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Home » Page 1493

Throwing some (laser-generated) shapes

SPIE lightshow.jpg
Light show at Photonics West

By Margaret Harris

The last time I saw a laser light show, I was six years old. Judging from last night’s “Cirque du Laisaire” event at the Photonics West conference (sponsored by the professional optics society, SPIE), the technology has moved on considerably since then.

Unfortunately, this photo doesn’t really do it justice. Lasers are hard to photograph at the best of times, and on this occasion I think my camera had been drinking too many of the event’s signature drink: the “Laser Martini”.

laser martini.jpg
Fancy a laser martini?

This violently blue concoction is made from (so the barmaid informed me) vodka, blue curacao, white cranberry juice and triple sec, with a twist of lemon. And, since another part of the evening’s entertainment was a clip from the James Bond film Goldfinger (you know, the bit where the villain tries to cut Agent 007 in half with a giant laser), it was of course served shaken, not stirred.

The highlight of the evening was the laser magic show, in which a magician called Latimer appeared to pick up a laser beam and spin it around his head. The trick didn’t get much applause, but there’s a reason for that; as the man next to me commented, “Right now, 400 physicists in this room are too busy trying to work out how the hell he did that.” You can watch a version of the show here.

Russian science in a state of ‘decline’

Despite launching the world’s first artificial satellite and being a leader in the nuclear arms race, science in Russia is in a state of decline and the country is losing its standing as a scientific powerhouse. So says a new report by the information-services provider Thomson Reuters. Entitled The New Geography of Science: Research and Collaboration in Russia, the report warns that the country’s research base “has a problem, and it shows little sign of a solution”.

Over the last five years, researchers in Russia have produced about 127,000 papers across all sciences, accounting for about 2.6% of the world’s output, according to data taken from Thomson Reuters’ Web of Science database. This share of publications is less than that for researchers in China and India, with 2.9% and 8.4%, respectively, but higher than fellow BRIC nation Brazil, whose scientists publish about 102,000 papers that account for 2.1% of the world’s output.

Russia still maintains a strong focus in the physical sciences but this too is in decline compared with other countries’ output. Between 1999 and 2003, physics articles published by researchers based in Russia accounted for 9.7% of the world’s output, with about 38,000 papers published. However, between 2004 and 2008 that number had shrunk to 7.4%, or 35,000 papers.

Still strong in nuclear science

Russia, however, is still potent in some areas of physics. Its strongest area is nuclear science, with Russian researchers publishing 3100 papers in the field between 2004 and 2008 – about 10.3% of the world’s output. Mineralogy is its second best field, followed by particle physics, which accounts for roughly 9.1% of the global total.

The main reason for the decline is the total neglect of fundamental science by the post-Soviet governments in Russia Andrei Starinets, Oxford University

“I think the overall tendency of a decline in Russia’s research output since the early 1990s is reflected correctly in this report,” says theoretical physicist Andrei Starinets from Oxford University, who co-authored a letter to Russian President Dmitry Medvedev last October along with 170 other Russian researchers working abroad that called for reforms to the country’s science base. “The main reason for the decline is the total neglect of fundamental science by the post-Soviet governments in Russia, especially from 1992 to 2002.”

Starinets agrees that the Russian government is, however, doing little to reverse the decline. “There is an absence of serious structural reforms in science in the country,” says Starinets. “Recent attempts by the government to improve the situation still fall short of the actual needs.”

Net widens for funding of arXiv

Librarians at Cornell University want more external funding to support their popular arXiv preprint server because the running costs are now “beyond a single institution’s resources”.

arXiv has become the most widely used preprint server among academics in the physical sciences. It received more than 60,000 new submissions in 2009, has about 400,000 registered users and provides 2.5 million article downloads per month. Its rate of expansion is so rapid that staff expect its budget – which covers personnel as well as operating expenses – to increase from $400,000 in 2010 to $500,000 in 2012. Most of the top 25 institutional users have already made financial commitments, but to meet the budget demands Cornell Library would like several hundred others to pledge support too.

