62 research outputs found
Comparative study of Cl₂, Cl₂/O₂, and Cl₂/N₂ inductively coupled plasma processes for etching of high-aspect-ratio photonic-crystal holes in InP
An extensive investigation has been performed on inductively coupled plasmaetching of InP. An important motivation for this work is the fabrication of high-aspect-ratio holes for photonic crystals. The essential chemistry is based on Cl₂ with the addition of N₂ or O₂ for sidewall passivation. The influence of different process parameters such as gas flows, temperature,pressure, ion energy, and inductively coupled plasma power on the hole geometry is presented. It is concluded that photonic crystals can be etched with Cl₂ only; however, temperature and pressure control is critical. Adding passivation gases largely broadens the window in the parameter space for hole etching. Most importantly, etching of narrow holes can be carried out at higher temperatures where the etching is mass limited and spontaneous etching of InP by Cl₂ occurs.Part of this research is supported by NanoNed, a technology program of the Dutch
Ministry of Economic Affairs
The DIRTY Model II: Self-Consistent Treatment of Dust Heating and Emission in a 3-D Radiative Transfer Code
In this paper and a companion paper we present the DIRTY model, a Monte Carlo
radiative transfer code, self-consistently including dust heating and emission,
and accounting for the effects of the transient heating of small grains. The
code is completely general; the density structure of the dust, the number and
type of heating sources, and their geometric configurations can be specified
arbitrarily within the model space. Source photons are tracked through the
scattering and absorbing medium using Monte Carlo techniques and the effects of
multiple scattering are included. The dust scattering, absorbing, and emitting
properties are calculated from realistic dust models derived by fitting
observed extinction curves in Local Group galaxies including the Magellanic
Clouds and the Milky Way. The dust temperature and the emitted dust spectrum
are calculated self consistently from the absorbed energy including the effects
of temperature fluctuations in small grains. Dust self-absorption is also
accounted for, allowing the treatment of high optical depths, by treating
photons emitted by the dust as an additional heating source and adopting an
iterative radiative transfer scheme. As an illustrative case, we apply the
DIRTY radiative transfer code to starburst galaxies wherein the heating sources
are derived from stellar evolutionary synthesis models. Within the context of
the starburst model, we examine the dependence of the UV to FIR SED, dust
temperatures, and dust masses predicted by DIRTY on variations of the input
parameters.Comment: 23 pages (emulateapj, single column), 17 figures. To appear in the
ApJ, in pres
Validity and responsiveness of the Clubfoot Assessment Protocol (CAP). A methodological study
BACKGROUND: The Clubfoot Assessment Protocol (CAP) is a multi dimensional instrument designed for longitudinal follow up of the clubfoot deformity during growth. Item reliability has shown to be sufficient. In this article the CAP's validity and responsiveness is studied using the Dimeglio classification scoring as a gold standard. METHODS: Thirty-two children with 45 congenital clubfeet were assessed prospectively and consecutively at ages of new-born, one, two, four months and two years of age. For convergent/divergent construct validity the Spearman's correlation coefficients were calculated. Discriminate validity was evaluated by studying the scores in bilateral clubfeet. The floor-ceiling effects at baseline (untreated clubfeet) and at two years of age (treated clubfeet) were evaluated. Responsiveness was evaluated by using effect sizes (ES) and by calculating if significant changes (Wilcoxons signed test) had occurred between the different measurement occasions. RESULTS: High to moderate significant correlation were found between CAP mobility I and morphology and the Dimeglio scores (r(s )= 0.77 and 0.44 respectively). Low correlation was found between CAP muscle function, mobility II and motion quality and the Dimeglio scoring system (r(s )= 0.20, 0.09 and 0.06 respectively). Of 13 children with bilateral clubfeet, 11 showed different CAP mobility I scores between right and left foot at baseline (untreated) compared with 5 with the Dimeglio score. At the other assessment occasions the CAP mobility I continued to show higher discrimination ability than the Dimeglio. No floor effects and low ceiling effects were found in the untreated clubfeet for both instruments. High ceiling effects were found in the CAP for the treated children and low for the Dimeglio. Responsiveness was good. ES from untreated to treated ranged from 0.80 to 4.35 for the CAP subgroups and was 4.68 for the Dimeglio. The first four treatment months, the CAP mobility I had generally higher ES compared with the Dimeglio. CONCLUSION: The Clubfoot Assessment Protocol shows in this study good validity and responsiveness. The CAP is more responsive when severity ranges between mild – moderate to severe, while the Dimeglio focuses more on the extremes. The ability to discriminate between different mobility status of the right and left foot in bilaterally affected children in this population was higher compared with the Dimeglio score implicating a better sensitivity for the CAP
Molecular velocity auto-correlation of simple liquids observed by NMR MGSE method
The velocity auto-correlation spectra of simple liquids obtained by the NMR
method of modulated gradient spin echo show features in the low frequency range
up to a few kHz, which can be explained reasonably well by a long
time tail decay only for non-polar liquid toluene, while the spectra of polar
liquids, such as ethanol, water and glycerol, are more congruent with the model
of diffusion of particles temporarily trapped in potential wells created by
their neighbors. As the method provides the spectrum averaged over ensemble of
particle trajectories, the initial non-exponential decay of spin echoes is
attributed to a spatial heterogeneity of molecular motion in a bulk of liquid,
reflected in distribution of the echo decays for short trajectories. While at
longer time intervals, and thus with longer trajectories, heterogeneity is
averaged out, giving rise to a spectrum which is explained as a combination of
molecular self-diffusion and eddy diffusion within the vortexes of hydrodynamic
fluctuations.Comment: 8 pages, 6 figur
Fitting the integrated Spectral Energy Distributions of Galaxies
Fitting the spectral energy distributions (SEDs) of galaxies is an almost
universally used technique that has matured significantly in the last decade.
