2014/09/01

Strange physics turns off laser

June 17, 2014 — Inspired by anomalies that arise in certain mathematical equations, researchers have demonstrated a laser system that paradoxically turns off when more power is added rather than becoming continuously brighter.


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The finding by a team of researchers at Vienna University of Technology and Princeton University, could lead to new ways to manipulate the interaction of electronics and light, an important tool in modern communications networks and high-speed information processing.

The researchers published their results June 13 in the journal Nature Communications.

Their system involves two tiny lasers, each one-tenth of a millimeter in diameter, or about the width of a human hair. The two are nearly touching, separated by a distance 50 times smaller than the lasers themselves. One is pumped with electric current until it starts to emit light, as is normal for lasers. Power is then added slowly to the other, but instead of it also turning on and emitting even more light, the whole system shuts off.

"This is not the normal interference that we know," said Hakan Türeci, assistant professor of electrical engineering at Princeton, referring to the common phenomenon of light waves or sound waves from two sources cancelling each other. Instead, he said, the cancellation arises from the careful distribution of energy loss within an overall system that is being amplified.

"Loss is something you normally are trying to avoid," Türeci said. "In this case, we take advantage of it and it gives us a different dimension we can use — a new tool — in controlling optical systems."

The research grows out of Türeci’s longstanding work on mathematical models that describe the behavior of lasers. In 2008, he established a mathematical framework for understanding the unique properties and complex interactions that are possible in extremely small lasers — devices with features measured in micrometers or nanometers. Different from conventional desk-top lasers, these devices fit on a computer chip.

That work opened the door to manipulating gain or loss (the amplification or loss of an energy input) within a laser system. In particular, it allowed researchers to judiciously control the spatial distribution of gain and loss within a single system, with one tiny sub-area amplifying light and an immediately adjacent area absorbing the generated light.

Türeci and his collaborators are now using similar ideas to pursue counterintuitive ideas for using distribution of gain and loss to make micro-lasers more efficient.

The researchers’ ideas for taking advantage of loss derive from their study of mathematical constructs called "non-Hermitian" matrices in which a normally symmetric table of values becomes asymmetric. T&uuml,Coque iPhone 5;reci said the work is related to certain ideas of quantum physics in which the fundamental symmetries of time and space in nature can break down even though the equations used to describe the system continue to maintain perfect symmetry.

Over the past several years, Türeci and his collaborators at Vienna worked to show how the mathematical anomalies at the heart of this work, called "exceptional points," could be manifested in an actual system. In 2012,coque s4, the team published a paper in the journal Physical Review Letters demonstrating computer simulations of a laser system that shuts off as energy is being added. In the current Nature Communications paper, the researchers created an experimental realization of their theory using a light source known as a quantum cascade laser.

The researchers report in the article that results could be of particular value in creating "lab-on-a-chip" devices — instruments that pack tiny optical devices onto a single computer chip. Understanding how multiple optical devices interact could provide ways to manipulate their performance electronically in previously unforeseen ways. Taking advantage of the way loss and gain are distributed within tightly coupled laser systems could lead to new types of highly accurate sensors, the researchers said.

"Our approach provides a whole new set of levers to create unforeseen and useful behaviors," Türeci said.

The work at Vienna, including creation and demonstration of the actual device, was led by Stefan Rotter at Vienna along with Martin Brandstetter, Matthias Liertzer, C. Deutsch, P. Klang, J. Schöberl, G. Strasser and K. Unterrainer. Türeci participated in the development of the mathematical models underlying the phenomena. The work on the 2012 computer simulation of the system also included Li Ge, who was a post-doctoral researcher at Princeton at the time and is now an assistant professor at City University of New York.

2014/08/26

New microscope sees what others can’t

May 8, 2014 — Microscopes don’t exactly lie, but their limitations affect the truths they can tell. For example, scanning electron microscopes (SEMs) simply can’t see materials that don’t conduct electricity very well, and their high energies can actually damage some types of samples.


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In an effort to extract a little more truth from the world of nanomaterials and nanostructures, researchers at the National Institute of Standards and Technology (NIST) have built the first low-energy focused ion beam (FIB) microscope that uses a lithium ion source.

