The Laser Glow of an Atom Cloud

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Random lasers are the "disco ball" version of normal lasers. Instead of beaming in one direction, the light is scattered out in all directions by particles in a powder, for example. Calculations described in the 1 May Physical Review Letters show that a cloud of cold atoms could form a new type of random laser, where individual atoms scatter and amplify the light. The work suggests that the atoms will begin lasing if the cloud can be made opaque, or "cloudy," enough, which the authors think is within reach of current technology. Because such a system is fundamentally simple, it may give insight into some of the mysteries of random lasers.
A normal laser is essentially a gain medium inside a reflective cavity. The light is amplified by the medium as it bounces back and forth between the cavity's mirrors. A random laser has no cavity. Instead, tiny "mirrors," or scatterers, are added to the gain medium, causing photons to bounce around and become amplified by the medium, before escaping in all directions. For example, a container of micron-sized particles floating in water in which a laser dye has also been dissolved can emit laser light if pumped with external light. Random lasers do not require the same precise manufacturing as normal lasers, so they could be inexpensive to produce. Potential applications include digital displays, light emitting paints, and temperature sensors.
There are, however, some uncertainties regarding the spectra and time variability of random lasers [see Focus story: "Photon Marathon"]. So Robin Kaiser of Nice Sophia Antipolis University and the National Center for Scientific Research (CNRS) in France and his colleagues considered a random laser made up of cold atoms. Such a system would not have the absorption and other difficult-to-model complexities of existing random lasers. However, "it was not clear that atoms would be able to play the roles of both gain medium and scatterers," says Nice team member William Guerin. A single atom scatters photons when it's in its ground state and amplifies photons (through stimulated emission) when it's in an excited state, but it can't do both at the same time. So the researchers needed to find the right balance of atoms in both ground and excited states.
The team modeled a two-level atomic system. In their scenario, a complicated three-photon transition--made possible by an external pump laser--keeps a sufficient fraction of the atoms in the excited state (a so-called population inversion) from which they can provide gain. For different pump frequencies and intensities, the researchers calculated the amount of scattering necessary to trap light in the cloud long enough for it to become true "coherent" laser light, with all the photons propagating in synch. They quantified the amount of scattering within the cloud with a number called the optical thickness [1]. A value between 200 and 250 would lead to lasing, they found, which is within reach of state-of-the-art cold atom traps that currently achieve 150, Guerin says.
The researchers performed a simulation showing that the laser light should be detectable over background light coming from the cloud. They are now trying to make such a random laser in the lab using a standard magneto-optical trap to hold the atoms. Guerin says there are tricks to compress the trap to increase the density and therefore the amount of scattering.
There may, however, be experimental challenges involving the non-uniform density of the cloud, says Diederik Wiersma of the European Laboratory for Non-linear Spectroscopy in Florence, Italy. But if it can work, the use of atoms, rather than micron-scale scatterers, "could enrich our understanding of the random laser phenomena," Wiersma says. Moreover, because cold atoms are capable of slowing down light, he speculates that they could be more efficient at trapping light than current techniques. --Michael Schirber Michael Schirber is a freelance science writer in Lyon, France.
References:[1] The optical thickness--a characterization of the "opaqueness" of the atoms--can be thought of as the diameter of the cloud measured in units of the "mean free path," which is the typical distance a photon travels before scattering. This refers to scattering with the pump laser turned off, when there is no lasing.
Related Information:Related info:
Focus story: Photon Marathon (2004)
more explanation of random lasers from Wiersma's lab web site
Threshold of a Random Laser with Cold Atoms Luis S. Froufe-Pérez, William Guerin, Rémi Carminati, and Robin Kaiser Phys. Rev. Lett. 102, 173903 (issue of 1 May 2009)

Unveiling The Structure Of Microcrystals

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http://www.sciencedaily.com/releases/2007/10/071004092126.htm

