58 research outputs found
2W/nm Peak-power All-Fiber Supercontinuum Source and its Application to the Characterization of Periodically Poled Nonlinear Crystals
We demonstrate a uniform high spectral brightness and peak power density
all-fiber supercontinuum source. The source consists of a nanosecond Ytterbium
fiber laser and an optimal length PCF producing a continuum with a peak power
density of 2 W/nm and less than 5 dB of spectral variation between 590 to 1500
nm. The Watt level per nm peak power density enables the use of such sources
for the characterization of nonlinear materials. Application of the source is
demonstrated with the characterization of several periodically poled crystals.Comment: 8 pages 4 figures v2 includes revisions to the description of the
continuum formatio
Novel microstructured fibres for supercontinuum generation
We report recent progress on the fabrication of photonic crystal fibre from ZBLAN and tellurite glasses and their application to generating broadband supercontinua
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Track A Basic Science
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138319/1/jia218438.pd
Do Femtonewton Forces Affect Genetic Function? A Review
Protein-Mediated DNA looping is intricately related to gene expression. Therefore any mechanical constraint that disrupts loop formation can play a significant role in gene regulation. Polymer physics models predict that less than a piconewton of force may be sufficient to prevent the formation of DNA loops. Thus, it appears that tension can act as a molecular switch that controls the much larger forces associated with the processive motion of RNA polymerase. Since RNAP can exert forces over 20 pN before it stalls, a âsubstrate tension switchâ could offer a force advantage of two orders of magnitude. Evidence for such a mechanism is seen in recent in vitro micromanipulation experiments. In this article we provide new perspective on existing theory and experimental data on DNA looping in vitro and in vivo . We elaborate on the connection between tension and a variety of other intracellular mechanical constraints including sequence specific curvature and supercoiling. In the process, we emphasize that the richness and versatility of DNA mechanics opens up a whole new paradigm of gene regulation to explore.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/41816/1/10867_2005_Article_9002.pd
Uniaxial compression of 2d and 3d packings : electrical conductivity measurements
We measured the conductivity of 2d and 3d packings of conducting grains under a uniaxial vertical pressure. The conductance as a function of stress is a power law with an exponent which is different from the microscopic Hertz exponent. We propose a simple theoretical explanation.Nous avons mesuré la conductivité d'empilements bi et tridimensionnels de grains conducteurs soumis à une pression uniaxiale verticale. La conductance en fonction de la contrainte suit une loi de puissance dont l'exposant est différent de l'exposant microscopique prévu par la loi de Hertz. Nous proposons une explication théorique simple
Compression de milieux granulaires modĂšles Ă deux dimensions
Nous prĂ©sentons une synthĂšse de travaux menĂ©s par notre Ă©quipe sur des empilements 2d de cylindres soumis Ă une compression verticale ĆdomĂ©trique.Nous avons Ă©tudiĂ© les effets du dĂ©sordre gĂ©omĂ©trique et du dĂ©sordre de composition (mĂ©lange de grains durs et mous) en utilisant en particulier des observations en photoĂ©lasticitĂ©. Ces expĂ©riences font apparaĂźtre deux rĂ©gimes en fonction de la force appliquĂ©e : un rĂ©gime de consolidation et un rĂ©gime consolidĂ© ; dans chacun d'eux la relation force macroscopique-dĂ©formation peut ĂȘtre exprimĂ©e par une loi en puissance dont lâexposant est trĂšs diffĂ©rent de l'exposant microscopique de Hertz et dĂ©pend beaucoup de la qualitĂ© des contacts entre grains.Nous avons Ă©galement fait lâanalyse critique de ces expĂ©riences en estimant lâimportance des effets de taille finie et ceux des parois. Le rĂŽle des frottements grain-paroi est discutĂ© et le rapport force transmise/force appliquĂ©e a Ă©tĂ© mesurĂ©. Nous avons testĂ© lâinfluence de la taille de l'Ă©chantillon en augmentant la largeur, la hauteur ou les deux simultanĂ©ment. Nous mettons ainsi en Ă©vidence le rĂŽle dâune longueur caractĂ©ristique supĂ©rieure Ă la taille des grains
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