar, was investigated using an established laboratory vertical Y-tube behavioural bioassay for assessing copepodid behaviour. Responses to artificial sea water conditioned with the odour of salmon, or to the known salmon-derived kairomone component, α-isophorone, in admixture with selected botanical materials previously known to interfere with invertebrate arthropod host location were recorded. Materials included oils extracted from garlic, Allium sativum (Amaryllidaceae), rosemary, Rosmarinus officinalis (Lamiaceae), lavender, Lavandula angustifolia (Lamiaceae), and bog myrtle, Myrica gale (Myricaceae), and individual components (diallyl sulphide and diallyl disulphide from garlic; allyl, propyl, butyl, 4-pentenyl and 2-phenylethyl isothiocyanate from plants in the Brassica genus). Removal of attraction to salmon-conditioned water (SCW) or α-isophorone was observed when listed materials were presented at extremely low parts per trillion (ppt), that is picograms per litre or 10−12 level. Significant masking of attraction to SCW was observed at a level of 10 ppt for diallyl disulphide and diallyl sulphide, and allyl isothiocyanate and butyl isothiocyanate. The potential of very low concentrations of masking compounds to disrupt Le. salmonis copepodid settlement on a host fish has been demonstrated in vitro
Edge pixel response studies of edgeless silicon sensor technology for pixellated imaging detectors
Silicon sensor technologies with reduced dead area at the sensor's perimeter are under development at a number of institutes. Several fabrication methods for sensors which are sensitive close to the physical edge of the device are under investigation utilising techniques such as active-edges, passivated edges and current-terminating rings. Such technologies offer the goal of a seamlessly tiled detection surface with minimum dead space between the individual modules. In order to quantify the performance of different geometries and different bulk and implant types, characterisation of several sensors fabricated using active-edge technology were performed at the B16 beam line of the Diamond Light Source. The sensors were fabricated by VTT and bump-bonded to Timepix ROICs. They were 100 and 200 μ m thick sensors, with the last pixel-to-edge distance of either 50 or 100 μ m. The sensors were fabricated as either n-on-n or n-on-p type devices. Using 15 keV monochromatic X-rays with a beam spot of 2.5 μ m, the performance at the outer edge and corners pixels of the sensors was evaluated at three bias voltages. The results indicate a significant change in the charge collection properties between the edge and 5th (up to 275 μ m) from edge pixel for the 200 μ m thick n-on-n sensor. The edge pixel performance of the 100 μ m thick n-on-p sensors is affected only for the last two pixels (up to 110 μ m) subject to biasing conditions. Imaging characteristics of all sensor types investigated are stable over time and the non-uniformities can be minimised by flat-field corrections. The results from the synchrotron tests combined with lab measurements are presented along with an explanation of the observed effects
Ballistic-electron-emission spectroscopy of Al<SUB>x</SUB>Ga<SUB>1-x</SUB>As/GaAs heterostructures: conduction-band offsets, transport mechanisms, and band-structure effects
We report an extensive investigation of semiconductor band-structure effects in
single-barrier AlxGa1-xAs/GaAs heterostructures using
ballistic-electron-emission spectroscopy (BEES). The transport mechanisms in these single-barrier
structures were studied systematically as a function of temperature and Al composition over the full
compositional range (0≤x≤1). The initial (Γ) BEES thresholds for
AlxGa1-xAs single barriers with 0≤x≤0.42 were extracted using a
model which includes the complete transmission probability of the metal-semiconductor interface and
the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous
measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements
at 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to
the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected
bias-induced band-bending. In the indirect band-gap regime (x>0.45), spectra show a weak
ballistic-electron-emission microscopy current contribution due to intervalley scattering through
AlxGa1-xAs X valley states. Low-temperature spectra show a marked reduction in
this intervalley current component, indicating that intervalley phonon scattering at the
GaAs/AlxGa1-xAs interface produces a significant fraction of this X valley
current. A comparison of the BEES thresholds with the expected composition dependence of the
AlxGa1-xAs Γ, L, and X points yields good agreement over the entire
composition range
Diffractive triangulation of radiative point sources
We describe a general method to determine the location of a point source of waves relative to a twodimensional
single-crystalline active pixel detector. Based on the inherent structural sensitivity of
crystalline sensor materials, characteristic detector diffraction patterns can be used to triangulate the
location of a wave emitter. The principle described here can be applied to various types of waves,
provided that the detector elements are suitably structured. As a prototypical practical application of
the general detection principle, a digital hybrid pixel detector is used to localize a source of electrons
for Kikuchi diffraction pattern measurements in the scanning electron microscope. This approach
provides a promising alternative method to calibrate Kikuchi patterns for accurate measurements of
microstructural crystal orientations, strains, and phase distributions
Regulation of Chromatin Remodeling by Inositol Polyphosphates
Chromatin remodeling is required for efficient transcription of eukaryotic genes. In a genetic selection for budding yeast mutants that were defective in induction of the phosphate-responsive PHO5 gene, we identified mutations inARG82/IPK2, which encodes a nuclear inositol polyphosphate kinase. In arg82 mutant strains, remodeling ofPHO5 promoter chromatin is impaired, and the adenosine triphosphate–dependent chromatin-remodeling complexes SWI/SNF and INO80 are not efficiently recruited to phosphate-responsive promoters. These results suggest a role for the small molecule inositol polyphosphate in the regulation of chromatin remodeling and transcription
ADGO 2.0: interpreting microarray data and list of genes using composite annotations
ADGO 2.0 is a web-based tool that provides composite interpretations for microarray data comparing two sample groups as well as lists of genes from diverse sources of biological information. Some other tools also incorporate composite annotations solely for interpreting lists of genes but usually provide highly redundant information. This new version has the following additional features: first, it provides multiple gene set analysis methods for microarray inputs as well as enrichment analyses for lists of genes. Second, it screens redundant composite annotations when generating and prioritizing them. Third, it incorporates union and subtracted sets as well as intersection sets. Lastly, users can upload their own gene sets (e.g. predicted miRNA targets) to generate and analyze new composite sets. The first two features are unique to ADGO 2.0. Using our tool, we demonstrate analyses of a microarray dataset and a list of genes for T-cell differentiation. The new ADGO is available at http://www.btool.org/ADGO2
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