Wednesday, October 12, 2011

Nanoscale nonlinear light source optical device can be controlled electronically

Not long after the development of the first laser in 1960 scientists discovered that shining a beam through certain crystals produced light of a different color; more specifically, it produced light of exactly twice the frequency of the original. The phenomenon was dubbed second harmonic generation.

The green laser pointers in use today to illustrate presentations are based on this science, but producing such a beautiful emerald beam is no easy feat. The green light begins as an infrared ray that must be first processed through a crystal, various lenses and other optical elements before it can illuminate that PowerPoint on the screen before you.

It was later discovered that applying an electrical field to some crystals produced a similar, though weaker, beam of light. This second discovery, known as EFISH -- for electric-field-induced second harmonic light generation -- has amounted mostly to an interesting bit of scientific knowledge and little more. EFISH devices are big, demanding high-powered lasers, large crystals and thousands of volts of electricity to produce the effect. As a result, they are impractical for all but a few applications.

In a paper published September 22 in Science, engineers from Stanford have demonstrated a new device that shrinks EFISH devices by orders of magnitude to the nanoscale. The result is an ultra-compact light source with both optical and electrical functions. Research implications for the device range from a better understanding of fundamental science to improved data communications.

Spring-loaded electrons

The device is based on the physical forces that bind electrons in orbit around a nucleus.

"It's like a spring," said Mark Brongersma, an associate professor of materials science and engineering at Stanford.

In most cases, when you shine a light on an atom, the added energy will pull the electron away from the positively charged nucleus very predictably, in a linear fashion, so that when the light is turned off and the electron springs back to its original orbit, the energy released is the same as the light that displaced it.

The key phrase here being: "in most cases." When the light source is a high-intensity laser shining on a solid, researchers discovered that the farther the electrons are pulled away from the nuclei the less linearly the light interacts with the atoms.

"In other words, the light-matter interaction becomes nonlinear," said Alok Vasudev, a graduate student and co-author of the paper. "The light you get out is different from the light you put in. Shine a strong near-infrared laser on the crystal and green light exactly twice the frequency emerges."

Engineering possibilities

"Now, Alok and I have taken this knowledge and reduced it to the nanoscale," said the paper's first author, Wenshan Cai, a post-doctoral researcher in Brongersma's lab. "For the first time we have a nonlinear optical device at the nanoscale that has both optical and electrical functionality. And this offers some interesting engineering possibilities."

For many photonic applications, including signal and information processing, it is desirable to electrically manipulate nonlinear light generation. The new device resembles a nanoscale bowtie with two halves of symmetrical gold leaf approaching, but not quite touching, in the center. This thin slit between the two halves is filled with a nonlinear material. The narrowness is critical. It is just 100 nanometers across.

"EFISH requires a huge electrical field. From basic physics we know that the strength of an electric field scales linearly with the applied voltage and inversely with the distance between the electrodes -- smaller distance, stronger field and vice versa," said Brongersma. "So, if you have two electrodes placed extremely close together, as we do in our experiment, it doesn't take many volts to produce a giant electrical field. In fact, it takes just a single volt."

"It is this fundamental science that allows us to shrink the device by orders of magnitude from the human scale to the nanoscale," said Cai.

Enter plasmonics

Brongersma's area of expertise, plasmonics, then enters the scene. Plasmonics is the study of a curious physical phenomenon that occurs when light and metal interact. As photons strike metal they produce waves of energy coursing outward over the surface of the metal, like the ripples when a pebble is dropped in a pond.

Engineers have learned to control the direction of the ripples by patterning the surface of the metal in such a way that almost all of the energy waves are funneled inward toward the slit between the two metallic electrodes.

The light pours into the crevice as if over the edge of a waterfall and there it intensifies, producing light some 80 times stronger than the already intense laser levels from which it came. The researchers next apply a modest voltage to the metal resulting in the tremendous electrical field necessary to produce an EFISH beam.

Practical applications

"This type of device may one day find application in the communications industry," says Brongersma. "Most of the masses of information and social media interaction we send through our data centers, and the future data we will someday create, are saved and transmitted as electrical energy -- ones and zeros."

