1,224 research outputs found
Next-To-Leading Order Determination of Fragmentation Functions
We analyse LEP and PETRA data on single inclusive charged hadron
cross-sections to establish new sets of Next-to-Leading order Fragmentation
Functions. Data on hadro-production of large- hadrons are also used
to constrain the gluon Fragmentation Function. We carry out a critical
comparison with other NLO parametrizations
Resonant excitonic emission of a single quantum dot in the Rabi regime
We report on coherent resonant emission of the fundamental exciton state in a
single semiconductor GaAs quantum dot. Resonant regime with picoseconde laser
excitation is realized by embedding the quantum dots in a waveguiding
structure. As the pulse intensity is increased, Rabi oscillation is observed up
to three periods. The Rabi regime is achieved owing to an enhanced light-matter
coupling in the waveguide. This is due to a \emph{slow light effect}
(), occuring when an intense resonant pulse propagates in a
medium. The resonant control of the quantum dot fundamental transition opens
new possibilities in quantum state manipulation and quantum optics experiments
in condensed matter physics.Comment: Submitted to Phys. Rev. Let
Penetration and cratering experiments of graphite by 0.5-mm diameter steel spheres at various impact velocities
Cratering experiments have been conducted with 0.5-mm diameter AISI 52100 steel spherical projectiles and 30-mm diameter, 15-mm long graphite targets. The latter were made of a commercial grade of polycrystalline and porous graphite named EDM3 whose behavior is known as macroscopically isotropic. A two-stage light-gas gun launched the steel projectiles at velocities between 1.1 and 4.5 km s 1. In most cases, post-mortem tomographies revealed that the projectile was trapped, fragmented or not, inside the target. It showed that the apparent crater size and depth increase with the impact velocity. This is also the case of the crater volume which appears to follow a power law significantly different from those constructed in previous works for similar impact conditions and materials. Meanwhile, the projectile depth of penetration starts to decrease at velocities beyond 2.2 km s 1. This is firstly because of its plastic deformation and then, beyond 3.2 km s 1, because of its fragmentation. In addition to these three regimes of penetration behavior already described by a few authors, we suggest a fourth regime in which the projectile melting plays a significant role at velocities above 4.1 km s 1. A discussion of these four regimes is provided and indicates that each phenomenon may account for the local evolution of the depth of penetration
Polarization measurements analysis II. Best estimators of polarization fraction and angle
With the forthcoming release of high precision polarization measurements,
such as from the Planck satellite, it becomes critical to evaluate the
performance of estimators for the polarization fraction and angle. These two
physical quantities suffer from a well-known bias in the presence of
measurement noise, as has been described in part I of this series. In this
paper, part II of the series, we explore the extent to which various estimators
may correct the bias. Traditional frequentist estimators of the polarization
fraction are compared with two recent estimators: one inspired by a Bayesian
analysis and a second following an asymptotic method. We investigate the
sensitivity of these estimators to the asymmetry of the covariance matrix which
may vary over large datasets. We present for the first time a comparison among
polarization angle estimators, and evaluate the statistical bias on the angle
that appears when the covariance matrix exhibits effective ellipticity. We also
address the question of the accuracy of the polarization fraction and angle
uncertainty estimators. The methods linked to the credible intervals and to the
variance estimates are tested against the robust confidence interval method.
From this pool of estimators, we build recipes adapted to different use-cases:
build a mask, compute large maps, and deal with low S/N data. More generally,
we show that the traditional estimators suffer from discontinuous distributions
at low S/N, while the asymptotic and Bayesian methods do not. Attention is
given to the shape of the output distribution of the estimators, and is
compared with a Gaussian. In this regard, the new asymptotic method presents
the best performance, while the Bayesian output distribution is shown to be
strongly asymmetric with a sharp cut at low S/N.Finally, we present an
optimization of the estimator derived from the Bayesian analysis using adapted
priors
Camera orientation, calibration and inverse perspective with uncertainties: A Bayesian method applied to area estimation from diverse photographs
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordSome map data is copyrighted by OpenStreetMap contributors and available from https://www.openstreetmap.org.Large collections of images have become readily available through modern digital catalogs, from sources as diverse as historical photographs, aerial surveys, or user-contributed pictures. Exploiting the quantitative information present in such wide-ranging collections can greatly benefit studies that follow the evolution of landscape features over decades, such as measuring areas of glaciers to study their shrinking under climate change. However, many available images were taken with low-quality lenses and unknown camera parameters. Useful quantitative data may still be extracted, but it becomes important to both account for imperfect optics, and estimate the uncertainty of the derived quantities. In this paper, we present a method to address both these goals, and apply it to the estimation of the area of a landscape feature traced as a polygon on the image of interest. The technique is based on a Bayesian formulation of the camera calibration problem. First, the probability density function (PDF) of the unknown camera parameters is determined for the image, based on matches between 2D (image) and 3D (world) points together with any available prior information. In a second step, the posterior distribution of the feature area of interest is derived from the PDF of camera parameters. In this step, we also model systematic errors arising in the polygon tracing process, as well as uncertainties in the digital elevation model. The resulting area PDF therefore accounts for most sources of uncertainty. We present validation experiments, and show that the model produces accurate and consistent results. We also demonstrate that in some cases, accounting for optical lens distortions is crucial for accurate area determination with consumer-grade lenses. The technique can be applied to many other types of quantitative features to be extracted from photographs when careful error estimation is important.Agence Nationale de la Recherche (ANR