Science Daily — Microcrystals take the form of tiny grains, so small that they resemble a powder. How can we determine their structure?
Until now, the technique of X-ray diffraction, normally used to study crystals, was not an appropriate solution. For the first time, researchers from the ESRF and the CNRS have used X-ray diffraction to determine the structure of microcrystal grains of only one cubic micrometre in size.
They gained a factor of a thousand on the size of the analysable samples thanks to new equipment created at the ESRF. This breakthrough opens up new possibilities of research to chemists, physicists and biologists.
The properties of a crystal are determined by the arrangement of its atom in space, its crystalline structure. Scientists use X-ray or neutron diffraction to study crystalline structure when the size of the crystal is more than 10 cubic micrometres. Below this limit, the solid material is considered a powder.
Scientists can apply powder diffraction to analyse such a material but this technique is not easy to exploit. Moreover, powder diffraction can only be used for materials with grain sizes of less than three millionths of a cubic micrometre. Due to these limitations, a determination of the structure of new synthetic solids in powder form is not always possible because the crystals are too small.
The teams from the ESRF and the Institute Lavoisier (CNRS/Université de Versailles Saint-Quentin) have used new set-up permitting X-ray diffraction on crystals of a size of one cubic micrometre, a volume a thousand times smaller than that ever attainable before. This new set-up consists of a focusing system for the ESRF beam, coupled with a goniometer, an instrument to position the sample with maximum precision.
The researchers studied the structure of an organic-inorganic hybrid compound (a microporous aluminium carboxylate), which could be used for gas absorption or to encapsulate various organic molecules. This study confirms that the new set-up allows pushing back the limits in crystal dimension accessible to X-ray diffraction.
“It is a revolution: what was considered a powder in the past has become a crystal today. Researchers can now bring forward samples left in their cupboards because the sizes had previously prevented their study. Now they will be able to elucidate the structures of these samples, with potentially great scientific advances on the horizon”, explains Thierry Loiseau, from the Institut Lavoisier.
Reference: A Microdiffraction Set-up for Nanoporous Metal-Organic-Framework-Type Solids. C. Volkringer, D. Popov, T. Loiseau, N. Guillou, G. Férey, M. Haouas, F. Taulelle, C. Mellot-Draznieks, M. Burghammer and C. Riekel, Nature Materials, 6 (2007) 760-4.
Note: This story has been adapted from material provided by European Synchrotron Radiation Facility.

Fausto Intilla
www.oloscience.com

New Giant Molecule Created

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http://www.sciencedaily.com/releases/2007/10/071005214924.htm

Science Daily — Ulrich Kortz, Professor of Chemistry at Jacobs University, and his team successfully synthesized a polyoxometalate with 100 Tungsten and 20 Cerium atoms that has a molar mass of about 30 kilo Dalton. With a maximum diameter of 4.2 nm the inorganic molecule is comparable in size to large complex bio-molecules or even small viruses.

Polyoxometalates are anionic metal-oxygen clusters of large structural diversity with chemical properties, which make them especially interesting for applications in catalysis, but also in materials science and nanotechnology.
Ulrich Kortz and his co-workers now achieved the synthesis of the tungstogermanate*, which belongs to the polyoxometalates, by condensation of the precursors [α-GeW9O34]10- and Cerium(III) ions in aqueous solution.
With about 600 atoms in total, amongst them 100 atoms of the heavy metal Tungsten, the new compound is the third largest molecular polytungstate ever synthesized. In addition it contains the largest number of atoms of the Rare Earth Cerium ever incorporated in such a compound.
„A single molecule of our new giant tungstate has many catalytically active centers and therefore a very high catalytic potential, which normally applies only to biological catalyst molecules. Being a lot less temperature and oxidatively sensitive than bio-catalysts though and in crystalline form applicable as a heterogenic solid catalyst in liquid phase reactions our new tungstogermanate is predestined for industrial purposes,“ says Ulrich Kortz about the possible applications of the newly created molecule.
“In addition our successful synthesis allows very good inferences about the mechanism of formation by stepwise self-assembly of the simple precursors in a classic one-pot synthesis, which is vitally important for the development of other so-called ‘molecular machines’, large molecules designed to have very specific functions,“ the Jacobs chemist concludes.
*Tungstogermanate [Ce20Ge10W100O376(OH)4(H2O)30]56-
The reaction conditions and the molecular structure were published in Angewandte Chemie (doi: 10.1002/anie.200701422).
Note: This story has been adapted from material provided by Jacobs University.