"Those ones and zeroes are just a switch; one is on, zero is off," said Cai. "As more energy-efficient optical information transport is rapidly gaining in importance, it is not a great leap to see why devices that can convert electrical to optical signals and back are of great value."

For the time being, however, the researchers caution that practical applications remain down the road, but they have created something new.

"It's a great piece of basic science," said Brongersma. "It is work that combines several disciplines -- nonlinear optics, electronics, plasmonics, and nanoscale engineering -- into a really interesting device that could keep us busy for awhile."

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

Journal Reference:

W. Cai, A. P. Vasudev, M. L. Brongersma. Electrically Controlled Nonlinear Generation of Light with Plasmonics. Science, 2011; 333 (6050): 1720 DOI: 10.1126/science.1207858

Ceramics researchers shed light on metal embrittlement

Why does a solid metal that is engineered for ductility become brittle, often suddenly and with dramatic consequences, in the presence of certain liquid metal impurities? The phenomenon, known as liquid metal embrittlement, or LME, has baffled metallurgists for a century.

Now, a team of ceramics researchers has shed light on LME by obtaining atomic-scale images of unprecedented resolution of the grain boundaries, or internal interfaces, where LME occurs.

In doing so, says Martin Harmer, professor of materials science and engineering at Lehigh University, the researchers have achieved the first direct observation in a metal system of a bilayer grain boundary phase transition.

The study suggests that interior interfaces can undergo transitions similar to the solid-to-liquid and liquid-to-gas phase transitions that occur in larger, "bulk" materials.

It also paves the way for scientists to prevent LME by strengthening the chemical bonds of the materials present at grain boundaries.

"This is a very exciting discovery," says Harmer, who directs Lehigh's Center for Advanced Materials and Nanotechnology. "It gives us a much clearer understanding of the atomic mechanism of LME and it promises to improve our ability to control and fine-tune the properties of metals and other materials during fabrication."

Harmer and his colleagues reported their findings Sept. 23 in Science magazine.

Their 18-month study was funded by the U.S. Navy. The group will continue its work, with a focus on rectifying LME-related problems in metals, with help from a five-year, $7.5 million grant through the Department of Defense's Multidisciplinary University Research Initiative program. That project involves researchers from Lehigh, Carnegie-Mellon, Clemson, Illinois and Kutztown universities.

The common ground of ceramics and metals

Many of the consequences of LME affect everyday life, says Harmer.

A steel highway signpost can crack because LME weakened it by the molten zinc alloy applied to the steel during fabrication. Mercury and gallium, both liquid at room temperature, cause normally corrosion-resistant aluminum to become brittle. And concerns over LME make nuclear power plant operators hesitate to switch from water to liquid metal coolant, whose higher boiling point and ability to absorb radiation give it superior and more reliable cooling properties.

Harmer, who has spent 30 years studying ceramics, became interested in LME after he and his students in 2006 identified six grain-boundary "complexions," each with a distinct rate of grain growth, in the ceramic alumina.

He described complexions, and their influence on material properties, in an article titled "The Phase Behavior of Interfaces," which was published April 8 in the Perspective section of Science.

The discovery of grain-boundary complexions in ceramics, Harmer says, prompted him to seek insight into the embrittlement of metals.

"Our ideas on complexions can be tested more rigorously with metals than with ceramics because metals are simpler systems than ceramics," he says.

Harmer's group examined a nickel-bismuth alloy using Lehigh's JEOL 2200 FS aberration-corrected scanning transmission electron microscope (STEM), which has unparalleled imaging capabilities. The group employed a technique called high-angle annular dark-field imaging (HAADF), which focuses a beam of electrons only 1 angstrom (0.1 nm) wide on a sample.

Previous studies had revealed the existence of four interfacial phases at grain boundaries (GB) in metals -- a clean, or intrinsic GB, a monolayer/submonolayer, a nanometer-thick intergranular film, and a complete GB wetting film.

The aberration-corrected STEM revealed two additional GB phases -- a bilayer and a trilayer.

"A bilayer had been seen before in a ceramic system," says Harmer, "but no one had seen such examples of the bi- and trilayers in metals."

The aberration-corrected STEM pinpointed the bilayer of bismuth atoms at the grain boundary as the source of a weak atomic-scale bond in the nickel-bismuth alloy.