Model predictions and fitting procedures have improved significantly over this
time, attempting to keep up with the vastly increased volume and quality of
available data. We review here the field of SED fitting, describing the
modelling of ultraviolet to infrared galaxy SEDs, the creation of
multiwavelength data sets, and the methods used to fit model SEDs to observed
galaxy data sets. We touch upon the achievements and challenges in the major
ingredients of SED fitting, with a special emphasis on describing the interplay
between the quality of the available data, the quality of the available models,
and the best fitting technique to use in order to obtain a realistic
measurement as well as realistic uncertainties. We conclude that SED fitting
can be used effectively to derive a range of physical properties of galaxies,
such as redshift, stellar masses, star formation rates, dust masses, and
metallicities, with care taken not to over-interpret the available data. Yet
there still exist many issues such as estimating the age of the oldest stars in
a galaxy, finer details ofdust properties and dust-star geometry, and the
influences of poorly understood, luminous stellar types and phases. The
challenge for the coming years will be to improve both the models and the
observational data sets to resolve these uncertainties. The present review will
be made available on an interactive, moderated web page (sedfitting.org), where
the community can access and change the text. The intention is to expand the
text and keep it up to date over the coming years.Comment: 54 pages, 26 figures, Accepted for publication in Astrophysics &
Space Scienc
Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains
We present a method for calculating the infrared emission from a population
of dust grains heated by starlight, including very small grains for which
stochastic heating by starlight photons results in high temperature transients.
Because state-to-state transition rates are generally unavailable for complex
molecules, we consider model PAH, graphitic, and silicate grains with realistic
vibrational mode spectra and realistic radiative properties. The vibrational
density of states is used in a statistical-mechanical description of the
emission process. Unlike previous treatments, our approach fully incorporates
multiphoton heating effects, important for large grains or strong radiation
fields. We discuss how the "temperature" of the grain is related to its
vibrational energy. By comparing with an "exact" statistical calculation of the
emission process, we determine the conditions under which the "thermal" and the
"continuous cooling" approximations can be used to calculate the emission
spectrum.
We present results for the infrared emission spectra of PAH grains of various
sizes heated by starlight. We show how the relative strengths of the 6.2, 7.7,
and 11.3um features depend on grain size, starlight spectrum and intensity, and
grain charging conditions. We show results for grains in the "cold neutral
medium", "warm ionized medium", and representative conditions in
photodissociation regions. Our model results are compared to observed ratios of
emission features for reflection nebulae and photodissociation regions, the
Milky Way, normal spiral galaxies, and starburst galaxies.Comment: Submitted to ApJ. 42 pages, 18 figures, Late
Interstellar Grains -- The 75th Anniversary
The year of 2005 marks the 75th anniversary since Trumpler (1930) provided
the first definitive proof of interstellar grains by demonstrating the
existence of general absorption and reddening of starlight in the galactic
plane. This article reviews our progressive understanding of the nature of
interstellar dust.Comment: invited review article for the "Light, Dust and Chemical Evolution"
conference (Gerace, Italy, 26--30 September 2004), edited by F. Borghese and
R. Saija, 2005, in pres
Complexity absorption : A processual strategic approach to corporate entrepreneurship strategy
The author Haina Zhang acknowledges the support of research grant received from the Lancaster University Research Committee (award reference: SGS21/33). The author is also grateful to Professor David Brown for his suggestions on earlier version of this paper.Peer reviewedPublisher PD
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