Fausto Intilla

www.oloscience.com

A New Look At The Proton

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http://www.sciencedaily.com/releases/2007/09/070925092529.htm

Science Daily — Dutch researcher Paul van der Nat investigated more than three million collisions between electrons and protons. In his PhD thesis he demonstrates -for the first time– that the spin contribution of quarks to the proton can be studied by examining collisions in which two particles (hadrons) are produced.

The spin of a particle can most easily be compared to the rotating movement of a spinning top.
In the HERMES experiment at the HERA particle accelerator in Hamburg, physicists are investigating how the spin of protons can be explained by the characteristics of their building blocks: quarks and gluons.
Van der Nat investigated a method to measure the contribution of the spin of the quarks to the total spin of the proton, independent of the contribution of the spin of the gluons. For this a quark is shot out of the proton by an electron from the particle accelerator, as a result of which two hadrons are formed.
The direction and amount of motion of these two hadrons is accurately measured. This method, which Van der Nat applied for the first time, turned out to be successful.
Spin is a characteristic property of particles, just like matter and electrical charge. Spin was discovered in 1925, by the Dutch physicists Goudsmit and Uhlenbeck. In 1987, scientists at CERN in Geneva discovered that only a small fraction of the proton's spin is caused by the spin of its constituent quarks.
The HERMES experiment was subsequently set up to find this missing quantity of spin, and has been running since 1995. It is expected that spin will play an increasingly important role in many applications. The MRI scanner is a well-known example of an application in which the spin of protons plays a key role.
Note: This story has been adapted from a news release issued by Netherlands Organization for Scientific Research.

Fausto Intilla

www.oloscience.com

Official Kilogram Losing Mass: Scientists Propose Redefining It As A Precise Number Of Carbon Atoms

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http://www.sciencedaily.com/releases/2007/09/070921110735.htm

Science Daily — How much is a kilogram?