"The bonding is so weak that the grains come apart almost like the opening of a slippery zipper," says Harmer.

"There is a very strong bond between bismuth and nickel, so it had never been clear why the alloy is prone to embrittlement. But the bonds between bismuth atoms are weak. We are the first group to see the formation of a bismuth bilayer that weakens this material."

A comprehensive study

Harmer described his group's study as "exhaustive." The researchers examined 12 independent interfaces and took care to exclude artificial "imaging artifacts" introduced by experimental error or by technology.

They also attempted to ensure that their images represented the 3-D nature of nickel-bismuth.

"When you project a 3-D image onto a 2-D film, distortions can result. To avoid this, we imaged at different depths on the sample. By looking sequentially at these images and their structural thickness, we were able to rule out artifacts that give the illusion of a bilayer."

In contrast with previous studies, most of which looked at synthetic bi-crystals, Harmer's group examined polycrystalline nickel which resembles industrial materials.

"Real grain boundaries are typically less symmetrical and have higher energy than synthetic bi-crystals," says Harmer, "and they show other differences as well."

The group plans next to attempt to experiment with the chemistry of nickel-bismuth GBs to produce a more ductile behavior.

"Perhaps combining the bismuth with other elements that bond at the interface will prove effective," says Harmer.

A new aberration-corrected microscope that Lehigh is acquiring in early 2012 -- the JEOL ARM2200F STEM -- will improve the group's ability to do atomic-scale chemical analysis of grain boundaries, says Harmer.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Lehigh University, via EurekAlert!, a service of AAAS.

Journal References:

J. Luo, H. Cheng, K. M. Asl, C. J. Kiely, M. P. Harmer. The Role of a Bilayer Interfacial Phase on Liquid Metal Embrittlement. Science, 2011; 333 (6050): 1730 DOI: 10.1126/science.1208774M. P. Harmer. The Phase Behavior of Interfaces. Science, 2011; 332 (6026): 182 DOI: 10.1126/science.1204204

Hints of universal behavior seen in exotic three-atom states

A novel type of inter-particle binding predicted in 1970 and observed for the first time in 2006, is forming the basis for an intriguing kind of ultracold quantum chemistry. Chilled to nano-kelvin temperatures, cesium atoms -- three at a time -- come together to form a bound state hundreds or even thousands of times larger than individual atoms. Unlike the case of ordinary atoms, wherein electrons are bound to a nucleus in a spectrum of energy levels on the order of an electron volt (that is, it would typically take an eV of energy to free the electron), the cesium triplets feature energy levels that are measured in trillionths of an electron volt (peV). Stranger still, a new experiment observing four such cesium states reports that the states' sizes are roughly the same. This has taken theorists by complete surprise.

In the seventeenth century Isaac Newton derived the classical force laws used to calculate the force between two objects. Calculating the behavior of three-body groupings such as the Moon/Earth/Sun system was much harder; indeed Newton never succeeded. Nowadays such problems can be studied with powerful computers, but only numerical simulations are possible, and not exact, analytical solutions.

In 1970, however, Russian physicist Vitaly Efimov predicted that under some special conditions, three bodies, such as atoms at ultralow temperatures, could be made to enter into stable states whose behavior could be calculated with remarkable ease. Then in 2006 exactly such states were actually observed by scientists at the University of Innsbruck. Now, these researchers have extended their work and demonstrated that the "three-body parameter," used to describe how the three participating bodies interact, varies in a consistent way regardless of the atomic species used.

Paul Julienne, a scientist at the Joint Quantum Institute (JQI), operated by the University of Maryland and the National Institute of Standards and Technology (NIST), contributed theoretical help to the Innsbruck scientists conducting the experiment, a team led by Rudolf Grimm. "None of the experts in three-body physics had expected this kind of universal behavior to show up in these 3-atom systems," Julienne said. "This behavior came as a big surprise." And the universality, in turn, might suggest the existence of some new kind of ultracold chemistry at work.

Efimov's 1970 work met with much skepticism, especially since his prediction specified that three particles could form stable partnerships even though none of the two-particle matchups were stable. That is, 3 particles could accomplish what 2 particles could not. This novel arrangement has been compared to the "Borromean Rings," a set of three rings used on heraldic symbol for the Borromeo family during the Italian Renaissance. The three rings hold together unless any one of the rings is removed.

