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  • richardmitnick 3:58 pm on September 17, 2014 Permalink | Reply
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    From physicsworld: “New plasmonic nanolaser is cavity-free” 

    physicsworld
    physicsworld.com

    Sep 17, 2014
    Tim Wogan

    A new design for a cavity-free nanolaser has been proposed by physicists at Imperial College London. The design builds on a proposal from the same team earlier this year to reduce the group velocity of light of a particular frequency to exactly zero in a metal–dielectric–metal waveguide. The laser, which has yet to be built, makes use of two such zero-velocity regions, and would achieve population inversion and create a laser beam without the need for an optical cavity. The researchers suggest that the design could have important applications in optical telecommunications and computing, as well as theoretical implications in reconciling the physics of lasers with plasmonics.

    graph
    Slowing light to a stop: nanolaser has no cavity

    The traditional design for a laser involves encasing a gain medium such as a gas in a cavity containing two opposing mirrors. The gain medium contains two electronic energy levels, and, in the natural state, the lower energy level is the more populated. However, by injecting electrical or light energy into the cavity, some electrons can be “pumped” into the upper state. At low pumping levels, atoms pushed to the upper level decay spontaneously back to the ground state with the emission of a photon. However, above a certain threshold, transitions back to the ground state are predominantly caused by an excited atom’s absorption of a second photon. The two photons are emitted perfectly in phase, and go on to excite emission from more atoms. The resulting beam of phase-coherent photons is the laser beam.

    Lasers have revolutionized modern science and technology, with tiny lasers can be found everywhere from cheap pointers to state-of-the-art telecommunications systems. While much smaller nanoscale lasers would be useful for creating chip-based optical circuits, the need for a cavity limits means that it is difficult to miniaturize a conventional laser beyond the wavelength of the light it produces. This limit is about one micron for the light used in telecommunications technologies.

    Plasmonic interactions

    Now, Ortwin Hess and colleagues have devised a new way of producing a sub-wavelength laser by removing the cavity altogether. The researchers designed a layered metal–dielectric–metal waveguide structure that supports plasmonic interactions between light and conduction electrons at the metal–dielectric interfaces. Such a plasmonic waveguide supports two “zero-velocity singularities” at closely spaced but distinct frequencies. Light of other frequencies will propagate through the semiconductor very slowly – allowing it plenty of time to interact with the gain material. While slow and stopped light might sound like unphysical concepts, they can occur when light interacts with plasmons. Injecting a pulse of this slow light, the researchers calculated, will pump carriers from a lower energy state to a higher state. This higher state would then decay to an intermediate state, which would then decay to produce the laser light. The presence of the zero-velocity singularities causes the laser light to be trapped in the material, where it drives the desired coherent stimulated emission.

    To produce a laser beam, however, some of the laser light must be able to leave the device. In previous work (see “Plasmonic waveguide stops light in its tracks”), Hess and colleagues proposed exciting a zero-velocity mode by passing the light through the cladding in the form of an evanescent wave – a special type of wave the frequency of which is a complex number. Radiation incident on the cladding would excite an evanescent wave, which would in turn excite the stopped-light mode in the dielectric inside. In their new paper, Hess and colleagues turn this idea on its head and use the evanescent field to allow laser light to escape. By varying the precise properties and thickness of the cladding layer, the proportion of light allowed to escape could be tuned, producing a laser beam of variable intensity.

    Biomedical applications

    Nicholas Fang, a nanophotonics expert at the Massachusetts Institute of Technology, believes that, if such cavity-free nanolasers could be produced, they could have major practical implications not only in computation and signalling, but also in less-obvious fields such as prosthetics: he suggests they could be embedded in synthetic tissue to provide sensors with output signals detectable by the nervous system. “Here you’d have a laser source that could be directly implantable,” he explains.

    Hess, meanwhile, is excited by the potential theoretical implications of the work. While the current research focuses on using plasmonic interactions to produce coherent light, he believes that it should also be possible to keep the plasmons themselves confined within the waveguide to produce a miniature surface plasmon laser or “spaser”.

    The research is described in Nature Communications.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

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  • richardmitnick 8:03 am on September 11, 2014 Permalink | Reply
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    From physicsworld.com: “Fractal-like honeycombs take the strain” 

    physicsworld
    physicsworld.com

    Sep 10, 2014
    Katherine Kornei

    Honeycomb lattices and fractal structures are found in a range of biological materials. Now, scientists in the US, the UK and France have combined the two types of pattern to create a strong and lightweight material that could be used in a range of applications, from aerospace to medicine. While the structures were made with centimetre-sized unit cells, the team believes that similar materials could be made on the nanoscale using carbon nanotubes.

    test
    Photograph of a fractal-like honeycomb structures being tested
    Under pressure

    Hexagonal honeycomb patterns often appear in nature, where strength, rigidity and lightness are called for. The shells of armadillos, the beaks of birds and, of course, the wax cells built by bees are just a few examples of nature’s honeycombs. Engineers have long known about the honeycomb’s strength and low density, and the structure has been used in applications as varied as satellite components and the scaffolding for growing new heart tissue.