It turns out that nobody can say for sure, at least not in a way that won't change ever so slightly over time. The official kilogram -- a cylinder cast 118 years ago from platinum and iridium and known as the International Prototype Kilogram or "Le Gran K" -- has been losing mass, about 50 micrograms at last check. The change is occurring despite careful storage at a facility near Paris.
That's not so good for a standard the world depends on to define mass.
Now, two U.S. professors -- a physicist and mathematician -- say it's time to define the kilogram in a new and more elegant way that will be the same today, tomorrow and 118 years from now. They've launched a campaign aimed at redefining the kilogram as the mass of a very large -- but precisely-specified -- number of carbon-12 atoms.
"Our standard would eliminate the need for a physical artifact to define what a kilogram is," said Ronald F. Fox, a Regents' Professor Emeritus in the School of Physics at the Georgia Institute of Technology. "We want something that is logically very simple to understand."
Their proposal is that the gram -- 1/1000th of a kilogram -- would henceforth be defined as the mass of exactly 18 x 14074481 (cubed) carbon-12 atoms.
The proposal, made by Fox and Theodore P. Hill -- a Professor Emeritus in the Georgia Tech School of Mathematics -- first assigns a specific value to Avogadro's constant. Proposed in the 1800s by Italian scientist Amedeo Avogadro, the constant represents the number of atoms or molecules in one mole of a pure material -- for instance, the number of carbon-12 atoms in 12 grams of the element. However, Avogadro's constant isn't a specific number; it's a range of values that can be determined experimentally, but not with enough precision to be a single number.
Spurred by Hill's half-serious question about whether Avogadro's constant was an even or odd number, in the fall of 2006 Fox and Hill submitted a paper to Physics Archives in which they proposed assigning a specific number to the constant -- one of about 10 possible values within the experimental range. The authors pointed out that a precise Avogadro's constant could also precisely redefine the measure of mass, the kilogram.
Their proposal drew attention from the editors of American Scientist, who asked for a longer article published in March 2007. The proposal has so far drawn five letters, including one from Paul J. Karol, chair of the Committee on Nomenclature, Terminology and Symbols of the American Chemical Society. Karol added his endorsement to the proposal and suggested making the number divisible by 12 -- which Fox and Hill did in an addendum by changing their number's final digit from 8 to 6. So the new proposal for Avogadro's constant became 84446886 (cubed), still within the range of accepted values.
Fast-forward to September 2007, when Fox read an Associated Press article on the CNN.com Web site about the mass disappearing from the International Prototype Kilogram. While the AP said the missing mass amounted to no more than "the weight of a fingerprint," Fox argues that the amount could be significant in a world that is measuring time in ultra-sub-nanoseconds and length in ultra-sub-nanometers.
So Fox and Hill fired off another article to Physics Archive, this one proposing to redefine the gram as 1/12th the mass of a mole of carbon 12 -- a mole long being defined as Avogrado's number of atoms. They now hope to generate more interest in their idea for what may turn out to be a competition of standards proposals leading up to a 2011 meeting of the International Committee for Weights and Measures.
At least two other proposals for redefining the kilogram are under discussion. They include replacing the platinum-iridium cylinder with a sphere of pure silicon atoms, and using a device known as the "watt balance" to define the kilogram using electromagnetic energy. Both would offer an improvement over the existing standard -- but not be as simple as what Fox and Hill have proposed, nor be exact, they say.
"Using a perfect numerical cube to define these constants yields the same level of significance -- eight or nine digits -- as in those integers that define the second and the speed of light," Hill said. "A purely mathematical definition of the kilogram is experimentally neutral -- researchers may then use any laboratory method they want to approximate exact masses."
The kilogram is the last major standard defined by a physical artifact rather than a fundamental physical property. In 1983, for instance, the distance represented by a meter was redefined by how far light travels in 1/299,792,458 seconds -- replacing a metal stick with two marks on it.
"We suspect that there will be some public debate about this issue," Fox said. "We want scientists and science teachers and others to think about this problem because we think they can have an impact. Public discussion may play an important role in determining how one of the world's basic physical constants is defined."
How important is this issue to the world's future technological development"
"When you make physical and chemical measurements, it's important to have as high a precision as possible, and these standards really define the limits of precision," Fox said. "The lack of an accurate standard leaves some inconsistency in how you state results. Having a unique standard could eliminate that."
While the new definition would do away with the need for a physical representation of mass, Fox says people who want a physical artifact could still have one -- though carbon can't actually form a perfect cube with the right number of atoms. And building one might take some time.
"You could imagine having a lump of matter that actually had exactly the right number of atoms in it," Fox noted. "If you could build it by some kind of self-assembly process -- as opposed to building it atom-by-atom, which would take a few billion years -- you could have new kilogram artifact made of carbon. But there's really no need for that. Even if you built a perfect kilogram, it would immediately be inaccurate as soon as a single atom was sloughed off or absorbed."
Note: This story has been adapted from a news release issued by Georgia Institute of Technology.

Fausto Intilla

www.oloscience.com

Nuclear Physicists Examine Oxygen's Limits

Source:

http://www.sciencedaily.com/releases/2007/09/070913170108.htm

Science Daily — Physicists at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University have made a unique measurement of an exotic oxygen nucleus, leading scientists one step closer to deciphering the behavior of the element at its limits of existence.

