Novel Semiconductor Structure Bends Light 'Wrong' Way -- Exciting Application Potential

Source:

http://www.sciencedaily.com/releases/2007/10/071014163634.htm

Science Daily — A Princeton-led research team has created an easy-to-produce material from the stuff of computer chips that has the rare ability to bend light in the opposite direction from all naturally occurring materials. This startling property may contribute to significant advances in many areas, including high-speed communications, medical diagnostics and detection of terrorist threats.

The new substance is in a relatively new class of materials called "metamaterials," which are made out of traditional substances, such as metals or semiconductors, arranged in very small alternating patterns that modify their collective properties. This approach enables metamaterials to manipulate light in ways that cannot be accomplished by normal materials.
Previous metamaterials were two-dimensional arrangements of metals, which limited their usefulness. The Princeton invention is the first three-dimensional metamaterial constructed entirely from semiconductors, the principal ingredient of microchips and optoelectronics.
"To be useful in a variety of devices, metamaterials need to be three-dimensional," said Princeton electrical engineering professor Claire Gmachl, one of the researchers on the study. "Furthermore, this is made from semiconductors, which are extremely functional materials. These are the things from which true applications are made."
The research team, led by Princeton engineering graduate student Anthony Hoffman, will publish its findings online Oct. 14 in the journal Nature Materials. Other Princeton researchers on the team include graduate students Leonid Alekseyev, Scott Howard and Kale Franz; former Council of Science and Technology fellow Dan Wasserman, now at the University of Massachusetts-Lowell; and former electrical engineering professor Evgenii Narimanov, now at Purdue University. The team also includes collaborators from Oregon State University and telecommunications firm Alcatel-Lucent.
Light waves and other forms of electromagnetic radiation bend whenever they pass from one medium to another. This phenomenon, called refraction, is readily observable when a straw placed into a glass of water appears to be bent or broken. Lenses in reading glasses or a camera work because of refraction.
All materials have an index of refraction, which measures the degree and direction that light is bent as it passes through them. While materials found in nature have positive refractive indices, the material recently invented by Princeton researchers has a negative index of refraction.
In the case of the straw in a glass, normal water would make the underwater portion of the straw appear to bend toward the surface. If water were able to refract light negatively, as the newly invented semiconductor does, the segment of straw under the water would appear as if it were bending away from the surface
Far more than a neat optical illusion, negative refraction holds promise for the development of superior lenses. The positive refractive indices of normal materials necessitate the use of curved lenses, which inherently distort some of the light that passes through them, in telescopes and microscopes. Flat lenses made from materials that exhibit negative refraction could compensate for this aberration and enable far more powerful microscopes that can "see" things as small as molecules of DNA.
In addition, the Princeton metamaterial is capable of negative refraction of light in the mid-infrared region, which is used in a wide range of sensing and communications applications. Its unique composition results in less lost light than previous metamaterials, which were made of extremely small arrangements of metal wires and rings. The semiconductors that constitute the new material are grown from crystals using common manufacturing techniques, making it less complex, more reliable and easier to produce.
"Currently, the typical infrared lens is a massive object -- the setups are bulky," Hoffman said. "This new material may enable more compact mid-infrared optics because we now have a new material with an entirely new set of optical parameters in our toolkit."
The research is part of a multi-institutional research center called Mid-Infrared Technologies for Health and the Environment (MIRTHE). Researchers at MIRTHE are developing compact sensors that detect trace amounts of gases in the atmosphere and human breath. These could one day be used in devices that monitor air quality and enhance homeland security, as well as in non-invasive and on-the-spot medical tests for diabetes and lung disease.
The research relies on a new type of laser that emits mid-infrared light. Gmachl, who directs the MIRTHE project, said the new material could be used to make the lasers better and smaller.
Next, the team plans to incorporate the new metamaterial into lasers. Additionally, the researchers will continue to modify the material in attempts to make features ever smaller in an effort to expand the range of light wavelengths they are able to manipulate.
The work was supported by the MIRTHE center and the Princeton Center for Complex Materials (PCCM), both sponsored by the National Science Foundation.
Note: This story has been adapted from material provided by Princeton University, Engineering School.

Fausto Intilla

www.oloscience.com

New Path For Designing Novel Nanomaterials Discovered

Source:

http://www.sciencedaily.com/releases/2007/10/071012095246.htm

Science Daily — A University of Arkansas researcher and his colleagues have found a novel way to “look” at atomic orbitals, and have directly shown for the first time that they change substantially when interacting at the interface of a ferromagnet and a high-temperature superconductor.
This finding opens up a new way of designing nanoscale superconducting materials and fundamentally changes scientific convention, which suggests that only electron spin and atomic charge – not atomic orbitals – influence the properties of superconducting nanostructures. It also has implications for interfaces between other complex oxide materials.
Jacques Chakhalian, assistant professor of physics in the J. William Fulbright College of Arts and Sciences, and his colleagues will publish their findings online at the Science Express Web site, published by the journal Science, Oct. 11.
Until now, materials science researchers believed that an electron’s charge and spin influenced the characteristics of conventional bulk materials. Atomic orbitals, which consist of the patterns of electron density that may be formed in an atom, were previously thought to be inactive.
“In conventional materials like copper or silicon, you could account for everything you could see through charge and spin,” Chakhalian said. Further, orbitals have proved difficult to “see” through physical experimentation, so it wasn’t possible to examine any changes in orbital symmetry that might be taking place at the interface.
Chakhalian’s work has focused on what happens at the interface between two different materials – for instance, superconductors and ferromagnets, two materials with properties that were thought to be incompatible with each other in bulk. In 2006, he and his colleagues created the first high-quality material to have both superconducting and ferromagnetic properties, and they used that material in this experiment.
Chakhalian and his colleagues worked with synchrotron radiation at the Advanced Photon Source, Argonne National Laboratory in Argonne, Ill., to examine the interface between a high-temperature superconducting material containing copper oxide and a ferromagnetic material containing manganese oxide. The synchrotron light is electromagnetic radiation of varying wavelengths that can be tuned to a specific wavelength and polarization for a particular experiment. Unlike conventional X-rays, which diffuse through space, the synchrotron light beams are sharply focused, like a laser beam with extreme brilliance.
The researchers forced the two materials into unusual quantum states. Using a technique called resonant X-ray absorption, they were able to “look” at the atomic orbitals at the interface and determine their symmetry in a non-destructive way.
They found that the atomic orbitals changed the nature of their symmetry at the interface and created a covalent bond between the copper and manganese atoms. This bonding does not exist in the bulk of the individual materials
“When you merge these two materials, the atomic orbitals at the interface become important. They start contributing to the electronic properties of the material,” Chakhalian said. “This opens a new way of designing materials. We can design quantum materials with engineered physical properties.”
The discovery may allow researchers to manipulate nanoscale superconductivity at the interface – opening the possibility of creating room-temperature semiconductors.
Generators that use superconducting materials generate electricity extremely efficiently, at half the size of conventional generators. General Electric estimates the potential market for superconducting generators to be between $20 billion and $30 billion over the next decade.
Chakhalian’s colleagues include J.W. Freeland and M. van Veenendaal of the Advanced Photon Source, Argonne National Laboratory, Argonne, Ill.; and H.-U. Habermeier, G. Cristiani, G. Khaliullin and B. Keimer of the Max Planck Institute for Solid State Research in Stuttgart, Germany.
Note: This story has been adapted from material provided by University of Arkansas, Fayetteville.

Fausto Intilla
www.oloscience.com

Stopping Atoms

Source:

http://www.sciencedaily.com/releases/2007/10/071003081905.htm

Science Daily — With atoms and molecules in a gas moving at thousands of kilometres per hour, physicists have long sought a way to slow them down to a few kilometres per hour to trap them.

A paper, published October 4 in the Institute of Physics' New Journal of Physics, demonstrates how a group of physicists from The University of Texas at Austin, US, have found a way to slow down, stop and explore a much wider range of atoms than ever before.
Inspired by the coilgun that was developed by the University's Center for Electromechanics, the group has developed an "atomic coilgun" that slows and gradually stops atoms with a sequence of pulsed magnetic fields.
Dr. Mark Raizen and his colleagues in Texas ultimately plan on using the gun to trap atomic hydrogen, which he said has been the Rosetta Stone of physics for many years and is the simplest and most abundant atom in the universe.
Work on slowing and stopping atoms has been at the forefront of advancement in physics for some time. In 1997, there were three joint-winners for the Nobel Prize in Physics for their combined contribution to laser cooling - a method using laser light to cool gases and keep atoms floating or captured in "atom traps".
These important advances had limited use because they only applied to atoms with 'closed two-level transition', excluding important elements such as hydrogen, iron, nickel and cobalt. In contrast, nearly all elements and a wide range of molecules are affected by magnetic forces, or are paramagnetic, which means that this latest research has much wider applicability.
Professor Raizen said, "Of particular importance are the doors being opened for our understanding of hydrogen. Precision spectroscopy of hydrogen's isotopes, deuterium and tritium, continues to be of great interest to both atomic and nuclear physics. Further study of tritium, as the simplest radioactive element, also serves as an ideal system for the study of Beta decay. "
Having successfully designed and used an 18-coil device to slow a supersonic beam of metastable neon atoms, the team is now developing a 64-stage device to further slow and stop atoms.
Note: This story has been adapted from material provided by Institute of Physics.

Fausto Intilla

www.oloscience.com

Portable Atomic Emission Detector Under Development

Source:

http://www.sciencedaily.com/releases/2007/09/070914172642.htm

Science Daily — Brad Jones, a professor of chemistry at Wake Forest University, is leading a team of researchers at four institutions to develop the first handheld, field instrument capable of detecting and identifying radioactive particles at the site of potential contamination.