According to a white paper on the arXiv website, Cornell is looking to introduce a three-tiered model in which the top 100 institutions, based on the previous year’s download activity, will be obliged to pay Cornell Library $4000 a year. For those ranking between 101 and 200 in downloads the suggested fee will be $3200 a year, while for those below 200 it will be $2300.

“Keeping an open-access resource like arXiv sustainable means not only covering its costs, but also continuing to enhance its value, and that kind of financial commitment is beyond a single institution’s resources,” says Oya Rieger, a Cornell University librarian who manages content on the server. “If a case can be made for any repository being community-supported, arXiv has to be at the top of the list.”

Unabated growth

It’s difficult for the library to maintain an indefinite commitment to unilateral support of a resource that provides so much benefit outside of the university Paul Ginsparg, Cornell University

Paul Ginsparg, the Cornell physicist who created arXiv as xxx.lanl.gov at the Los Alamos National Laboratory back in 1991, and who won a MacArthur Fellowship for his efforts, shares the same view. “It’s difficult for the library to maintain an indefinite commitment to unilateral support of a resource that provides so much benefit outside of the university,” he told physicsworld.com. “The usage levels, submission rates, external enthusiasm, visibility, impact, etc continue to grow unabated, so it’d be useful to assess whether this can be made a community-supported resource in the long term.”

Indeed, Cornell Library has long-term plans for even more community input. Along with consistent underwriting from the library’s own budget, it would like to seek support from other libraries, societies, endowments or funding agencies, such as the US National Science Foundation. It is also looking to strengthen ties with collaborations – such as the INSPIRE project by the particle-accelerator labs CERN, SLAC, DESY and Fermilab for collating high-energy-physics literature – to improve its service.

“arXiv has become such a useful resource to researchers in many areas of physics, [so] it doesn’t seem at all unreasonable that the burden of financing it should be spread a little more evenly among its users,” says Alison Wright, editor of the journal Nature Physics.

Important resource

Gene Sprouse, editor-in-chief of the American Physical Society, agrees. “This important resource for the community of scientists costs money to run even though everyone, including me, would like information to be free,” he says. He points out that the APS’s 10-year-old open-access journal Physical Review Special Topics: Accelerators and Beams uses a similar funding model to that proposed by Cornell Library in which cash comes from an international consortium of accelerator labs. “The primary users of arXiv form a similar small community with a common interest, so there is a good chance for success, as with our journal,” he says.

However, the “institutions pay” model is not the only way to fund open access. The Institute of Physics (which owns physicsworld.com) and the Deutsche Physikalische Gesellschaft publish New Journal of Physics, which offers free access to all papers by charging authors publication fees. Tim Smith, who is the senior publisher in charge of New Journal of Physics, suggests that arXiv could consider charging a small submission fee to authors to cover costs. “It would be very interesting to see how the physics community would respond to a submission fee on arXiv,” says Smith.

One benefit of this approach is that income would be proportional to the number of papers processed – and processing papers is often the most costly part of an online publishing operation. According to arXiv’s own figures, about $7 per submission would cover the 2010 budget.

50 years of the laser

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View of the Moscone convention centre and San Francisco Bay

By Margaret Harris

Fifty years ago, lasers were “a solution looking for a problem”. Today there seems to be no limit to their reach. There are lasers in space and lasers underground; lasers in the lab, factory, hospital and office; lasers that could scarcely singe a fly and lasers that cut through metal as if it were butter. Scientists use lasers in precision measurements of systems that range from atoms to planets. Medical doctors use them to perform delicate surgery. Nearly everyone uses them to listen to music or read other kinds of data. For astronomers, lasers can be a tool for making an artificial star in the sky; for fusion physicists, they may someday be the key to creating a very different kind of artificial star, this time down here on Earth.

Oh, and they look cool, too.

(more…)

Plasmas are cool for dental disinfection

Painless jets of plasma could come to replace the drill in the fight against harmful bacteria in dentistry. That is according to researchers in Germany who have created a futuristic device that they say should be available to dentists within three years.

Cavities can form in teeth as bacteria penetrate through their enamel coating to an inner region that consists of a pulpy material known as dentin. When this material begins to decay, dentists normally drill away the damaged enamel and dentin, chemically disinfect the area, and then protect it with a resin filling.

