3,797 research outputs found
Low loss Ge-on-Si waveguides operating in the 8â14 ”m atmospheric transmission window
Germanium-on-silicon waveguides were modeled, fabricated and characterized at wavelengths ranging from 7.5 to 11 ”m. Measured waveguide losses are below 5 dB/cm for both TE and TM polarization and reach values of ⌠1 dB/cm for ℠10 ”m wavelengths for the TE polarization. This work demonstrates experimentally for the first time that Ge-on-Si is a viable waveguide platform for sensing in the molecular fingerprint spectral region. Detailed modeling and analysis is presented to identify the various loss contributions, showing that with practical techniques losses below 1 dB/cm could be achieved across the full measurement range
The adsorption and desorption of ethanol ices from a model grain surface
Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures > 50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3 +/- 1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K. (c) 2008 American Institute of Physics
An investigation into the use of Prolog for Chinese-English translation
In order to undertake machine translation from Chinese to English, it is necessary to accomplish three tasks. Firstly, the Chinese characters need to be handled on the computer system in use - in this case an IBM PC/XT. Secondly, the grammar for the language has to be represented and here Prolog has been used with a Definite Clause Grammar. Finally, the lexicon must be stored in a manner that facilitates efficient retrieval. Arity Prolog provides a hash table that achieves this task. This report describes the current state of a project aimed at producing a Chinese English machine translation system
Universal Scaling of Optimal Current Distribution in Transportation Networks
Transportation networks are inevitably selected with reference to their
global cost which depends on the strengths and the distribution of the embedded
currents. We prove that optimal current distributions for a uniformly injected
d-dimensional network exhibit robust scale-invariance properties, independently
of the particular cost function considered, as long as it is convex. We find
that, in the limit of large currents, the distribution decays as a power law
with an exponent equal to (2d-1)/(d-1). The current distribution can be exactly
calculated in d=2 for all values of the current. Numerical simulations further
suggest that the scaling properties remain unchanged for both random injections
and by randomizing the convex cost functions.Comment: 5 pages, 5 figure
Fine structure in the gamma-ray sky
The EGRET results for gamma-ray intensities in and near the Galactic Plane
have been analysed in some detail. Attention has been concentrated on energies
above 1 GeV and the individual intensities in a longitude bin have
been determined and compared with the large scale mean found from a nine-degree
polynomial fit.
Comparison has been made of the observed standard deviation for the ratio of
these intensities with that expected from variants of our model. The basic
model adopts cosmic ray origin from supernova remnants, the particles then
diffusing through the Galaxy with our usual 'anomalous diffusion'. The variants
involve the clustering of SN, a frequency distribution for supernova explosion
energies, and 'normal', rather than 'anomalous' diffusion.
It is found that for supernovae of unique energy, and our usual anomalous
diffusion, clustering is necessary, particularly in the Inner Galaxy. An
alternative, and preferred, situation is to adopt the model with a frequency
distribution of supernova energies. The results for the Outer Galaxy are such
that no clustering is required.Comment: 10 pages, 4 figures, 1 table, accepted for publication in J.Phys.G:
Nucl.Part.Phy
Linking habitat mosaics and connectivity in a coral reef seascape
Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 109 (2012): 15372-15376, doi:10.1073/pnas.1206378109.Tropical marine ecosystems are under mounting anthropogenic pressure from overfishing and habitat destruction, leading to declines in their structure and function on a global scale. While maintaining connectivity among habitats within a seascape is necessary for preserving population resistance and resilience, quantifying movements of individuals within seascapes remains challenging. Traditional methods of identifying and valuing potential coral reef fish nursery habitats are indirect, often relying on visual surveys of abundance and correlations of size and biomass among habitats. We used compound-specific stable isotope analyses to determine movement patterns of commercially important fish populations within a coral reef seascape. This approach allowed us to quantify the relative contributions of individuals from inshore nurseries to reef populations and identify migration corridors among important habitats. Our results provided direct measurements of remarkable migrations by juvenile snapper of over 30 km between nurseries and reefs. We also found significant plasticity in juvenile nursery residency. While a majority of individuals on coastal reefs had used seagrass nurseries as juveniles, many adults on oceanic reefs had settled directly into reef habitats. Moreover, seascape configuration played a critical but heretofore unrecognized role in determining connectivity among habitats. Finally, our approach provides key quantitative data necessary to estimate the value of distinctive habitats to ecosystem services provided by seascapes.This research was based on work supported by Award Nos. USA 00002 and KSA 00011 made by King Abdullah University of Science and Technology (KAUST). Additional funding was provided by Woods Hole Oceanographic Institution and an International Society for Reef Studies-Ocean Conservancy Coral Reef Fellowship. K. McMahon received support from the National Science Foundation Graduate Research Fellowship Program
Failure to up-regulate VEGF165b in maternal plasma is a first trimester predictive marker for pre-eclampsia
Pre-eclampsia is a pregnancy-related condition characterized by hypertension,
proteinuria and endothelial dysfunction. VEGF165b, formed by
alternative splicing of VEGF (vascular endothelial growth factor) pre-mRNA,
inhibits VEGF165-mediated vasodilation and angiogenesis, but has not
been quantified in pregnancy. ELISAs were used to measure
means±S.E.M. plasma VEGF165b, sEng (soluble endoglin) and
sFlt-1 (soluble fms-like tyrosine kinase-1). At 12Â weeks of
gestation, the plasma VEGF165b concentration was significantly
up-regulated in plasma from women who maintained normal blood pressure
throughout their pregnancy (normotensive group,
4.90±1.6 ng/ml; P<0.01, as
determined using a Mann-Whitney U test) compared with
non-pregnant women (0.40±0.22 ng/ml). In contrast, in
patients who later developed pre-eclampsia, VEGF165b levels were
lower than in the normotensive group (0.467±0.209 ng/ml),
but were no greater than non-pregnant women. At term, plasma VEGF165b
concentrations were greater than normal in both pre-eclamptic
(3.75±2.24 ng/ml) and normotensive
(10.58 ng/ml±3.74 ng/ml;
P>0.1 compared with pre-eclampsia) pregnancies.
Patients with a lower than median plasma VEGF165b at
12Â weeks had elevated sFlt-1 and sEng pre-delivery. Concentrations of
sFlt-1 (1.20±0.07 and 1.27±0.18 ng/ml) and sEng
(4.4±0.18 and 4.1±0.5 ng/ml) were similar at
12Â weeks of gestation in the normotensive and pre-eclamptic groups
respectively. Plasma VEGF165b levels were elevated in pregnancy, but
this increase is delayed in women that subsequently develop pre-eclampsia. In
conclusion, low VEGF165b may therefore be a clinically useful first
trimester plasma marker for increased risk of pre-eclampsia
- âŠ