    Now, Ashkan Vaziri and colleagues at Northeastern University, along with researchers at the University of Oxford and the Université de Lyon, have shown that fractal-like structures based on honeycombs are even more resistant to deformation than conventional honeycomb materials.

    Hierarchical structures

    Fractals – in which the same patterns appear on different length scales – are also found in a variety of naturally occurring materials, including the buds on certain types of broccoli, pinecone seeds and nautilus shells. “Hierarchical structures are ubiquitous in nature and can be observed at many different scales in organic materials and biological systems,” explains Vaziri. Honeycombs on their own are not fractals because their characteristic shape only occurs on one length scale. However, a hierarchical fractal structure can be built upon a hexagonal honeycomb by successively replacing each vertex of three edges with another, smaller hexagon (see the image below).

    Vaziri and collaborators looked at how the mechanical properties of these hierarchical hexagonal honeycombs varied as a function of how many times the fractal order was repeated. The team used both MATLAB computer models and experimental testing to study the structural performance of the hierarchical hexagonal honeycombs. Specifically, the researchers looked at the elements of the structure that can undergo stretching, shear and bending. “Our goal is to develop novel, hierarchical structures that are far superior to the classical cellular structures in terms of their mechanical response,” Vaziri told physicsworld.com.

    Reaching a limit

    The computer simulations focused on the elastic modulus of each structure, which measures a material’s ability to resist deformation. To make a meaningful comparison between structures comprising different numbers of hexagonal hierarchies, the team adjusted the thickness of each structure to ensure that they all had the same density.

    “Generally, increasing the density of the cellular structure while preserving its topology improves the mechanical properties of the structure,” Vaziri explains. “To solely investigate the effect of hierarchical order on the mechanical properties of the hierarchical structure, we preserve the relative density while increasing the hierarchical order.”

    The simulations predict that the elastic moduli of the structures increase with hierarchical order, up to a certain threshold. Furthermore, the calculations suggest that materials with desirable elastic moduli can be manufactured without having to resort to extremely high orders of hierarchy. This is good news from a practical point of view, because it would be difficult to achieve high orders of hierarchy using today’s 3D printing technologies.

    Making it real

    The next step for Vaziri’s team was to test its findings in the lab. The researchers used a 3D printer to manufacture extruded polymer shells of five honeycomb structures, each with a successively higher order of hierarchy. The thickness of the honeycomb walls was maintained at 2 mm because of limitations of the 3D printing process. To maintain a constant density, the size of the unit cells was adjusted instead of the thickness.
    Photograph of fractal-like honeycomb unit cells

    comb
    Honeycombs within honeycombs: unit cells made by a 3D printer

    The hexagonal edge lengths of the extruded structures ranged from 0.6 to 2.2 cm. The researchers tested the compressive response and elastic modulus of each structure, recording how each structure’s resistance to deformation varied as a function of its hierarchy. The results revealed that, as predicted by the simulations, structures with a higher order of hierarchy had increasingly larger elastic moduli, to a certain limit.

    Even though Vaziri and his team focused on unit cells that were on the centimetre length scale, they are confident that their findings can be applied to smaller scales. “The unit cells of the hierarchical honeycombs can be built with single- or multi-walled carbon nanotubes,” Vaziri claims. Deformation-resistant structures assembled from carbon nanotubes would have widespread applications in biological engineering and materials science.

    The structures are described in Physical Review Letters.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 3:49 pm on September 5, 2014 Permalink | Reply
    Tags: , , , , physicsworld.com, Pleiades cluster,   

    From physicsworld.com: “Pleiades distance debate resolved, say radio astronomers” 

    physicsworld
    physicsworld.com

    Aug 29, 2014
    Tim Wogan

    A long-running debate about the distance to the Pleiades cluster of stars has been resolved, claims a team of radio astronomers in the US. The researchers conclude that the cluster is almost exactly as far away as originally thought. This contradicts analyses of data from the Hipparcos satellite cluster, which suggested that the cluster is 13 parsecs closer than astronomical models predict. The astronomer who made those Hipparcos calculations, however, is standing by the original results, claiming that there are errors and unjustified assumptions in the new research.

    pleides
    Pleiades cluster

    The Pleiades is the star cluster most obvious to the naked eye in the night sky, and has been known since antiquity. In modern astronomy, the distance to the Pleiades is used to calibrate the cosmic-distance ladder, allowing the distances to star clusters and galaxies that are further away to be inferred. For this reason, it is important to know this distance precisely, and multiple calculations of it have been made using various methods.

    ladder

    Light green boxes: Technique applicable to star-forming galaxies.
    Light blue boxes: Technique applicable to Population II galaxies.
    Light Purple boxes: Geometric distance technique.
    Light Red box: The planetary nebula luminosity function technique is applicable to all populations of the Virgo Supercluster.
    Solid black lines: Well calibrated ladder step.
    Dashed black lines: Uncertain calibration ladder step.