The team’s new approach opens up the possibility of creating a whole category of FIBs using any one of up to 20 different elements, greatly increasing the options for imaging, sculpting, or characterizing materials.

Although the new microscope’s resolution isn’t yet as good as a SEM or a helium ion microscope (HIM), it can image nonconductive materials and can more clearly visualize the chemical composition on the surface of a sample than the higher-energy SEMs and FIBs. And,Coque Galaxy Note 3, by analyzing the energy with which the ions scatter, the researchers have shown that the microscope should be able to not only see that adjacent materials are chemically different, but also identify the elements that make them up.

Jabez McClelland and his colleagues at NIST applied Nobel Prize-winning laser cooling techniques to make the first low-energy FIB using lithium ions in 2011. Since then, they have been working to refine the technique to increase the beam’s brightness and collimation, i.e., getting all the ions to move in the same direction to make it more useful for imaging applications.

The new instrument first cools a gas of neutral lithium atoms to a temperature of about 600 microkelvins, just a few millionths of a degree above absolute zero, using lasers and a magneto-optical trap (MOT) to hold the atoms. Another laser ionizes the atoms and then electric fields accelerate them, straightening out their flight and focusing the beam on a target.

The NIST FIB can produce lithium ion beams with energies in the range of 500 electron volts to 5,000 electron volts (compared to about30,000 electron volts for HIMs.) The NIST team can reduce the beam’s energy even further, but repulsive interaction effects at the source limit how small they can focus the beam when the accelerating field is weaker.

As detailed in their paper, the team demonstrated how their microscope could help to solve a common problem in nanoimprint lithography, a process for stenciling patterns on silicon chips. This technique requires etching into the silicon through the spaces in the lithography stencil to transfer the pattern.

"Before manufacturers can etch the silicon, they have to make sure the spaces are free of chemical residue," says McClelland. "Commonly, they use a process called plasma etching to clean that residue off, but they have to be careful not to overdo it or they can damage the substrate and ruin the chip. Our FIB scope could check to see if the plasma has done its work without damaging the chip. A scanning electron microscope couldn’t do this because it’s difficult to see the thin residue, and the high-energy beam is likely to charge up and/or melt the stencil and make the problem worse."

The group has big plans for the microscope. One future project they’re planning to do is trying to unravel exactly how lithium batteries work by injecting lithium ions into the materials and watching how they affect the behavior of the batteries. This and other applications will add to the capabilities of NIST’s nanotechnology user facility, the Center for Nanoscale Science and Technology, where the work is being carried out.

A few former members of the group have started their own company to develop a low-energy cesium FIB for milling and sculpting features on the order of single nanometers, a huge leap in nanofabrication if successful.

"This new form of microscopy we’ve developed promises to provide a new tool for nanotechnology with good surface sensitivity, elemental contrast and high resolution,Accessoires iPhone 5s," says McClelland. "The applications range from nanofabrication process control to nanomaterial development and imaging of biomaterials."

2014/08/26

Jennifer Lawrence toujours au top pour Miss Dior


Jennifer Lawrence toujours au top pour Miss Dior

 

Les clichés pour la nouvelle campagne des sacs Miss Dior ont été dévoilés. Jennifer Lawrence, l’égérie, y est sublime.

Jennifer Lawrence, l’actrice de la saga Hunger Games, est depuis la fin de l’année 2012 l’égérie publicitaire des sacs Miss Dior, pour la marque Christian Dior. L’actrice de 23 ans succède ainsi à Mila Kunis et Marion Cotillard. En ce moment, on peut la voir sur les affiches de la campagne Automne-Hiver 2013-2014 et elle est sublime.

Sur les clichés, Jennifer Lawrence est fraîche et distinguée.  Vêtue d’un simple manteau en laine grise, dont le col est relevé, la jeune actrice nous hypnotise par son regard à la fois franc et naïf. Jennifer Lawrence est très naturelle sur les photos et très légèrement maquillée. Son teint est parfait, à tel point que l’on croirait à un visage de poupée. Ses cheveux, tirés en arrière, laisse échapper quelques mèches. Pendu à son épaule, on retrouve le célèbre sac matelassé de la marque de luxe française.

