An international research project involving the University of Adelaide has revealed that the magnetic field in the centre of the Milky Way is at least 10 times stronger than the rest of the Galaxy.
The evidence is significant because it gives astronomers a lower limit on the magnetic field, an important factor in calculating a whole range of astronomical data.
Researchers from the Max-Planck-Institute for Nuclear Physics, the University of Adelaide, Monash University and the United States have recently published their findings in Nature.
Dr Roland Crocker, the lead author, and Dr David Jones both worked on the project while based at Monash University and the University of Adelaide’s School of Chemistry and Physics. The two physicists are now based at the Max-Planck-Institute for Nuclear Physics in Heidelberg, Germany.
“This research will challenge current thinking among astronomers,” Dr Crocker says. “For the last 30 years there has been considerable uncertainty of the exact value of the magnetic field in the centre of the Milky Way. The strength of this field enters into most calculations in astronomy, since almost all of space is magnetised,” he says.
Dr Jones says the findings will affect diverse fields, from star formation theory to cosmology.
“If our Galactic Centre’s magnetic field is stronger than we thought, this raises additional questions of how it got so strong when fields in the early universe are, in contrast, quite weak. We know now that more than 10% of the Galaxy’s magnetic energy is concentrated in less than 0.1% of its volume, right at its centre,” he says.
Dr Jones completed his PhD at Adelaide, studying the Galactic Centre magnetic field under the supervision of Dr Raymond Protheroe, Associate Professor of Physics at the University of Adelaide, and Dr Crocker, a former postdoctoral researcher at the University.
“The Milky Way just glows in radio waves and in gamma-rays produced by collisions of energetic particles, and is brightest near its centre. Knowing the magnetic field there helps us understand the source of the radio and gamma-rays better,” says Dr Protheroe.
Northwestern University researchers have developed compact, mid-infrared laser diodes that generate more light than heat — a breakthroughs in quantum cascade laser efficiency.
The results are an important step toward use of quantum cascade lasers in a variety of applications, including remote sensing of hazardous chemicals.
The research, led by Manijeh Razeghi, the Walter P. Murphy Professor of Electrical Engineering and Computer Science at the McCormick School of Engineering and Applied Science, was published online in the journal Nature Photonics on Jan. 10.
After years of research and industrial development, modern laser diodes in the near-infrared (approximately 1 micron) wavelength range are now extremely efficient. However the mid-infrared (greater than 3 microns) is much more difficult to access and has required the development of new device architectures.
The quantum cascade laser (QCL) is a diode laser that is designed on the quantum mechanical level to produce light at the desired wavelength with high efficiency. Unlike traditional diode lasers, the device is unipolar, requiring only electrons to operate. A significant effort has been spent trying to understand and optimize the electron transport, which would allow researchers to improve the laser quality and efficiency.
Despite the special nature of these devices, laser wafer production is done using standard compound semiconductor growth equipment. By optimizing the material quality in these standard tools, researchers at the Center for Quantum Devices (CQD) at Northwestern, led by Razeghi, have made significant breakthroughs in QCL performance.
Previous reports regarding QCLs with high efficiency have been limited to efficiency values of less than 40 percent, even when cooled to cryogenic temperatures.
After removing design elements unnecessary for low-temperature operation, researchers at CQD have now demonstrated individual lasers emitting at wavelengths of 4.85 microns with efficiencies of 53 percent when cooled to 40 Kelvin.
“This breakthrough is significant because, for the very first time, we are able to create diodes that produce more light than heat,” says Razeghi. “Passing the 50 percent mark in efficiency is a major milestone, and we continue to work to optimize the efficiency of these unique devices.”
Though efficiency is currently the primary goal, the large demonstrated efficiencies also can be exploited to enable power scaling of the QCL emitters. Recent efforts in broad area QCL development have allowed demonstration of individual pulsed lasers with record output powers up to 120 watts, which is up from 34 watts only a year ago.
Transforming lead into gold is an impossible feat, but a similar type of “alchemy” is not only possible, but cost-effective too. Three Penn State researchers have shown that certain combinations of elemental atoms have electronic signatures that mimic the electronic signatures of other elements.
According to the team’s leader A. Welford Castleman Jr., Eberly Distinguished Chair in Science and Evan Pugh Professor in the Departments of Chemistry and Physics, “the findings could lead to much cheaper materials for widespread applications such as new sources of energy, methods of pollution abatement, and catalysts on which industrial nations depend heavily for chemical processing.”
The researchers also showed that the atoms that have been identified so far in these mimicry events can be predicted simply by looking at the periodic table. The team used advanced experimentation and theory to quantify these new and unexpected findings. “We’re getting a whole new perspective of the periodic table,” said Castleman.