    The generally accepted distance was about 134 parsecs (about 437 light-years) until, in 1999, Floor van Leeuwen of the Institute of Astronomy in Cambridge, UK, used data from the European Space Agency’s Hipparcos satellite to produce what was the most precise calculation to date. The result was obtained with trigonometric parallax, using the apparent shift in the position of the target star relative to distant “fixed” stars as the Earth orbits the Sun. This is independent of any stellar models, depending only on the fundamental laws of geometry.

    ESA Hipparcus spacecraft
    ESA/HIPPARCOS

    Controversial analysis

    Van Leeuven arrived at a distance of about 120 parsecs. He refined his analysis in 2009, reaching a similar conclusion. This figure was highly controversial as the potential theoretical implications of such an unexpected discovery were huge, putting into question the amount of helium in the stars making up the Pleiades and even suggesting that hitherto unknown physics governs the early lives of stars.

    In the new research, Carl Melis of the University of California, San Diego and colleagues at several other US institutions did their own trigonometric-parallax measurement of five selected stars in the Pleiades cluster using very long baseline radio interferometry [the radio astronomy resource is not named]. In this technique, measurements are made by linked radio antennas spread across the world, giving the total resolution of a telescope the size of the Earth. The researchers found that the distances of all five stars were in broad agreement with the original figure, with the lowest value being 134.8 parsecs and the highest being 138.4.

    Melis says that, taken together with all the other measurements of the distance to the Pleiades cluster that back current theoretical models, these measurements demonstrate conclusively that the Hipparcos data were erroneous. “We’ve already come to that conclusion,” says Melis. “This is just reiterating it, and really hitting the hammer on the head of the nail and driving it into the coffin.”

    Not convinced

    Van Leeuwen, however, is not convinced. Hipparcos catalogued more than 100,000 stars, including multiple clusters like the Pleiades, and found answers in line with predictions for the others. The distance to the Pleiades was calculated from separate measurements of more than 50 stars, and Van Leeuwen says that, for that distance to be incorrect, Hipparcos would have needed to give wrong answers in these specific measurements. He adds that there is no convincing explanation for how this could have occurred.

    He questions several technical details of the new measurements, such as the fact that the proper motions (the velocities relative to the Sun) of the Pleiades stars vary widely, whereas the proper motions of the stars within a cluster should be almost the same. “As soon as you bring the proper motions in line with each other,” he says, “all the parallaxes will change.” He also says that the Hipparcos figure can be explained. “There is no new physics needed,” he says. “The only thing that’s needed is a re-assessment of the depths of the convection layers in these stars, which have conveniently been assumed to be fixed and constant during the main sequence phase.”
    Waiting on Gaia

    In 2013 ESA launched Gaia, a successor to Hipparcos with much higher specifications, such as higher-sensitivity cameras, that will measure the parallaxes of thousands of stars in the Pleiades cluster. The design principles are conceptually similar, which leads Melis and colleagues to suggest that the unidentified error they believe distorted the Hipparcos measurements of the Pleiades could also affect Gaia. Nevertheless, Melis suspects that “the Gaia measurement is not going to be the same as the Hipparcos measurement…Hopefully then the Hipparcos community is going to have to face the fact that Hipparcos did not produce the correct result.”

    ESA Gaia satellite
    ESA Gaia Camera
    ESA/Gaia and its camera

    The research is published in Science.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 8:41 am on September 3, 2014 Permalink | Reply
    Tags: , , , , , LUNA, , physicsworld.com   

    From physicsworld.com: “Big Bang ruled out as origin of lithium-6″ 

    physicsworld
    physicsworld.com

    Sep 2, 2014
    Hamish Johnson

    Collisions between hydrogen and helium nuclei deep under a mountain in Italy have confirmed a mystery of cosmic proportions: why the amount of lithium-6 observed in today’s universe is so different from the amount that theory predicts was produced shortly after the Big Bang. Working at the Laboratory for Underground Nuclear Astrophysics (LUNA) at Gran Sasso, an international team of researchers has measured for the first time how fast lithium-6 is produced under conditions similar to those when the universe was a few minutes old. The measured rate suggests that almost all lithium-6 was actually produced well after the Big Bang – something that current theories of nucleosynthesis cannot explain.

    The only three elements created in the early universe before stars and galaxies began to form were hydrogen, helium and lithium. According to Big Bang nucleosynthesis (BBN) theory, protons and neutrons combined to form these three elements just a few minutes after the Big Bang. The snag is that while the theory does a good job of predicting the observed abundances of hydrogen and helium isotopes in the universe, it fails miserably when it comes to the two stable lithium isotopes: lithium-6 and lithium-7.