Nul besoin d’artifice pour l’actrice qui est très naturelle. Son expression et son attitude semble dire : « Ce sac est le must de la mode, toute fashionista qui se respecte se doit d’en avoir un »

Lire aussi : Natalie Portman magnifique pour la publicité de Miss Dior Chérie

jennifer lawrence pour Miss Dior
Jennifer lawrence pour Miss Dior

La campagne shootée par le photographe Daniel Jackson, qui a également photographié Cate Blanchett pour Giorgio Armani, ne comporte pour le moment que deux clichés. Sur le premier l’actrice est de face et sur le second, de profil. Sur ce second cliché, Jennifer Lawrence porte la même tenue, mais a échangé le sac noir par le même modèle en rose vieux. Mains dans les poches, attitude effrontée, l’actrice a tout d’un mannequin. Quel plaisir de voir l’actrice si élégante parce que sur le tapis rouge ce n’est pas toujours ça !

Jennifer Lawrence sublime pour Miss Dior
Jennifer Lawrence sublime pour Miss Dior

Une actrice à suivre

A seulement 23 ans, depuis le 15 août 2013, Jennifer Lawrence a déjà de nombreux blockbusters à son actif. En 2011, elle incarne Mystique, la créature bleue qui peut changer d’apparence, dans X-Men : Le commencement. Elle reprendra d’ailleurs le rôle en 2014 dans le film dirigé par Bryan Singer X-Men : Days Of Future Past,sandales compensées. Jennifer Lawrence est également Katniss Everdeen dans la saga Hunger Games,BLOG MODE, dont le premier volet est sorti en 2013. Le film est tiré de la trilogie littéraire de Suzanne Collins. Le deuxième film est attendu avec impatience par les fans pour le 27 novembre 2013 en France. L’actrice a également joué au côté de Bradley Cooper dans le film Happiness Therapy, qui a reçu de très bonnes critiques. Pour son rôle de Tiffany, Jennifer Lawrence remporte l’Oscar de la meilleure actrice en 2013.

En mars 2013, Jennifer Lawrence est classée parmi les 100 personnes les plus influentes du monde par le Time Magazine. Une consécration pour une si jeune actrice !

 

2014/08/23

Like a hall of mirrors, nanostructures trap photons inside ultrathin solar cells

Apr. 22, 2014 — In the quest to make sun power more competitive, researchers are designing ultrathin solar cells that cut material costs. At the same time they’re keeping these thin cells efficient by sculpting their surfaces with photovoltaic nanostructures that behave like a molecular hall of mirrors.


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« We want to make sure light spends more quality time inside a solar cell, » said Mark Brongersma, a professor of materials science and engineering at Stanford and co-author of a review article in Nature Materials.

Brongersma and two Stanford colleagues — associate professor of materials science and engineering Yi Cui and professor of electrical engineering Shanhui Fan — surveyed 109 recent scientific papers from teams around the world.

Their overview revolves around a basic theme: looking at the many different ways that researchers are trying to maximize the collisions between photons and electrons in the thinnest possible layers of photovoltaic materials. The goal is to reveal trends and best practices that will help drive developments in the field.

Solar energy is produced when photons of light collide with the electrons in a photovoltaic crystal. As loose electrons move through the crystal, they generate an electrical current.

Today’s solar cells are already thin. They are made up of layers of photovoltaic materials, generally silicon, that average 150 to 300 micrometers, which is roughly the diameter of two to three human hairs.

As engineers continue to shave down those dimensions they have to develop new molecular traps and snares to ensure that photons don’t simply whiz through their ultrathin solar cells before the electrical sparks can fly.

« A lot of the excitement now is about using the principles of photonics to manage light waves in the most efficient way, » Fan said. « There are perhaps hundreds of groups in the world working on this. »

The review article provides a high level view of how scientists are trying to design structures to facilitate interactions between the infinitesimal instigators of solar current, the photons and the electrons.