Local disorder and optical properties in V-shaped quantum wires : towards one-dimensional exciton systems
The exciton localization is studied in GaAs/GaAlAs V-shaped quantum wires
(QWRs) by high spatial resolution spectroscopy. Scanning optical imaging of
different generations of samples shows that the localization length has been
enhanced as the growth techniques were improved. In the best samples, excitons
are delocalized in islands of length of the order of 1 micron, and form a
continuum of 1D states in each of them, as evidenced by the sqrt(T) dependence
of the radiative lifetime. On the opposite, in the previous generation of QWRs,
the localization length is typically 50 nm and the QWR behaves as a collection
of quantum boxes. These localization properties are compared to structural
properties and related to the progresses of the growth techniques. The presence
of residual disorder is evidenced in the best samples and explained by the
separation of electrons and holes due to the large in-built piezo-electric
field present in the structure.Comment: 8 figure
Dynamic cratering of graphite : experimental results and simulations
The cratering process in brittle materials under hypervelocity impact (HVI) is of major relevance for debris shielding in spacecraft or high-power laser applications. Amongst other materials, carbon is of particular interest since it is widely used as elementary component in composite materials. In this paper we study a porous polycrystalline graphite under HVI and laser impact, both leading to strong debris ejection and cratering. First, we report new experimental data for normal impacts at 4100 and 4200 m s-1 of a 500-μm-diameter steel sphere on a thick sample of graphite. In a second step, dynamic loadings have been performed with a high-power nanosecond laser facility. High-resolution X-ray tomographies and observations with a scanning electron microscope have been performed in order to visualize the crater shape and the subsurface cracks. These two post-mortem diagnostics also provide evidence that, in the case of HVI tests, the fragmented steel sphere was buried into the graphite target below the crater surface. The current study aims to propose an interpretation of the results, including projectile trapping. In spite of their efficiency to capture overall trends in crater size and shape, semi-empirical scaling laws do not usually predict these phenomena. Hence, to offer better insight into the processes leading to this observation, the need for a computational damage model is argued. After discussing energy partitioning in order to identify the dominant physical mechanisms occurring in our experiments, we propose a simple damage model for porous and brittle materials. Compaction and fracture phenomena are included in the model. A failure criterion relying on Weibull theory is used to relate material tensile strength to deformation rate and damage. These constitutive relations have been implemented in an Eulerian hydrocode in order to compute numerical simulations and confront them with experiments. In this paper, we propose a simple fitting procedure of the unknown Weibull parameters based on HVI results. Good agreement is found with experimental observations of crater shapes and dimensions, as well as debris velocity. The projectile inclusion below the crater is also reproduced by the model and a mechanism is proposed for the trapping process. At least two sets of Weibull parameters can be used to match the results. Finally, we show that laser experiment simulations may discriminate in favor of one set of parameters
Vector vortex solitons and soliton control in vertical-cavity surface-emitting lasers
The properties of vector vortex beams in vertical-cavity-surface emitting
lasers with frequency-selective feedback is investigated. They are interpreted
as high-order vortex solitons with a spatially non-uniform, but locally linear
polarization state. In contrast to most schemes to obtain vector vortex beams
relying on imprinting the polarization structure, vector vortex solitons form
spontaneously due to the near polarization degeneracy in vertical-cavity
devices. We observe radially, hyperbolic and spiral polarization configurations
depending on small residual anisotropies in the system and multi-stability
between different states. In addition, we demonstrate flip-flop operation of
laser solitons via in principle local electronic nonlinearities. Combining the
two themes might open up a route for a simple device enabling fast switching
between different vector vortex beams for applications. The investigations
connect nicely the fields of nonlinear science, singular optics, structured
light and semiconductor laser technology
Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium
We compare the performance of several dust models in reproducing the dust
spectral energy distribution (SED) per unit extinction in the diffuse
interstellar medium (ISM). We use our results to constrain the variability of
the optical properties of big grains in the diffuse ISM, as published by the
Planck collaboration.
We use two different techniques to compare the predictions of dust models to
data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared
emission spectrum to recover the dust extinction and the intensity of the
interstellar radiation field (ISRF). Second, we infer the ISRF intensity from
the total power emitted by dust per unit extinction, and then predict the
emission spectrum. In both cases, we test the ability of the models to
reproduce dust emission and extinction at the same time.
We identify two issues. Not all models can reproduce the average dust
emission per unit extinction: there are differences of up to a factor
between models, and the best accord between model and observation is obtained
with the more emissive grains derived from recent laboratory data on silicates
and amorphous carbons. All models fail to reproduce the variations in the
emission per unit extinction if the only variable parameter is the ISRF
intensity: this confirms that the optical properties of dust are indeed
variable in the diffuse ISM.
Diffuse ISM observations are consistent with a scenario where both ISRF
intensity and dust optical properties vary. The ratio of the far-infrared
opacity to the band extinction cross-section presents variations of the
order of ( in extreme cases), while ISRF intensity varies
by ( in extreme cases). This must be accounted for in
future modelling.Comment: A&A, in pres
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