168 research outputs found
FGF receptor signalling is required to maintain neural progenitors during Hensen's node progression
Previous analyses of labelled clones of cells within the developing nervous system of the mouse have indicated that descendants are initially dispersed rostrocaudally followed by more local proliferation, which is consistent with the progressing node's contributing descendants from a resident population of progenitor cells as it advances caudally. Here we electroporated an expression vector encoding green fluorescent protein into the chicken embryo near Hensen's node to test and confirm the pattern inferred in the mouse. This provides a model in which a proliferative stem zone is maintained in the node by a localized signal; those cells that are displaced out of the stem zone go on to contribute to the growing axis. To test whether fibroblast growth factor (FGF) signalling could be involved in the maintenance of the stem zone, we co-electroporated a dominant-negative FGF receptor with a lineage marker, and found that it markedly alters the elongation of the spinal cord primordium. The results indicate that FGF receptor signalling promotes the continuous development of the posterior nervous system by maintaining presumptive neural progenitors in the region near Hensen's node. This offers a potential explanation for the mixed findings on FGF in the growth and patterning of the embryonic axis
Direct machining of curved trenches in silicon with femtosecond accelerating beams
International audienceControl of the longitudinal profile of ablated structures during laser processing is a key technological requirement. We report here on the direct machining of trenches in silicon with circular profiles using femtosecond accelerating beams. We describe the ablation process based on an intensity threshold model, and show how the depth of the trenches can be predicted in the framework of a caustic description of the beam
Caustics and Rogue Waves in an Optical Sea
There are many examples in physics of systems showing rogue wave behaviour,
the generation of high amplitude events at low probability. Although initially
studied in oceanography, rogue waves have now been seen in many other domains,
with particular recent interest in optics. Although most studies in optics have
focussed on how nonlinearity can drive rogue wave emergence, purely linear
effects have also been shown to induce extreme wave amplitudes. In this paper,
we report a detailed experimental study of linear rogue waves in an optical
system, using a spatial light modulator to impose random phase structure on a
coherent optical field. After free space propagation, different random
intensity patterns are generated, including partially-developed speckle, a
broadband caustic network, and an intermediate pattern with characteristics of
both speckle and caustic structures. Intensity peaks satisfying statistical
criteria for rogue waves are seen especially in the case of the caustic
network, and are associated with broader spatial spectra. In addition, the
electric field statistics of the intermediate pattern shows properties of an
optical sea with near-Gaussian statistics in elevation amplitude, and
trough-to-crest statistics that are near-Rayleigh distributed but with an
extended tail where a number of rogue wave events are observed.Comment: 10 pages, 5 figures, to be published in Scientific Report
Spherical light, arbitrary nonparaxial accelerating beams and femtosecond laser micromachining of curved profiles
International audienceWe review our recent results applying caustics wave theory to the generation of arbitrary curved accelerating beams and their use in the field of femtosecond laser materials processing. We report experimental realization of highly nonparaxial accelerating beams with circular, parabolic and quartic trajectories that extend over more than 95 degrees of arc as well as spherical optical fields. We also report femtosecond laser curved edge profiling
Micromachining along a curve: Femtosecond laser micromachining of curved profiles in diamond and silicon using accelerating beams
International audienceWe report femtosecond laser micromachining of micron-size curved structures using tailored accelerating beams. We report surface curvatures as small as 70âÎŒm in both diamond and silicon, which demonstrates the wide applicability of the technique to materials that are optically transparent or opaque at the pump laser wavelength. We also report the machining of curved trenches in silicon. Our results are consistent with an ablation-threshold model based on calculated local beam intensity, and we also observe asymmetric debris deposition which is interpreted in terms of the optical properties of the incident accelerating beam
Filamentation of high-angle nondiffracting beams and applications to ultrafast laser processing
International audienceWe report on filamentation of nondiffracting beams and show that the intense light-matter interaction regime achieved on long distances allows for an enhanced control on ultrashort laser deep ablation
Femtosecond laser material processing with nondiffracting light
International audienceAlthough versatile and widely used, femtosecond laser micro-nanomachining faces a challenge for the fabrication of high aspect ratio or deep structures. The control of the profile along the longitudinal dimension is extremely difficult. In this context, our approach is based on controlling the direction of light rather than shaping the intensity pattern in one plane. Here we review our recent work in this field using Bessel beams and accelerating beams. In Bessel beams, light propagates along the generatrix of a cone, with a fixed radial wavevector. Contrary to Gaussian beams, the femtosecond filamentation regime of Bessel beams can be stationary, and can generate extended plasma tracks in dielectrics. This allowed us to generate nanochannels in glass with aspect ratio up to 100:1 [1]. Numerical simulations of the nonlinear propagation in this regime show the importance of the conical structure in generating extremely dense plasmas [2]. Airy beams and more generally accelerating beams constitute another family of beams that possess quasi-nondiffracting behaviour and their nonlinear propagation can also be stationary. Their primary intensity lobe propagates along a curved trajectory that can be arbitrarily shaped even in the nonparaxial regime. Since this lobe is adjacent to a region where no light propagates, accelerating beams can be used for direct curved edge profiling and trench processing in transparent and opaque materials [3,4]. Our results show that controlling the linear and nonlinear propagation of ultrashort femtosecond laser pulses by nondiffracting beams is a key new technological approach for laser processing
Faisceaux non-diffractants et accélérants. Applications à la micro et nanostructuration par laser femtoseconde
National audienceLors des processus d'ablation par laser, le contrÎle des profils des flancs d'ablation est un aspect important. Nous avons récemment développé une approche permettant de répondre à ce problÚme, à partir du contrÎle de la direction de la lumiÚre dans des faisceaux femtoseconde. Nous adressons des modes particuliers (stationnaires) de la filamentation dans les diélectriques afin de contrÎler la répartition spatiale du dépÎt d'énergie dans la matiÚre. Des structures à haut rapport de forme et l'usinage direct de profils courbes seront présentés
Effectiveness of Protease Inhibitor Monotherapy versus Combination Antiretroviral Maintenance Therapy: A Meta-Analysis
The unparalleled success of combination antiretroviral therapy (cART) is based on the combination of three drugs from two classes. There is insufficient evidence whether simplification to ritonavir boosted protease inhibitor (PI/r) monotherapy in virologically suppressed HIV-infected patients is effective and safe to reduce cART side effects and costs
Search for dark matter produced in association with bottom or top quarks in âs = 13 TeV pp collisions with the ATLAS detector
A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fbâ1 of protonâproton collision data recorded by the ATLAS experiment at âs = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements
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