The finding, published in Physical Review Letters, confirms a relatively new theoretical model that predicts dramatic changes in structure as one looks at heavier and heavier oxygen nuclei.
In the experiment, researchers measured a never-before-seen energy state of oxygen 23 -- one of the heaviest oxygen isotopes that exist.
"It was very exciting to see an experiment that was able to observe this [energy] state very close to where we predicted," said Alex Brown, a professor a NSCL who was involved in the shaping of the theory.
Atomic nuclei are composed of protons and neutrons, only certain combinations of which can exist. Each element -- determined by the number of protons in its nucleus -- comes in a variety of flavors with different numbers of neutrons, creating isotopes. The search for the maximum number of neutrons that can fit into a given element's nucleus lies at the forefront of nuclear physics research.
Moving towards the limit of nuclear stability often leads to strange behavior, such as unexpected changes in nuclear structure.
"We thought we understood the nuclear forces well," said Andreas Schiller, an assistant professor at Ohio University and lead researcher on the study. "But it turns out, when we go to extreme ratios of neutrons and protons, the forces in those areas still hold surprises."
While oxygen 23 contains 8 protons and 15 neutrons, stable form of oxygen, making up the bulk of the oxygen found on Earth, has only 8 neutrons.
A few years ago, scientists tweaked an older version of the theory of atomic nuclei to try to explain some startling phenomena among the heavier oxygen isotopes. The new calculations predicted more dramatic changes in structure among the heavier oxygen isotopes. The experiment, which was conducted at NSCL, confirms these predictions.
Looking at the excited states of a nucleus -- reached by adding extra energy into it -- s a good way to understand the forces inside it, said Michael Thoennessen, associate director of nuclear science at NSCL and co-author of the paper.
The result paves the road to studying the neighboring oxygen 24 -- the heaviest possible oxygen isotope.
Many more mysteries remain to be explored, physicists say. As many as 8,000 nuclei are predicted to exist, but so far only 2,000 have been observed.
The experiment, funded by the National Science Foundation, was the first to yield new information from two tailored NSCL tools, which came on line only recently. One device, the Modular Neutron Array, detects neutrons with high efficiency, and the other, the sweeper magnet, uses NSCL's superconducting magnet technology to allow a higher percentage of sought-after particles to pass.
These devices make it possible to explore isotopes farther towards the extreme edges of existence, by making experimental run times up to seven times shorter.
"Without them you couldn't do the experiments," Thoennessen said.
Note: This story has been adapted from a news release issued by Michigan State University.

Fausto Intilla

www.oloscience.com

Nanotechnology Fights E. Coli

Source:

http://www.sciencedaily.com/releases/2007/08/070830164955.htm

Science Daily — Single-walled carbon nanotubes (SWCNTs) can kill bacteria like the common pathogen E. coli by severely damaging their cell walls, according to a recent report from Yale researchers in the American Chemical Society (ACS) journal Langmuir.

"We began the study out of concerns for the possible toxicity of nanotubes in aquatic environments and their presence in the food chain," said Menachem Elimelech, professor and chair of chemical and environmental engineering at Yale and senior author on the paper. "While nanotubes have great promise for medical and commercial applications there is little understanding of how they interact with humans and the environment."
"The nanotubes are microscopic carbon cylinders, thousands of times smaller than a human hair that can be easily taken up by human cells," said Elimelech. "We wanted to find out more about where and how they are toxic."
This "nanoscience version of a David-and-Goliath story" was hailed in an ACS preview of the work as the first direct evidence that "carbon nanotubes have powerful antimicrobial activity, a discovery that could help fight the growing problem of antibiotic resistant infections."
Using the simple E. coli as test cells, the researchers incubated cultures of the bacteria in the presence of the nanotubes for up to an hour. The microbes were killed outright -- but only when there was direct contact with aggregates of the SWCNTs that touched the bacteria. Elimelech speculates that the long, thin nanotubes puncture the cells and cause cellular damage.
The study ruled out metal toxicity as a source of the cell damage. To avoid metal contaminants in commercial sources, the SWCNTs were rigorously synthesized and purified in the laboratory of co-author Professor Lisa Pfefferle.
"We're now studying the toxicity of multi-walled carbon nanotubes and our preliminary results show that they are less toxic than SWCNTs," Elimelech said. "We are also looking at the effects of SWCNTs on a wide range of bacterial strains to better understand the mechanism of cellular damage."
Elimelech projects that SWCNTs could be used to create antimicrobial materials and surface coatings to improve hygiene, while their toxicity could be managed by embedding them to prevent their leaching into the environment.
Other authors on the paper are Seoktae Kang and Mathieu Pinault. The project was funded by a research grant from the National Science Foundation.
Citation: Langmuir 23(17): 8670-8673 (August 28, 2007).
Note: This story has been adapted from a news release issued by Yale University.

Fausto Intilla

www.oloscience.com