799 research outputs found
Strong field dynamics with ultrashort electron wave packet replicas
We investigate theoretically electron dynamics under a VUV attosecond pulse
train which has a controlled phase delay with respect to an additional strong
infrared laser field. Using the strong field approximation and the fact that
the attosecond pulse is short compared to the excited electron dynamics, we
arrive at a minimal analytical model for the kinetic energy distribution of the
electron as well as the photon absorption probability as a function of the
phase delay between the fields. We analyze the dynamics in terms of electron
wave packet replicas created by the attosecond pulses. The absorption
probability shows strong modulations as a function of the phase delay for VUV
photons of energy comparable to the binding energy of the electron, while for
higher photon energies the absorption probability does not depend on the delay,
in line with the experimental observations for helium and argon, respectively.Comment: 14 pages, 8 figure
Optomechanically induced transparency
Coherent interaction of laser radiation with multilevel atoms and molecules
can lead to quantum interference in the electronic excitation pathways. A
prominent example observed in atomic three-level-systems is the phenomenon of
electromagnetically induced transparency (EIT), in which a control laser
induces a narrow spectral transparency window for a weak probe laser beam. The
concomitant rapid variation of the refractive index in this spectral window can
give rise to dramatic reduction of the group velocity of a propagating pulse of
probe light. Dynamic control of EIT via the control laser enables even a
complete stop, that is, storage, of probe light pulses in the atomic medium.
Here, we demonstrate optomechanically induced transparency (OMIT)--formally
equivalent to EIT--in a cavity optomechanical system operating in the resolved
sideband regime. A control laser tuned to the lower motional sideband of the
cavity resonance induces a dipole-like interaction of optical and mechanical
degrees of freedom. Under these conditions, the destructive interference of
excitation pathways for an intracavity probe field gives rise to a window of
transparency when a two-photon resonance condition is met. As a salient feature
of EIT, the power of the control laser determines the width and depth of the
probe transparency window. OMIT could therefore provide a new approach for
delaying, slowing and storing light pulses in long-lived mechanical excitations
of optomechanical systems, whose optical and mechanical properties can be
tailored in almost arbitrary ways in the micro- and nano-optomechanical
platforms developed to date
One-pot Synthesis in Polyamines for Preparation of Water-soluble Magnetite Nanoparticles with Amine Surface Reactivity
Magnetite nanoparticles with hydrophilic surface coating are prepared in polyamine solvents. The resulting products are highly stable in polar solvent. The surface amine groups are available for secondary reactions
Interspecies and Interregional Analysis of the Comparative Histologic Thickness and Laser Doppler Blood Flow Measurements at Five Cutaneous Sites in Nine Species
Studies in dermatology, cutaneous pharmacology, and toxicology utilize skin from different animal species and body sites. However, regional differences exist in topical chemical percutaneous absorption studies in man and in animals. The objective of this study was to compare epidermal thickness and number of cell layers across species and body sites using both formalin-fixed paraffin and frozen sections. Cutaneous blood flow determined by laser Doppler velocimetry (LDV)was compared to histologic data. Six animals of each of the following species were used: monkeys, pigs, dogs, cats, cows, horses, rabbits, rats, and mice. Cutaneous blood flow was determined and 6-mm skin biopsies were taken directly from the following sites: buttocks, ear, humeroscapular joint, thoracolumbar junction, and abdominal area. When the two histologic methods were compared across all species and body sites, the thickness of the epidermis was significantly greater, and the thickness of the stratum corneum significantly less, in paraffin sections versus frozen sections (p < 0.05). There were no differences in the number of viable cell layers determined by both methods. The values for LDV-determined blood flow did not significantly correlate (p > 0.05) to epidermal or stratum corneum thickness. However, regional and species differences were noted in all these parameters. In conclusion, these data indicate that thickness and LDV blood flow are independent and must be evaluated separately when comparisons are made between species and body sites. This work provides a data base for future comparative studies in which a knowledge of skin thickness or blood flow might be important variables
Exocomet signatures around the A-shell star Leo?