Efimov's prediction applies not just to atoms but to any 3 particles. For example, helium-6, a semi-stable nucleus consisting of 2 protons and 4 neutrons, can be made by from a helium-4 nucleus and 2 extra neutrons. The 2 neutrons cannot form a stable composite; neither can He-4 plus 1 extra neutron. But the three-body He4-n-n system is stable, at least for a while.

Such Borromean nuclei have been known for some time, but atoms have turned out to be more useful in pursuing the novel interactions called for in Efimov's theory. That's because atoms can be chilled to nano-kelvin temperatures in traps and made to interact with great precision. As atoms cool down, they get larger -- at least in a quantum sense: as waves, their equivalent wavelength can be many times larger than their nominal particle size (a hydrogen atom is about 0.1 nm across). Furthermore, by applying an external magnetic field, subtle interactions among neutral atoms can be achieved.

Such interactions, called Feshbach resonances, were used to bring cesium atoms together, three at a time, in Efimov states. These atoms were part of a vapor held at temperatures of tens of nano-K. In 2006 the Innsbruck team reported seeing one such troika of atoms. Now, in the 16 September 2011 issue of Physical Review Letters, the Innsbruck-JQI-Durham researchers are reporting the observation of three more state of 3 atoms bound together.

These trimers are quantum objects; they have no classical counterpart. The weak binding of the super-cold Cs atoms is described in terms of a parameter, a, called the scattering length. If a is positive and large (much larger than the nominal range of the force between the atoms), weak binding of atoms can happen. If a is negative, a slight attraction of two atoms can occur but not binding. If, however, a is large, negative, and three atoms are present, then the Efimov state can appear. Indeed an infinite number of such states can occur. The Efimov state has an energy spectrum, as if it were a chemical element all by itself, with each binding energy level scaling with the value of a. This kind of universal behavior was expected.

The effective size of these Efimov-triplets is referred to as the three-body parameter. In the case of the four cesium states seen so far, the value is just about the same, about 50 nm, or about 500 times the size of a hydrogen atom. This, combined with the three-body-parameter values observed in experiments for lithium and possibly for other elements being studied right now, suggests that while adjusting for the size of the respective atoms all the species are behaving in the same way. This kind of universality was totally unexpected.

"It is really amazing how the new research field developed since we found the first traces of Efimov states, "said Grimm. "Now things have become reality, things we did not even dream about five years ago."

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Joint Quantum Institute, via EurekAlert!, a service of AAAS.

Journal Reference:

M. Berninger, A. Zenesini, B. Huang, W. Harm, H.-C. Nägerl, F. Ferlaino, R. Grimm, P. Julienne, J. Hutson. Universality of the Three-Body Parameter for Efimov States in Ultracold Cesium. Physical Review Letters, 2011; 107 (12) DOI: 10.1103/PhysRevLett.107.120401

Research leads to enhanced kit to improve design and processing of plastics

The National Physical Laboratory (NPL) has developed a world-leading pvT (pressure-volume-temperature) and thermal conductivity test kit.

The kit is based on more than nine years of extensive research at NPL. It can be used to help improve the design and processing of plastics, including the injection moulding process used to make specialised polymers and everyday plastic items such as CDs

NPL's equipment can measure the thermo-physical properties of polymers. It can help improve the injection moulding process by allowing designers to find the exact pvT and shrinkage properties of a material. Although plastics are the main material tested, other more unusual materials such as soap and even chocolate have also been analysed.

The pvT instrument operates at pressures ranging from 200 bar to 2500 bar, and is the only equipment in the world that can test materials at ultra fast cooling rates of up to 280 °C/min and down to temperatures approaching -100 °C. NPL found that at higher pressures polymers can conduct heat up to 20% more efficiently, leading to faster cooling rates and shorter cycle times.

A thermal conductivity measurement facility is also incorporated into the instrument. Research on the thermal conductivity properties of polymers such as HDPE (high-density polyethylene) and PBT (polybutylene terephthalate) is vital to manufacturers and it was found that they can increase their production rates and gain a higher profit by filling a polymer with glass -- as this cools faster, reducing the time that the polymer needs to stay in the mould. The less time the polymer stays in the mould, the faster the output rate of products.