Scientists in Homburg and Leipzig now say that low-temperature plasmas can kill these bacteria, potentially stopping cavities forming in the initial phase, without the patient feeling pain in the process. “Plasmas may provide us with new possibilities for destroying bacterial biofilms, thereby preserving more dentin,” says Stefan Rupf of Saarland University’s Clinic of Operative Dentistry.

Cool comfort

Plasmas – which are clouds of highly ionized gas – are already used by medical professionals to sterilize surgical equipment and even parts of the human body during operations. In dentistry, however, it is problematic to use plasmas because they can damage the dental pulp inside a tooth as their formation usually requires temperatures of at least 100 °C.

Rupf and his colleagues exploited a low-temperature plasma jet developed at the Leibniz Institute of Surface Modification (IOM). “We use a pulsed plasma that switches on and off with high frequency,” explains Axel Schindler, leader of the IOM group that developed the low-temperature source. “Only in the on phase does the microwave energy heat the plasma, and we use a high flow of gases that cools the surface when the plasma is switched off.”

The cool plasma was tested on bacteria both in cultures and adhered to dentin slices. Growth was completely inhibited in the two types of bacteria, while bacterial colonies were reduced 10,000-fold in the dentin slices. The adherence to tubules allowed some colonies to survive on all the dentin slices. However, more bacteria died when exposed to the plasma jet for longer, which convinces Rupf that this could help dentists reduce or avoid drilling.

The plasma warmed the dentin surface to about 44 °C, but this should not damage the tooth interior. “The pulp is not critically heated up,” Schindler says. “When this reaches patients, they will feel a gas flow that is a little bit warm.”

Dental block

Schindler hopes to find an industrial partner and commercialize dental plasma jets within three years. While laser equipment for similar applications is already available for about €10,000, the IOM researcher believes a plasma tool would cost just a few thousand euros. “Microwave excitation is cheap, simple and safe – the same technology is used to heat up food,” he says.

Gerrit Kroesen of the Eindhoven University of Technology believes dentists and other medical professionals will indeed use plasmas for disinfection, although not anytime soon. “All kinds of qualification and certification issues have to be tackled,” he told physicsworld.com.

This work is to be published in the Journal of Medical Microbiology.

Sun’s appetite for dark matter may affect Earth’s orbit

Calculations made by a physicist in Italy suggest that the observable changes to Earth’s orbit could be caused by the Sun’s appetite for dark matter. This latest research predicts that over the next few billion years the orbits of the planets should shrink considerably, with the Earth to Sun distance halving over this timescale.

Physicists believe that some 23% of the mass-energy content of the universe is made up of dark matter, a non-luminous substance that interacts gravitationally with ordinary matter. This dark matter is spread throughout the universe but clumps together at higher densities in the vicinity of visible bodies, thereby forming a “halo” around the Milky Way. Some researchers also believe that the solar system is home to an especially dense lump of dark matter.

Accreting since birth

The Italian physicist Lorenzo Iorio calculated the effects of this dark matter on the orbits of the planets. To do this he assumed that between 2 and 5% of the Sun’s mass is in the form of dark matter, an upper limit imposed by measurements of properties such as solar luminosity and energy flux, and that the Sun accumulated this dark matter continuously over its 4.5 billion year history as it moved through the galactic halo. These assumptions lead to a fractional increase in the Sun’s mass of about one part in 1012 each year.

First, Iorio calculated how much further out the planets would have been at the birth of the solar system 4.5 billion years ago. The outermost planets, he concluded, would have been the furthest removed from their present positions, the orbital radius of Neptune, for example, being greater by up to 10 astronomical units (1 au is approximately equal to the Earth’s semi-major axis, some 150 million kilometres). Earth, in contrast, would have been no more than 0.32 au further out.

Iorio also worked out how much the planetary orbits will have shrunk by the time the Sun is expected to reach its maximum size as a red giant star, in some 7.5 billion years’ time. He calculated that Neptune could be some 16 au closer in, while the Earth’s radius could be halved. Given that there is currently disagreement between astronomers over whether or not the Earth’s orbit will be engulfed by the expanding Sun, Iorio says that the effect of solar dark matter would be to increase the probability of this encroachment.