    As far as lithium-7 is concerned, numerous observations suggest that there is much less of it in the universe than predicted with BBN, with the theory that underlies the prediction having been confirmed in 2006 by experiments done at LUNA by Daniel Bemmerer of Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in Germany and colleagues. Now, Bemmerer and an international team of physicists have turned their attention to lithium-6, which accounts for about 7% of the lithium here on Earth.

    db
    Recreating the minutes after the Big Bang

    A thousand times more abundant

    The BBN model predicts that lithium-6 should account for about two out of every 100,000 lithium nuclei in “metal-poor” stars, which are believed to be among the first stars to have formed and so should reflect the composition of the early universe. However, observations made in 2006 by Martin Asplund of the Australian National University and colleagues suggest that the abundance of lithium-6 is more than a thousand times greater in such stars, accounting for about 5% of all the lithium present. The question, therefore, is whether the calculations or the observations were wrong.

    The production of lithium-6 by BBN should be dominated by one nuclear reaction, namely the collision and subsequent fusion of deuterium (hydrogen-2) with helium-4 to create lithium-6 and a gamma ray. Bemmerer and colleagues have now used the 400 kV accelerator at LUNA to study this interaction at two collision energies that would have occurred in the early universe. They did this by firing an intense beam of helium-4 nuclei at a target of deuterium gas and monitoring the collisions for the gamma rays associated with the production of lithium-6.

    Minimizing the background

    The probability that this specific fusion process occurs is very low, and so an important experimental challenge for the physicists was to see the weak gamma-ray signal among all the other radiation produced by the collisions, as well as background signals from naturally occurring radioactive materials and cosmic rays. By going deep underground, LUNA’s researchers were able to reduce the cosmic-ray background, while the effect of naturally occurring radon gas was minimized by flushing the experimental area with nitrogen gas.

    “For the first time, we could actually study the lithium-6 production in one part of the Big Bang energy range with our experiment.”
    Daniel Bremmerer, HZDR

    After carefully analysing tiny bumps in the gamma-ray spectra acquired during two experimental runs, the team calculated the rate at which lithium-6 is produced by fusion – finding it to be more or less as was expected. “For the first time, we could actually study the lithium-6 production in one part of the Big Bang energy range with our experiment,” says Bemmerer. The team then used BBN to calculate the ratio of lithium-6 to lithium-7 that should have been present in the early universe. The result is of the same order of magnitude as previously calculated, albeit a bit smaller, which makes the observation of high levels of lithium-6 in metal-poor stars even more mysterious. “Should unusual lithium concentrations be observed in the future, we know, thanks to the new measurements, that it cannot be down to the primordial nucleosynthesis,” says Bremmerer.
    Hints of new physics?

    As for the origin of most of the lithium-6 in the universe, this latest measurement reinforces the argument that it could not have been forged in the early universe. One possibility is that the isotope is produced in stellar flares. A much more radical suggestion is that the excess of lithium-6 was created by hitherto unknown physical processes, making cosmic measurements of the isotope a potential probe of physics beyond the Standard Model of particle physics.

    The research is reported in Physical Review Letters.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 6:14 am on August 23, 2014 Permalink | Reply
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    From physicsworld.com: “Strontium’s nuclear ‘spin symmetry’ revealed” 

    physicsworld
    physicsworld.com

    Aug 22, 2014
    Gabriel Popkin

    A new measurement, made by an international team of researchers using the world’s most precise clock, shows that the quantum spins of atomic nuclei can help determine an atomic collision’s strength. This phenomenon arises due to a particular type of “spin symmetry” of nuclear spins, the first direct evidence of which has now been found. The finding could help researchers better understand phenomena such as superconductivity and quantum magnetism.

    spin
    Probing strontium: seeing the first evidence of “spin symmetry

    The measurement piggybacks on a recently developed atomic clock based on the element strontium, which has two electrons in its outermost shell. Typically the spins – or magnetic moments – of these electrons point in opposite directions and cancel each other, giving the atoms zero overall electronic spin. This makes electronic spin largely irrelevant for how such atoms interact with each other. Conversely, the strontium nucleus has a non-zero spin – for the isotope strontium-87 this spin can take any of 10 values. But because the nucleus resides in the atom’s centre, inside many layers of electrons, physicists have long thought nuclear spin should not affect how atoms interact when they collide.
    Strongly interacting

    In 2010, however, theorist Ana Maria Rey at the research institute JILA in Boulder, Colorado in the US, realized that the nuclear spin could have an effect, due to a property called “SU(N) spin symmetry“. According to that symmetry, two colliding atoms with different nuclear spins should interact much more strongly than those colliding with the same nuclear spins, although the particular values of the spins should not matter. No existing device at the time of Rey’s prediction afforded the precision and control needed to measure this effect.