The team’s findings will be published in the 28 December 2009 early on-line issue of the journal Proceedings of the National Academy of Sciences, and at a later date in the print edition of the journal.
Castleman and his team — which includes Samuel Peppernick, a former Penn State graduate student who now is a postdoctoral researcher at the Pacific Northwest National Laboratory, and Dasitha Gunaratne, a Penn State graduate student — used a technique, called photoelectron imaging spectroscopy, to examine similarities between titanium monoxide and nickel, zirconium monoxide and palladium, and tungsten carbide and platinum.
“Photoelectron spectroscopy measures the energy it takes to remove electrons from various electronic states of atoms or molecules, while simultaneously capturing snapshots of these electron-detachment events with a digital camera,” said Castleman. “The method allows us to determine the binding energies of the electrons and also to observe directly the nature of the orbitals in which the electrons resided before they were detached. We found that the amount of energy required to remove electrons from a titanium-monoxide molecule is the same as the amount of energy required to remove electrons from a nickel atom. The same is true for the systems zirconium monoxide and palladium and tungsten carbide and platinum. The key is that all of the pairs are composed of isoelectronic species, which are atoms with the same electron configuration.”
Castleman noted that, in this case, the term isoelectronic refers to the number of electrons present in the outer shell of an atom or molecule.
The team looked at images of the photoelectron spectroscopy data. Bright spots in the images, which correspond to the energy of the electrons emitted during their removal from the atoms’ outer shells, appeared to be similar between the pairs of species in the three systems studied. Likewise, graphical displays of energy peaks were similar between the pairs, and theoretical calculations also resulted in the pairs having matching energy levels.
Castleman explained that the molecules titanium monoxide, zirconium monoxide, and tungsten carbide are superatoms of nickel, palladium, and platinum, respectively. Superatoms are clusters of atoms that exhibit some property of elemental atoms. Former work in Castleman’s lab has involved investigating the notion of superatoms. One of his previous experiments showed that a cluster of 13 aluminum atoms behaves like a single iodine atom. Adding a single electron to this aluminum-atom system results in the cluster behaving like a rare-gas atom. Further, he showed that a cluster of 14 aluminum atoms has a reactivity similar to an alkaline earth atom.
Now, Castleman’s new research takes the superatom idea to a new level and provides a sound quantitative foundation for the concept of superatoms. “It looks like we can predict which combinations of elemental atoms mimic other elemental atoms,” he said. “For example, by looking at the periodic table, you can predict that titanium monoxide will be a superatom of nickel. Simply start at titanium, which has four outer-shell electrons, and move six elements to the right, because atomic oxygen possesses six outer-shell electrons. The element you end up on is nickel, whose 10 outer-shell electrons make it isoelectronic with the 10 outer-shell electron molecule resulting from the combination of titanium and oxygen. We thought this finding must be a curious coincidence, so we tried it with other atoms and we found that a pattern emerged.”
Castleman said that he doesn’t know if the pattern will occur across the entire periodic table or if it will be confined to only a part of it. Right now, he and his team are working through the transition-metal atoms. In the future, they plan to take the research a step further to investigate whether or not the superatoms are chemically similar to their respective single atoms. “Platinum is used in nearly all catalytic converters in automobiles, but it is very expensive,” said Castleman. “In contrast, tungsten carbide, which mimics platinum, is cheap. A significant amount of money can be saved if catalytic-converter manufacturers are able to use tungsten carbide instead of platinum. Likewise, palladium is used in certain combustion processes, yet it is mimicked by zirconium monoxide, which is less expensive by a factor of 500. Our new findings are exciting from both a scientific as well as a practical point of view.”
Source: Science Daily
An international team of applied scientists from Harvard, Hamamatsu Photonics, and ETH Zürich have demonstrated compact, multibeam, and multi-wavelength lasers emitting in the invisible part of the light spectrum (infrared). By contrast, typical lasers emit a single light beam of a well-defined wavelength. The innovative multibeam lasers have potential use in applications related to remote chemical sensing pollution monitoring, optical wireless, and interferometry.
The research was led by postdoctoral researcher Nanfang Yu and Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering, both at the Harvard School of Engineering and Applied Sciences (SEAS); Hirofumi Kan, General Manager of the Laser Group at Hamamatsu Photonics; and Jérôme Faist, Professor at ETH Zürich. The findings appeared online in the October 23 issue of Applied Physics Letters and will appear as a December 7 cover story.
“We have demonstrated devices that can create highly directional laser beams pointing in different directions either at the same or at different wavelengths,” says Capasso. “This could have major implications for parallel high-throughput monitoring of multiple chemicals in the atmosphere or on the ground and be used, for example, for studying hazardous trace gases and aerosols, monitoring greenhouse gases, detecting chemical agents on the battlefield, and mapping biomass levels in forests.”