Just about consistent with observations

One counterintuitive effect of the Earth’s orbital shrinkage, according to Iorio’s modelling, is that the Earth’s semi-major axis will actually increase by some 2–5 cm per year. He says that this is possible because the solar dark matter causes the trajectory of the Earth’s orbit to continually shrink and therefore only approximate an ellipse. So while the distance of closest approach of the Earth to the Sun in each cycle decreases, the distance taken to be the semi-major axis can increase. In fact, this conclusion is just about consistent with observations, combined from a variety of different sources, showing an increase in the astronomical unit of between 5 and 9 cm per year.

Iorio also calculated how planetary accretion of dark matter would affect the motion of the planetary satellites but found this to be negligible – with the Moon’s orbital radius a mere 160 m greater at the beginning of the solar system. In fact, this mechanism is not able to explain a number of observed changes in the orbits of planetary satellites, including different variations in the orbital periods of some of the moons of Jupiter and a decrease in the semi-major axis of the Earth’s artificial satellite LAGEOS.

Indeed, Iorio emphasizes that his work does depend on a number of assumptions about the nature of dark matter, in particular the rate at which it might accrete within the solar system. Philippe Jetzer, an astrophysicist at the University of Zurich points out that the situation is further complicated by the fact that the Sun and the planets can also accrete ordinary matter. “With our present knowledge one cannot conclude that dark matter is accumulating at any significant rate within the solar system,” he says.

The research is reported at arXiv:1001.1697.

Legendary oceanographer scoops Crafoord prize

American oceanographer Walter Munk has been awarded this year’s Craaford Prize in Geophysics by the Royal Swedish Academy of Sciences.

Munk is honoured “for his pioneering and fundamental contributions to our understanding of ocean circulation, tides and waves, and their role in the Earth´s dynamics”.

The celebrated American researcher is perhaps best known for his holistic approach to ocean science, in particular his grasp of the tide’s significance on a number of scales.

“Walter is without question the scientist who has contributed more than anyone else to oceanography over the past 50 years,” says Leif Anderson, a marine chemist at the University of Gothenburg, Sweden, who was a member of the prize-awarding panel.

The King and Queen of Sweden will present Munk with a cheque for 4 million Swedish Krona, (about $550,000), at a ceremony in Stockholm in May.

Ex-banker

Born in Vienna, Austria in 1917, Walter Munk and his family moved to New York State in 1932 as his family envisioned a career in banking for their promising son.

Munk soon realized, however, that his real passion was the natural sciences, so he enrolled at California Institute of Technology where he obtained a BSc in physics in 1939.

After a brief spell with the US Army as a private, Munk was excused from action to conduct military research at the Scripps Institution of Oceanography at University of California. He remained at the institution after the Second World War and completed his PhD in oceanography in 1947 and was awarded a professorship in 1954.

In the latter half of the 20th century, Munk went on to revolutionize marine science by rigorously applying mathematics to the world’s oceans to link them with the wider global dynamics. One example is his work on mixing processes in the oceans, which helped to improve our picture of how heat is distributed in the global energy balance.

“He provided us with a theoretical basis to explaining ocean circulation which will be invaluable as climate models become more sophisticated,” says Anderson.

Surfin’ USA

Munk is also hailed by surfing communities around the globe, which are grateful for the advances in tide and wave forecasting enabled by the oceanographer’s research. Erik Huss, a spokesperson for the Royal Swedish Academy of Sciences, describes Munk as a “legendary figure” amongst surfers who have honoured him with gifts.

Despite his 93 years, Munk is apparently still an active figure in the research community having co-authored no fewer than four research articles last year.

The Crafoord Prize in astronomy and mathematics, biosciences, geosciences or polyarthritis research is awarded by the Royal Swedish Academy of Sciences annually according to a rotating scheme.

The prize money comes courtesy of the Anna-Greta and Holger Crafoord´s Fund which was set up in 1980. Holger, who died in 1982, had been a successful Swedish industrialist before suffering badly from rheumatoid arthritis in later life.