    That changed in January 2014, when Rey’s JILA colleague Jun Ye and others reported building a strontium clock that beat all previous clocks in precision and stability. The clock uses a population of weakly interacting strontium atoms cooled to around one-millionth of a degree above absolute zero and trapped in a lattice made by interlacing laser beams. The researchers determined the clock’s tick rate by illuminating the atoms with a red laser at a precise frequency, and averaging the rate of electron transitions between two of the atoms’ quantum-energy states. Rey theorized that this rate should change slightly depending on whether two interacting atoms had the same or different nuclear spins, and that Ye’s newest clock would be sensitive to this change.

    Super stability

    Ye’s team has now measured the predicted frequency shift. Depending on whether colliding strontium atoms have similar or different nuclear spins, the rate at which Ye’s clock ticked changed by around one part in 1016, which is like adding or subtracting 14 seconds to the age of the solar system. Detecting this minute effect would be impossible without the clock’s ultra-stable laser, which can maintain a coherent quantum state long enough to average the electronic transition frequency over hundreds or thousands of strontium atoms. “That’s really the secret weapon, that we have a laser that can maintain extremely long coherence times,” says Ye.

    ac
    Perfect timing: JILA’s strontium clock

    Understanding and controlling for the spin-symmetry effect will be crucial for building even more precise clocks, one of which will eventually replace the current cesium standard. Beyond better clocks, the team’s methods will also enable physicists to study phenomena that emerge from the quantum behaviour of large ensembles of particles, including superconductivity and quantum magnetism, Ye says. “It’s really fantastic how this clock technology is now used to investigate quantum many-body systems,” says Florian Schreck, a physicist at the University of Amsterdam who calls the result a breakthrough. “I think many people will follow in the steps of this team.”

    The work was published in Science.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 7:35 am on August 14, 2014 Permalink | Reply
    Tags: , Harvard Physics, NMR, physicsworld.com   

    From physicsworld.com: “Going mobile with NMR spectroscopy” 

    physicsworld
    physicsworld.com

    Aug 13, 2014
    Gabriel Popkin

    Nuclear magnetic resonance (NMR) spectroscopy could be about to go mobile, thanks to a team of researchers in the US that has shrunk the electronic components needed for the spectroscopic technique down to fit on an integrated circuit the size of a grain of sand. The team’s chip, combined with compact, state-of-the-art magnets, could lead to portable devices that can help identify chemicals in lab reactions and on industrial production lines.

    nmr
    Mini me: the miniaturized NMR chip

    NMR spectroscopy, a technology that has helped visualize the chemical structures of countless compounds, allows scientists to gather information from the spins – the inherent magnetic moments – of atomic nuclei. When compounds with certain nuclei, like those of hydrogen or the isotope carbon-13, are placed in a strong magnetic field, the nuclear spins align with or against the magnetic field. If the nuclei are then bombarded with electromagnetic radiation at a frequency determined by the magnetic-field strength, the directions of the nuclear spins will precess. It is then possible to measure the precession frequencies of the spins of nuclei in a sample to determine how a molecule’s atoms are arranged.

    Mini spectroscopy

    While scientists have used NMR spectroscopy since the 1950s, the necessary hardware has typically been bulky, requiring superconducting magnets larger than a person and electronics the size of a kitchen cabinet. Recently, smaller permanent magnets that are good enough for NMR have come on to the market and some of the electronic components have been integrated onto semiconductor chips, which has enabled table-top systems that can probe small molecules. But a miniaturized integrated system with a full range of NMR spectroscopy capabilities had not been developed.

    Now, though, a team based at Harvard University has done just that. The researchers placed a radio-frequency (RF) transmitter and receiver along with a component known as an “arbitrary pulse sequencer” onto a silicon chip with a surface area of 4 mm2. The scientists then combined their chip with a cube-shaped magnet around the size of a large grapefruit and were able to analyse a variety of compounds. To do the analyses, samples are placed inside a small hole in the centre of the magnet.

    The key advance was miniaturizing and integrating the pulse sequencer, which controls the timing, shapes and amplitudes of the RF pulses directed at the sample being measured, says Donhee Ham, the Harvard physicist who led the research. “The arbitrary pulse sequencer is the brain of the entire chip,” he says.

    Multidimensional probes

    The electronics the team developed are an improvement on those of previous portable systems, which have so far only implemented simplified NMR techniques that cannot fully resolve complex molecular structures, says Ham. The more sophisticated technique of “multidimensional” NMR spectroscopy can be extremely useful when trying to probe structures beyond the most basic molecules. With the new integrated pulse sequencer, the researchers can “control the RF transmitter in any way we desire, so the transmitter can produce any RF pulse sequence”, according to Ham – a requirement for multidimensional NMR spectroscopy.

    In addition to enabling portable spectroscopy, the team’s miniaturized electronics could be coupled with larger magnets to greatly speed up the NMR process. By incorporating dozens of the chips into a large superconducting magnet, researchers could scan many samples at once rather than one at a time, which can be a laborious process. Such a “high throughput” spectroscopy scheme could accelerate drug discovery, Ham says.