Research face enormous challenges in trying to architect nanostructures attuned to catch light. Sunlight consists of many colors. When we see rainbow, what we see is result of atmospheric moisture acting as a prism to bend light into its constituent colors. Creating different nanostructures to catch the pot of photons at the end of each color of the rainbow is part of what this research is about.

Nevertheless, scientists are already reporting some success

« We are seeing systems that use one one-hundredth as much photovoltaic material as today’s solar cells while getting 60 percent to 70 percent of the electrical output, » Brongersma said.

The most common photovoltaic material is a refined form of silicon similar to that found in computer chips. This material accounts for 10 percent to 20 percent of a solar cell’s cost. Lowering those expenses 100-fold would therefore have a considerable effect on the overall cost-efficiency of solar energy production.

But Cui says lowering material costs is only part of the push behind ultrathin solar. Another benefit is flexibility. Because of the thickness of the light-catching silicon layer, today’s solar cells must be kept rigid lest their crystal lattice be damaged and the flow of electrons disrupted.

« But at 10 micrometers of thickness silicon has a high degree of mechanical flexibility, » said Cui, citing a dimension less than one-tenth the thickness of the photovoltaic layer inside today’s solar cells.

Cui, who has made just such an experimental material, shows a movie of flapping this thin silicon like a piece of paper and cutting it with a scissors (see separate videos; flapping http://www.youtube.com/watch?v=e71Z0Kt3bsQ and cutting http://www.youtube.com/watch?v=PFyKfkwPRfs). Those thin silicon strips incorporate some of the photon-trapping nanostructures described in the Nature Materials article,Coques iPhone 5. Cui says the light-to-energy conversion efficiency of thin silicon is approaching that of the rigid silicon in today’s solar cells.

Flapping silicon isn’t just a science project. Such flexibility would pay a dividend when it comes to installation,Accessoires & Coques Pour iPhone, which accounts for roughly one-third of the total cost of a rooftop solar array. « These thin silicon cells can be embedded into flexible plastic, making installation like rolling out a carpet, » Cui said.

Yet even as researchers succeed in getting more from less, many hurdles remain according to Fan, who develops computer models to study how different nanostructures and materials will affect photon-electron interactions.

« There are an infinite number of structures, so it isn’t possible to model them all, » he said, alluding to what he called the « theoretical bottlenecks » that impede scientific understanding of this ethereal realm where light and matter intersect.

« For instance, right now, we really don’t have a way to know when we’ve gotten the most out of our photons, » Fan said.

2014/08/16

Arke, D-reizen en … genomineerd voor beste Website van het jaar.

De 12 genomineerden voor de Website van het Jaar 2012 in de categorie Vrije tijd & Reizen zijn bekend,Housse iPhone 5. Van 17 september tot en met 26 oktober kan weer gestemd worden. In totaal zijn er meer dan 250 websites genomineerd voor de Beste Website van het Jaar verkiezing. De winnaar wordt op 15 november bekend gemaakt. Het onderzoek wordt dit jaar voor het elfde achtereenvolgende jaar uitgevoerd door marktonderzoeksbureau MetrixLab in samenwerking met Emerce.

Er zijn twee prijzen te verdelen in elke categorie. Een prijs voor de beste website met de hoogste gemiddelde score voor inhoud, navigatie en ontwerp. De site met het hoogst aantal stemmen wint de prijs voor populairste website. Vorig jaar ging Wintersporters.nl aan de haal met de beste Website van het Jaar 2011 en Vakantieveilingen had het meest aantal stemmen. Dat is ook niet zo gek als je weet hoeveel Facebook fans ze hebben opgebouwd. Die gaan dus weer veel kans maken dit jaar.

Naast Arke en D-reizen zijn dit jaar de volgende bedrijven genomineerd: Expedia.nl, Hotelaanbiedingen.nl, Hotelspecials.nl, KLM,étui Samsung Galaxy Note 3, Neckermann.nl, Oad, TravelBird.nl, Vakantieveilingen.nl, Wintersporters.nl en Zoover.

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