Splashing stones drive supersonic flow

There is something satisfying about the plonk and splash of a stone falling through the smooth surface of a pond. But now this seemingly ordinary, albeit pleasing, phenomenon appears to be anything but mundane. A sinking stone, it turns out, can create a supersonic jet of air.

When a stone plunges into water, a cylindrical sheet of water called the “crown splash” is sent up into the air. As the stone begins to sink, it pulls a cylindrical cavity of air down with it. The surrounding water then pinches middle of the cavity to create an hourglass shape. The cavity begins to collapse and the upward rush of air causes the spectacular finale – a jet of water that shoots high up above the surface of the pond.

Detlef Lohse and colleagues at the University of Twente and the University of Valencia have now shown that this final rush of air moves faster than the speed of sound.

Cloud of smoke

The team watched this happen by pulling a 40 mm diameter disc below the surface of a water tank that was covered in a cloud of smoke. The smoke was sucked into the air cavity, allowing the physicists to study its shape using a high-speed camera that can also track the motion of individual smoke particles.

Lohse’s team found that the shape of the hourglass constriction resembles a “de Laval nozzle”, which is used in supersonic jet and rocket engines. The team was able to measure the diameter of the nozzle and found that it shrinks rapidly as time progresses, which boosts the speed of the escaping air.

By tracking the smoke particles, the team was able to measure air speed as a function of nozzle diameter up to speeds of about 10 m/s – much less than 343 m/s, which is the speed of sound in air. Undeterred, the team was able to use this data along with numerical simulations to predict the speed of the air for much smaller nozzle diameters. They concluded that the air could accelerate to beyond the speed of sound when the diameter of the neck shrinks to about 2% of its original size.

Shockwave spotted

The team also saw a shock wave develop above the nozzle, which they say is further evidence for supersonic flow. It creates two jets of water – one that flows downward and the other upward beyond the surface. The latter often exceeds the height from which the splashing object is dropped.

Many of these effects can be seen without smoke or cameras – so find yourself a stone and pond, and break the speed of sound.

The work is described in Physical Review Letters.

Ions trapped by optical fields

 

Physicists in Germany claim to have trapped single ions using lasers for the first time – an achievement that could open the door to advanced simulations of quantum systems.

In the past, the trapping of atomic particles has followed a basic rule: use radio-frequency (RF) electromagnetic fields for ions, and optical lasers for neutral particles, such as atoms. This is because RF fields can only exert electric forces on charges; try to use them on neutral particles and there’s little effect. A laser, on the other hand, can draw the dipole moments of neutral particles towards the centre of its beam. But the resultant optical trap is relatively weak, and so ions – which are sensitive to stray electric fields – easily escape.

Now, Tobias Schaetz and colleagues at the Max-Planck Institute for Quantum Optics in Garching, Germany, claim to have got around this problem. They were unable to speak with physicsworld.com because they are planning to submit to a journal with an embargo policy that discourages communication with journalists, but in a preprint available on the arXiv server they describe an “experimental proof-of-principle” of how stronger, more focused lasers can optically trap ions for over a millisecond.

Single magnesium ion

In the experiment, Schaetz’s group cooled a single 24Mg+ ion in a standard RF trap before superimposing it with the field from a strong laser. The physicists then gradually reduced the RF field to zero so the ion was contained by the optical field alone. Finally, they observed the fluorescence light of the ion through a CCD camera to check it had been trapped successfully.

Over several experiments, they calculated the trapping lifetime as about 1.8 ms. “The lifetime of the ion in the optical dipole trap is limited by photon scattering and is thus expected to be improvable by state of the art techniques,” they note.

Optical traps for ions could benefit simulations on quantum systems. If optical traps are superimposed on RF traps, there comes the ability to simulate in two or three dimensions, or collide particles in new ways. “The biggest advantage of being able to trap an ion optically, instead of [using] an RF electrode, is that one can then study the collision between a neutral atom and an ion at low energy,” explains Yu-Ju Lin, a researcher at the Joint Quantum Institute at the University of Maryland, US. “If an ion is still trapped by an RF electrode, it always has an RF-induced micro-motion – i.e. kinetic energy – which limits how low the energy of the collisions [can go].”