    The team’s work “represents a further step towards the complete miniaturization of an NMR spectrometer”, says Giovanni Boero of the Swiss Federal Institute of Technology in Lausanne. But Boero says that the integration of the pulse sequencer is a technical advance rather than a game changer. “It is not a revolutionary paper, but it is an important work in the frame of the worldwide effort towards the goal of performing NMR spectroscopy using a low-cost, highly portable system.”

    The work is published in the Proceedings of the National Academy of Sciences.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 9:32 am on August 7, 2014 Permalink | Reply
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    From physicsworld.com: “Molecular seeds sprout identical carbon nanotubes” 

    physicsworld
    physicsworld.com

    Aug 7, 2014
    Hamish Johnston

    The first effective technique for growing a batch of single-walled carbon nanotubes (SWCNTs) that all have the same molecular structure has been developed by scientists in Switzerland. The new process involves using “seed molecules” on a platinum substrate to grow SWCNTs with the desired structure. The breakthrough could be extremely important to those developing electronic devices based on SWCNTs because nanotubes with different structures can have very different electronic properties.

    tube
    Up, up and away: growing a nanotube from a seed molecule

    An SWCNT can be thought of as an atomically thin sheet of carbon that has been rolled up to form a tube about 1 nm thick, resembling a drinking straw. The carbon sheet always has the same honeycomb structure, which it shares with graphene. However, there are about a hundred different ways that the edges of the sheet can join together to make a tube, and this defines whether an SWCNT conducts electricity like a metal or a semiconductor. In the case of semiconducting nanotubes, the size of the electronic band gap also depends on how the edges are joined.

    Electronic devices based on SWCNTs could, in principle, be used to create transistors and other components that are smaller, faster and more energy efficient than those based on silicon. But before that can happen, scientists have to come up with reliable ways of producing batches of SWCNTs with identical structures.
    Costly separation

    Careful control of how SWCNTs are prepared can limit the number of different structures to as few as five. Then SWCNTs with the desired structure can be separated from a mixture. However, this is a very costly process with a structurally pure sample of SWCNTs costing about $1000 per milligram from a chemical supplier. As a result, scientists are very keen on developing methods for producing batches containing just one structure.

    This latest work was done by Juan Ramon Sanchez-Valencia and colleagues at the Swiss Federal Laboratories for Material Sciences and Technology (Empa) in Zürich.

    grow
    Grown from seed

    The new technique is based on the fact that, unlike a drinking straw, the tips of SWCNTs are capped by carbon atoms and each species has a cap with a different structure. The team used the established technique of organic chemical synthesis to create cap molecules with the same structure as the cap of the desired structural species of SWCNT. These cap molecules are placed on a platinum surface, which is heated in the presence of a carbon-rich gas such as ethylene. The platinum surface acts as a catalyst, pulling carbon atoms from the gas and passing them to the cap molecules. This steady supply of carbon molecules attaches itself to the bottom of a cap and pushes it up from surface, creating an SWCNT with the desired structure.

    Metallic armchairs

    The cap molecules were designed to seed SWCNTs with the “(6,6) armchair” structure. This much-studied type of nanotube is of interest to device designers because it conducts electricity like a metal. The SWCNTs were grown to several hundred nanometres in length before they were analysed using scanning tunnelling microscopy (STM) and Raman spectroscopy. This revealed that the SWCNTS were all of the same type and were free of structural defects.

    “The clever thing about this is that they predesign the cap and that cap then defines the nanotube type,” explains SWCNT expert James Tour at Rice University in the US, who was not involved in the research. Although the team did not show that the technique can create other types of SWCNTs by using different cap molecules, Tour says that this possibility “seems to be implied and it is likely that that would be the case”.

    Making tonnes of nanotubes

    An important benefit of the new technique is that 1 kg of seed molecules could, in principle, produce 5 tonnes of SWCNTs, each 10 μm in length. On the downside, a platinum surface measuring about 30 km2 would be needed to grow such a quantity of SWCNTs.

    An additional challenge facing anyone wanting to use the technique to produce commercial quantities of SWCNTs is how to deal with the entanglement of neighbouring nanotubes. This occurs before the SWCNTs reach a usable length, and disentangling nanotubes can be a tricky process.

    The new technique is described in Nature.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 9:09 pm on August 4, 2014 Permalink | Reply
    Tags: , , , physicsworld.com   

    From physicsworld.com: “Silicon nanorods bend light in new directions” 

    physicsworld
    physicsworld.com

    Aug 4, 2014
    Tim Wogan

    Ultrathin coatings that arbitrarily manipulate the phase and polarization of electromagnetic waves have been created by researchers in the US. The coatings are made from silicon nanorods using a technique that is compatible with industrial processes such as photolithography. The researchers say that the coatings could be used in new types of optical components that are much less bulky than traditional lenses. The technique could even be used to bend light in ways not possible with conventional lenses.

    swirl
    Swirling nanorods: an axicon lens made of silicon

    Fermat’s principle – the rule that light travels along the path of least time – says that electromagnetic waves travel along the path on which they accumulate the least phase. In a medium of higher refractive index, the wavelength shortens and so a wave accumulates more phase across the same distance. A wave therefore bends towards the normal to reduce the distance travelled in the medium and the phase accumulated.