We present an intensive monitoring of high-resolution spectra of the Ca {\sc
ii} K line in the A7IV shell star Leo at very short (minutes, hours),
short (night to night), and medium (weeks, months) timescales. The spectra show
remarkable variable absorptions on timescales of hours, days, and months. The
characteristics of these sporadic events are very similar to most that are
observed toward the debris disk host star Pic, which are commonly
interpreted as signs of the evaporation of solid, comet-like bodies grazing or
falling onto the star. Therefore, our results suggest the presence of solid
bodies around Leo. To our knowledge, with the exception of Pic,
our monitoring has the best time resolution at the mentioned timescales for a
star with events attributed to exocomets. Assuming the cometary scenario and
considering the timescales of our monitoring, our results indicate that
Leo presents the richest environment with comet-like events known to date,
second only to Pic.Comment: A&A letters, proof-correcte
Fully Parallel Hyperparameter Search: Reshaped Space-Filling
Space-filling designs such as scrambled-Hammersley, Latin Hypercube Sampling
and Jittered Sampling have been proposed for fully parallel hyperparameter
search, and were shown to be more effective than random or grid search. In this
paper, we show that these designs only improve over random search by a constant
factor. In contrast, we introduce a new approach based on reshaping the search
distribution, which leads to substantial gains over random search, both
theoretically and empirically. We propose two flavors of reshaping. First, when
the distribution of the optimum is some known , we propose Recentering,
which uses as search distribution a modified version of tightened closer
to the center of the domain, in a dimension-dependent and budget-dependent
manner. Second, we show that in a wide range of experiments with unknown,
using a proposed Cauchy transformation, which simultaneously has a heavier tail
(for unbounded hyperparameters) and is closer to the boundaries (for bounded
hyperparameters), leads to improved performances. Besides artificial
experiments and simple real world tests on clustering or Salmon mappings, we
check our proposed methods on expensive artificial intelligence tasks such as
attend/infer/repeat, video next frame segmentation forecasting and progressive
generative adversarial networks
DECK: A new model for a distributed executive kernel integrating communication and multithreading for support of distributed object oriented application with fault tolerance support
DECK (Distributed Executive Communication Kernel) is a communication layer that provides support for multithreading and fault tolerance support. The approach retained in DECK is close to other distributed communication kernels like PM2, Athapascan, Nexus, TPVM or Chant in its way to integrate communication and multithreading to efficiently overlap communication by computation and provide low latency remote thread creation mechanisms. However, DECK differs from these communication kernels from the services offered and its modular architecture.
The main goal of DECK is to implement a new model for the design of distributed executive kernel to efficiently use the new underlying hardware architectures (SMP architectures and fast communication adapters like Myrinet or memory oriented adapter like SCI) and provide a portable layer that abstract the problems linked with the integration of communication and multithreading while offering support for heterogeneity.
A great lack in the current implementation of communication libraries or distributed executive kernel is the support for basic services at the thread level and support for fault tolerance support. Indeed, communication library like PVM or MPI are often used as communication layer to ensure portability and take benefits of specific implementation to ensure a good efficiency on specific architectures however the support for fault tolerance support, multithreading, scalability and interoperability are usually not offered.
In the case of DECK, we propose a model where a distributed application can dynamically instantiate clusters of processes among an heterogeneous network of computers or parallel machines and this using multiple communication protocols or communication interfaces to ensure good performances regarding the underlying hardware architecture. The programming model proposed offer both classic synchronous and asynchronous remote service calls for thread creation and message passing for synchronization and data exchange.
These basic functionalities, that form the low level communication and execution layer of DECK, are enforced by a service layer that propose the basic fault tolerant services like naming and group services or data management services for the marshaling and un-marshalling of complex data structures. The layered and modular approach followed by DECK enable many other extensions while keeping a high degree of portability and efficiency.Sistemas Distribuidos - Redes ConcurrenciaRed de Universidades con Carreras en Informática (RedUNCI
Radio emission of extensive air shower at CODALEMA: Polarization of the radio emission along the v*B vector
Cosmic rays extensive air showers (EAS) are associated with transient radio
emission, which could provide an efficient new detection method of high energy
cosmic rays, combining a calorimetric measurement with a high duty cycle. The
CODALEMA experiment, installed at the Radio Observatory in Nancay, France, is
investigating this phenomenon in the 10^17 eV region. One challenging point is
the understanding of the radio emission mechanism. A first observation
indicating a linear relation between the electric field produced and the cross
product of the shower axis with the geomagnetic field direction has been
presented (B. Revenu, this conference). We will present here other strong
evidences for this linear relationship, and some hints on its physical origin.Comment: Contribution to the 31st International Cosmic Ray Conference, Lodz,
Poland, July 2009. 4 pages, 8 figures. v2: Typo fixed, arxiv references adde
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