The device will enable authorities to quickly test dust, soil, water and crops in the event of a terrorist attack such as a “dirty” bomb.
The three-year project is funded by the National Science Foundation in conjunction with the Department of Homeland Security, which asked scientists to submit proposals for radioactivity detection devices. Jones, who specializes in creating spectroscopic instruments, saw the potential to adapt a design he originally conceived years ago to permit rapid field testing for lead in blood samples.
Jones’ “Tungsten Coil Atomic Emission Spectrometer” is constructed using the metal coil filament from a standard slide projector bulb powered by a 12-volt battery, such as the type used to start boats or automobiles. Environmental samples of suspect particles are dissolved in liquid, and droplets are placed on the coil.
The samples are dried at low voltage and the residue vaporized at 3,000 degrees, producing a flash of light. Each metal displays a unique color signature, which is captured by a fiber optic sensor connected to a laptop computer. Test results are then charted on a graph showing each sample’s wavelength and intensity, allowing scientists to identify specific elements and amounts of radioactivity.
“It’s just a natural application,” Jones says, noting that the radioisotopes likely to be stolen from medical or industrial facilities and used by terrorists are also the most brightly emitting elements in atomic spectrometry. “But, the proposed device represents a new way of thinking in the field of nuclear forensics. Atomic emission spectrometry is traditionally a laboratory-based technique using very large, very expensive instruments. With immediate on-site results, residents could be given timely information about a potential threat or reassured that none existed rather than waiting for samples to be transported to laboratories for analysis.”
Portability may also lead to new applications of atomic spectrometry in the field, Jones adds, such as testing for contamination by pesticides and other pollutants.
Instrument manufacturer Teledyne Leeman Labs is interested in the production and marketing of the device once Jones’ research group perfects their prototype. Jones has collaborated with the company for more than a decade.
Other members of the research team include Clifton P. Calloway Jr., associate professor of chemistry at Winthrop University in Rock Hill, S.C.; Arthur L. Salido, assistant professor of chemistry at Western Carolina University in Cullowhee; and Joaquim A. Nobrega, professor of chemistry at the Federal University of Sao Carlos in Brazil.
Note: This story has been adapted from a news release issued by Wake Forest University.

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

Accepted Notion Of Neutron's Electrical Properties Overturned By New Research

Source:

http://www.sciencedaily.com/releases/2007/09/070917151054.htm

Science Daily — For two generations of physicists, it has been a standard belief that the neutron, an electrically neutral elementary particle and a primary component of an atom, actually carries a positive charge at its center and an offsetting negative charge at its outer edge.
The notion was first put forth in 1947 by Enrico Fermi, a Nobel laureate noted for his role in developing the first nuclear reactor. But new research by a University of Washington physicist shows the neutron's charge is not quite as simple as Fermi believed.
Using precise data recently gathered at three different laboratories and some new theoretical tools, Gerald A. Miller, a UW physics professor, has found that the neutron has a negative charge both in its inner core and its outer edge, with a positive charge sandwiched in between to make the particle electrically neutral.
"Nobody realized this was the case," Miller said. "It is significant because it is a clear fact of nature that we didn't know before. Now we know it."
The discovery changes scientific understanding of how neutrons interact with negatively charged electrons and positively charged protons. Specifically, it has implications for understanding the strong force, one of the four fundamental forces of nature (the others are the weak force, electromagnetism and gravity).
The strong force binds atomic nuclei together, which makes it possible for atoms, the building blocks of all matter, to assemble into molecules.
"We have to understand exactly how the strong force works, because it is the strongest force we know in the universe," Miller said.
The findings are based on data collected at the Thomas Jefferson National Accelerator Facility in Newport News, Va., the Bates Linear Accelerator at the Massachusetts Institute of Technology and the Mainz Microtron at Johannes Gutenberg University in Germany.
The three labs examine various aspects of the properties and behavior of subatomic particles, and Miller studied data they collected about neutrons. His analysis was published online Sept. 13 in Physical Review Letters. The work was funded in part by the U.S. Department of Energy.
Since the analysis is based on data gathered from direct observations, the picture could change even more as more data are collected, Miller said.
"A particle can be electrically neutral and still have properties related to charge. We've known for a long time that the neutron has those properties, but now we understand them more clearly," he said.
He noted that the most important aspect of the finding confirms that a neutron carries a negative charge at its outer edge, a key piece of Fermi's original idea.
The strong force that binds atomic nuclei is related to nuclear energy and nuclear weapons, and so it is possible the research could have practical applications in those areas.
It also could lend to greater understanding of the interactions that take place in our sun's nuclear furnace, and a greater understanding of the strong force in general, Miller said.
"We already know that without the strong force you wouldn't have atoms -- or anything else that follows from atoms," he said.
This research was published online Sept. 13 in Physical Review Letters. The work was funded in part by the U.S. Department of Energy.
Note: This story has been adapted from a news release issued by University of Washington.

Fausto Intilla
www.oloscience.com