    Manipulative metasurface

    In a conventional optical component such as a lens, phase accumulates continuously as the wave propagates and this determines the nature of the wave that emerges from the lens. However, if the phase of a wave could be changed discontinuously at a surface (called a metasurface), then the wave could, in principle, be manipulated in ways not possible with conventional optics.

    While this is straightforward in theory, the challenge facing physicists is how to create such a phase discontinuity using real materials. In 2011 researchers at Harvard University led by Federico Capasso and Zeno Gaburro covered a surface with V-shaped gold antennas so that the surface could be used to introduce any desired phase shift to optical waves passing through it. While this allows the arbitrary redirection of visible light, there are two major problems with this approach. First, the metallic nature of the surface means that most of the visible light is lost as it travels through the surface. Second, thin layers of metal are very difficult to work with and incompatible with the complementary metal-oxide semiconductor (CMOS) process used to make modern electronic devices.

    In the new research, Mark Brongersma and colleagues at Stanford University in California use lossless silicon optical antennas. When illuminated by a particular frequency of light (which can be selected by varying its diameter), the antenna will resonate strongly. This causes the light wave to pick up a phase shift that depends on the relative orientations of its polarization axes to the antenna. By appropriately tailoring the orientations and distances between the antennas, the surface can impart any desired phase shift to the light. This allowed the researchers to reproduce the functions of a bulk lens with a single layer of nanorods just 100 nm thick.

    Axicons and Bessel beams

    The team was able to create various types of “lenses” using this technique. These include traditional focusing lenses and an axicon. The latter is a specialized type of conical lens that transforms an ordinary laser beam into a Bessel beam – a ring-shaped beam used in optical tweezers and eye surgery.

    Optics expert John Pendry of Imperial College London is impressed. “If anyone in the electronics or photonics game wanted to use a material, it would have to be silicon,” he explains. “You can lay down silicon extremely flat and shape it very precisely. Metals are nowhere near silicon in terms of the precision and the control you can exert over them; so, if you can translate a technology like metasurfaces into a silicon environment, you’re on to a real winner because you can hook on to this bandwagon that’s been rolling for half a century now.”

    In the experiment, the metasurfaces were fabricated by electron-beam lithography, but team member Erez Hasman, now at the Technion-Israel Institute of Technology in Haifa, says that commercial companies could produce large quantities using industrial processes such as photolithography. “I think that Intel or other companies based on CMOS technology can implement such a metasurface now,” he says.

    “The theoretical concept is not surprising at this point, but the fact that they built it and it works is interesting,” agrees Andrea Alù, an expert on metasurfaces at the University of Texas at Austin. He looks forward to the development of optical components that are not possible with normal lenses. Hasman suggests that one of the first such uses might be to interface waveguides with free space. “In general, the modes of a laser resonator or a waveguide are very complex and different from the modes of free space,” he says. Coupling the two together to allow signals to pass between them, he explains, is very difficult using a lens or a prism but should be no problem using the 2D metasurface.

    The research is published in Science.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 3:28 pm on July 31, 2014 Permalink | Reply
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    From physicsworld.com: A SKA For Astronomy 

    physicsworld
    physicsworld.com

    Jul 31, 2014

    The Square Kilometre Array (SKA) promises to usher in a new era in radio astronomy. Astronomers will use the telescope to probe the early universe by looking as far back in time as the first 100 million years after the Big Bang. It will also be employed to search for life and planets, as well as to study the nature of dark energy. [The] video [in this article] takes you on a tour of the sites in Australia and southern Africa that will host the SKA, featuring artists’ impressions of the impressive telescope equipment. The film will also transport you to the headquarters of the SKA Organisation in the UK, where scientists and engineers describe the challenges and opportunities that lie ahead.

    vla
    The Very Large Array, a radio interferometer in New Mexico, USA

    When completed, the SKA will be the world’s largest radio telescope, with a total collecting area of one million square metres. Construction of the first phase is scheduled to begin in 2018. This will see an array of 254 dishes being built in South Africa’s Karoo region covering the bulk of the high and mid-frequencies of the radio spectrum. Meanwhile, the Murchison region in Western Australia will host the low-frequency section of the array with 96 dishes accompanied by approximately 250,000 individual dipole antennas.

    Engineers involved in the SKA project are full of impressive facts about the scale of the technology infrastructure. For instance, they say that the number of data being collected by the array will be equivalent to 10 times the global Internet traffic. And given its processing capabilities, the array will be able to survey the sky 10,000 times faster than any existing radio telescope and at a sensitivity that is 50 times greater. To put the latter figure in perspective, it means that the SKA would be able to detect an airport radar signal on a planet tens of light-years away.