Angela Dawson a Higher Research Scientist for NPL's Materials Division, said: "pvT testing kits are essential for improving design and processing of ubiquitous, everyday plastics and for more specialised polymers with advanced applications. NPL is the only laboratory where manufacturers can send materials for testing using this advanced equipment and this work has improved the reliability and accuracy of measuring pvT data."

The research was funded by the National Measurement System (NMS) -- and operated by BIS (the Department of Business Innovation and Skills).

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by National Physical Laboratory, via EurekAlert!, a service of AAAS.

Edible carbon dioxide sponge: All-natural nanostructures could address pressing environmental problem

 A year ago Northwestern University chemists published their recipe for a new class of nanostructures made of sugar, salt and alcohol. Now, the same team has discovered the edible compounds can efficiently detect, capture and store carbon dioxide. And the compounds themselves are carbon-neutral.

The porous crystals -- known as metal-organic frameworks (MOFs) -- are made from all-natural ingredients and are simple to prepare, giving them a huge advantage over other MOFs. Conventional MOFs, which also are effective at adsorbing carbon dioxide, are usually prepared from materials derived from crude oil and often incorporate toxic heavy metals.

Other features of the Northwestern MOFs are they turn red when completely full of carbon dioxide, and the carbon capture process is reversible.

The findings, made by scientists working in the laboratory of Sir Fraser Stoddart, Board of Trustees Professor of Chemistry in the Weinberg College of Arts and Sciences, are published in the Journal of the American Chemical Society (JACS).

"We are able to take molecules that are themselves sourced from atmospheric carbon, through photosynthesis, and use them to capture even more carbon dioxide," said Ross S. Forgan, a co-author of the study and a postdoctoral fellow in Stoddart's laboratory. "By preparing our MOFs from naturally derived ingredients, we are not only making materials that are entirely nontoxic, but we are also cutting down on the carbon dioxide emissions associated with their manufacture."

The main component, gamma-cyclodextrin, is a naturally occurring biorenewable sugar molecule that is derived from cornstarch.

The sugar molecules are held in place by metals taken from salts such as potassium benzoate or rubidium hydroxide, and it is the precise arrangement of the sugars in the crystals that is vital to their successful capture of carbon dioxide.

"It turns out that a fairly unexpected event occurs when you put that many sugars next to each other in an alkaline environment -- they start reacting with carbon dioxide in a process akin to carbon fixation, which is how sugars are made in the first place," said Jeremiah J. Gassensmith, lead author of the paper and also a postdoctoral fellow in Stoddart's laboratory. "The reaction leads to the carbon dioxide being tightly bound inside the crystals, but we can still recover it at a later date very simply."

The fact that the carbon dioxide reacts with the MOF, an unusual occurrence, led to a simple method of detecting when the crystals have reached full capacity. The researchers place an indicator molecule, which detects changes in pH by changing its color, inside each crystal. When the yellow crystals of the MOFs are full of carbon dioxide they turn red.

The simplicity of the new MOFs, allied with their low cost and green credentials, have marked them as candidates for further commercialization. Ronald A. Smaldone, also a postdoctoral fellow in Stoddart's group and a co-author of the paper, added, "I think this is a remarkable demonstration of how simple chemistry can be successfully applied to relevant problems like carbon capture and sensor technology."

The National Science Foundation, the U.S. Department of Energy, the Engineering and Physical Sciences Research Council in the U.K., the King Abdulaziz City of Science and Technology (KACST) in Saudi Arabia and the Korea Advanced Institute of Science and Technology (KAIST) in Korea supported the research.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Northwestern University. The original article was written by Megan Fellman.

Journal Reference:

Jeremiah J. Gassensmith, Hiroyasu Furukawa, Ronald A. Smaldone, Ross S. Forgan, Youssry Y. Botros, Omar M. Yaghi, J. Fraser Stoddart. Strong and Reversible Binding of Carbon Dioxide in a Green Metal–Organic Framework. Journal of the American Chemical Society, 2011; 110913144109022 DOI: 10.1021/ja206525x