‘Excellent experimental physics’

Andrew Steane, a quantum physicist at the University of Oxford, UK, calls the research “excellent experimental physics”, although he notes that it is an extension of previous ideas and experiments. “Such [an optical trap] is of course already widely used in experiments on neutral atoms,” he adds. “Applying that idea to ions extends the toolbox available to experiments in basic atomic physics and quantum mechanics.”

The research is available as a preprint at arXiv:1001.2953.

Jostling balls reveal secrets of ultrathin films

Depositing extremely thin crystalline layers of different materials on a substrate – a process known as “epitaxy” – is key to the manufacture of semiconductor devices. But physicists have long been puzzled as to why some materials form flat ordered layers, whereas others grow as rough mound-like structures. Although researchers have created models of epitaxy that have helped the microelectronics industry, it has proved impossible to see how individual atoms move from site to site along a surface while forming a layer because the atoms are so small and move so fast.

Now, however, Itai Cohen and colleagues at Cornell University in the US have gained key insights into epitaxy by creating crystalline layers made of “colloidal” particles, which are bigger than atoms and so move much more slowly. They did this by bonding a closely packed layer of plastic balls 1 µm in diameter onto a flat substrate, which was then submerged in an aqueous solution of the balls. These balls were free to sediment onto the crystalline surface, where they slowly formed a thin layer.

In order to simulate the attractive interaction that causes atoms to crystallize into layers, the team then added much smaller polymer chains to the mixture, which coil up into spheres with a diameter of about 100 nm. Both the polymer spheres and the larger plastic balls are jostled about by random thermal fluctuations in the water (Brownian motion). But when two plastic balls come to within about 100 nm of each other, the polymer chains can no longer fit between them. Because there are no polymers between the balls to push them apart, the balls start to close in on each other – an imbalance that looks like a short-range attractive force.

Balls in motion

The team took a series of optical micrographs of the surface over several days, combining them to create a “movie” that shows how the plastic balls move (see above). The data reveal that when individual spheres land on the surface they move around randomly. But if the spheres get close to each other, they can coalesce to form a crystalline “island” that can grow by attracting other spheres.

In an ideal world, these islands would grow to cover the entire substrate, while remaining just one sphere thick. In practice, however, spheres tend to land on top of an existing island – and if these spheres cannot move over the edge of the island and onto the substrate, then the island will turn into a mound. Understanding and controlling the processes that cause such “step-edge barriers” are crucial to developing techniques for creating perfectly flat films.

As the interaction between atoms can extend out several atomic radii, atoms at an edge of a mound end up feeling an overall inward pull that keeps them on the island. However, the bonds between the plastic balls in the Cornell team’s model system are much shorter (relative to the ball’s radius) and the physicists did not expect to see such a barrier. But they did, which inspired the team to study the barrier using optical tweezers to place a sphere near a step edge and watching what happened next.

The Cornell team worked out that a sphere travelling across a step edge to the nearest lattice site has to move three times further than a sphere moving to a neighbouring site in any other direction. Because the motion of the spheres is described by a random walk, travelling over a step edge can take up to nine times longer than moving in another direction – which means that a sphere is more likely to remain on an island than leave it.

Cohen told physicsworld.com that such diffusion-related effects have not, as far as he is aware, ever been considered in models of atomic epitaxy, but they could be relevant because atoms are jostled about on a substrate by lattice vibrations. As a result, atoms could be “doubly cursed” when it comes to processes that work against the formation of perfectly flat films.

Biasing a random walk

On a more positive note, Cohen says that films of both spheres and atoms could be smoothed out by biasing the direction of diffusion. In the case of the spheres, this could be done by simply increasing the density mismatch between the particles and the fluid and letting gravity do the job. It may be possible to do the same for atoms by applying electric fields.

Other phenomena that could be studied using the spheres include surface reconstruction – a distortion of the surface lattice that often occurs when the film and substrate are different materials. This could be simulated by using two different-sized spheres for the substrate and film – something that the team is now investigating.

Cohen also points out that crystals made of micron-sized spheres are interesting in their own right. Indeed, they can be made with lattice constants on par with optical wavelengths. This means that they could find uses as photonic crystals in optical telecommunication devices.

The work is reported in Science 327 445.

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