    “I think it’s fair to say that the SKA really represents the next step in the evolution of low-frequency radio astronomy,” says Jeff Wagg, a SKA project scientist featured in the film. “Observing the universe at low radio frequency not only tells us about the evolution of gas in our own galaxy and other galaxies, but also tells us about the evolution of star formation in the universe.”

    In addition, the film takes a look at some of the precursor telescope arrays that are being developed in both host nations as a means of testing some of the SKA technologies. South Africa has the MeerKAT array, which is currently under construction in the Karoo and had the first of its 64 antenna inaugurated in March. Meanwhile, Australia has the Murchison Widefield Array (MWA), which is already up and running. It also has the Australian Square Kilometre Array Pathfinder (ASKAP), which astronomers are currently commissioning and testing.

    SKA MeerKAT Array
    Elements of the MeerKAT Array

    SKA Murchison Widefield Array
    Murchison Widefield Array

    Australian Square Kilometer Array Pathfinder Project
    Australian Square Kilometre Array Pathfinder

    “There are some really exciting images coming out of those instruments right now,” says SKA engineer Roshene McCool, referring to developments at ASKAP and MeerKAT. McCool says that as well as being important scientific instruments in their own right, the SKA precursor projects will also return a lot of practical information about building telescope arrays in these environments. “The design and the construction of those telescopes has built both infrastructure and also human capital in those areas so that we have skilled people who understand what is actually quite a specialized area,” she says.

    See the full article, with video, here.

    [I see no mention in this article of interferometry. Are we just supposed to assume that tey can pull this off? For this reder, thi is a missing piece.]

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics


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  • richardmitnick 8:12 pm on July 30, 2014 Permalink | Reply
    Tags: , , physicsworld.com   

    From physicsworld.com: “High-gain optical transistors flipped by just one photon” 

    physicsworld
    physicsworld.com

    Jul 30, 2014
    Hamish Johnston

    Two independent teams of physicists in Germany have created the first high-gain optical transistors that can be switched using a single photon. Based on ultracold atomic gases, the devices make use of the “Rydberg blockade“, whereby the creation of an atom in a highly excited state has a huge effect on the ability of the surrounding gas to transmit light. The research might lead to the development of all-optical logical circuits that could operate much faster than conventional electronics. The transistors could also find use in photon-based quantum-information systems of the future.

    rb
    Blocked drain: one photon flips atomic transistor

    Communications and computing systems that use only light to transmit and process information have the potential to be faster and much more energy-efficient than those that use electronic signals. While optical-fibre communications is already widespread, the switching and processing of optically encoded data is usually done by converting light pulses to an electronic signal, which can then be easily processed. The electronic signal is then converted back to a light pulse.

    Making photons interact

    This time-consuming and energy-hungry process is necessary because photons do not readily interact with each other, which makes the design of all-optical components a major challenge that is currently being addressed by physicists and engineers. During the past few years, several research groups have made important breakthroughs in this area by showing that photons can be made to interact with each other in specially prepared samples of ultracold atomic gas.

    Now, two independent teams led by Sebastian Hofferberth of the University of Stuttgart and Stephan Dürr of the Max Planck Institute of Quantum Optics near Munich have created devices in which a single “gate” photon can switch off a stream of as many as 20 photons. This gain of 20 is a huge improvement on previous attempts at optical switches, which either needed pulses of several gate photons to achieve gains greater than one or offered gains of much less than one for single-gate photons.

    Both teams based their gates on gases of rubidium atoms that were cooled to temperatures below 1 mK. Normally, the gas is transparent to a beam of “source” photons, which can travel through the device and emerge via the “drain” – gate, source and drain being terms used to describe the control, input and output channels, respectively, of a conventional field-effect transistor.
    Blocking the drain

    When a gate photon is fired into the gas, it is absorbed by one atom, which puts that atom into a highly excited Rydberg state with one electron in an extremely large orbital. The large distance between this electron and the nucleus gives the atom a very large electric dipole moment, which shifts the energy levels of nearby atoms. This shift causes the gas to become opaque to light from the source, effectively switching the transistor off. The Rydberg state endures for about 1 μs, which is a surprisingly long time for an atomic system. This allowed Dürr and colleagues to use their transistor to switch off a stream of 20 source photons, while Hofferberth’s team prevented 10 photons from reaching the drain of its device.

    “This effect should make it possible – at least in principle – to cascade such transistors to solve complex computational tasks,” says Dürr. He also points out that the experiments offer physicists a new and non-destructive way of studying the physics of Rydberg states. The ability to operate at the single-photon level also means that the transistors could find use in quantum-information applications such as secure quantum-communication systems or powerful quantum computers.

    Another interesting aspect of the devices is that the gate photon is re-emitted by the gas when the Rydberg states decay – an effect that has been observed in other experiments. In principle, this means that the transistors could also be used as storage devices for quantum information.

    Both experiments are described in separate papers in Physical Review Letters.

    See the full article here.

    PhysicsWorld is a publication of the Institute of Physics. The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics


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