The more versatile laser is a descendant of the quantum cascade laser (QCL), invented and first demonstrated by Capasso, Faist, and their collaborators at Bell Labs in 1994. Commercially available QCLs, made by stacking ultra-thin atomic layers of semiconductor materials on top of one another, can be custom designed to emit a well -defined infrared wavelength for a specific application or be made to emit simultaneously multiple wavelengths. To achieve multiple beams, the researchers patterned the laser facet with metallic structures that behave as highly directional antennas and then beam the light in different directions.
“Having multibeam and multi-wavelength options will provide unprecedented flexibility. The ability to emit multiple wavelengths is ideal for generating a quantitative map of the concentration of multiple chemicals in the atmosphere,” explains Kan. “Profiles of these atmospheric components — as a function of altitude or location — are critically important for environmental monitoring, weather forecasting, and climate modeling.”
The team’s co-authors included graduate students Mikhail A. Kats and Markus Geiser, research associates Christian Pflügl, all from SEAS, and Qi Jie Wang, now an assistant professor at Nanyang Technical University in Singapore; researchers Tadataka Edamura, Shinichi Furuta, and Masamichi Yamanishi, all from Hamamatsu Photonics; and researchers Milan Fischer, Andreas Wittmann, both from the Institute of Quantum Electronics, ETH Zürich.
The work was partially supported by Air Force Office of Scientific Research and Harvard’s Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Infrastructure Network.
Source: Science Daily
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Physicists at the California Institute of Technology (Caltech) have developed a nanoscale device that can be used for force detection, optical communication, and more. The device exploits the mechanical properties of light to create an optomechanical cavity in which interactions between light and motion are greatly strengthened and enhanced.
These interactions, notes Oskar Painter, associate professor of applied physics at Caltech, and the principal investigator on the research, are the largest demonstrated to date.
The device and the work that led to it are described in a recent issue of the journal Nature.
The fact that photons of light, despite having no mass, nonetheless carry momentum and can interact with mechanical objects is an idea that dates back to Kepler and Newton. The mechanical properties of light are also known to limit the precision with which one can measure an object’s position, since simply by using light to do the measurement, you apply a force and disturb the object.
It was important to consider these so-called back-action effects in the design of devices to measure weak, classical forces. Such considerations were part of the development of gravity-wave detectors like the Laser Interferometer Gravitational-Wave Observatory (LIGO). These sorts of interferometer-based detectors have also been used at much smaller scales, in scanning probe instruments used to detect or image atomic surfaces or even single electron spins.
To get an idea of how these systems work, consider a mirror attached to a floppy cantilever, or spring. The cantilever is designed to respond to a particular force—say, a magnetic field. Light shining down on the mirror will be deflected when the force is detected—i.e., when the cantilever moves—resulting in a variation in the light beam’s intensity that can then be detected and recorded.
“LIGO is a huge multikilometer-scale interferometer,” notes Painter. “What we did was to take that and scale it all the way down to the size of the wavelength of light itself, creating a nanoscale device.”
They did this, he explains, because as these interferometer-based detectors are scaled down, the mechanical properties of light become more pronounced, and interesting interactions between light and mechanics can be explored.
“To this end, we made our cantilevers many, many times smaller, and made the optical interaction many, many times larger,” explains Painter.
They call this nanoscale device a zipper cavity because of the way its dual cantilevers—or nanobeams, as Painter calls them—move together and apart when the device is in use. “If you look at it, it actually looks like a zipper,” Painter notes.
“Zipper structures break new ground on coupling photonics with micromechanics, and can impact the way we measure motion, even into the quantum realm,” adds Kerry Vahala, Caltech’s Ted and Ginger Jenkins Professor of Information Science and Technology and professor of applied physics, and one of the paper’s authors. “The method embodied in the zipper design also suggests new microfabrication design pathways that can speed advances in the subject of cavity optomechanics as a whole.”
To create their zipper cavity device, the researchers made two nanobeams from a silicon chip, poking holes through the beams to form an effective optical mirror. Instead of training a light down onto the nanobeams, the researchers used optical fibers to send the light “in plane down the length of the beams,” says Painter. The holes in the nanobeams intercept some of the photons, circulating them through the cavity between the beams rather than allowing them to travel straight through the device.
Or, to be more precise, the circulating photons actually create the cavity between the beams. As Painter puts it: “The mechanical rigidity of the structure and the changes in its optical response are predominantly governed by the internal light field itself.”
Such an interaction is possible, he adds, because the structure is precisely designed to maximize the transfer of momentum from the input laser’s photons to the mechanical nanobeams. Indeed, a single photon of laser light zipping through this structure produces a force equivalent to 10 times that of Earth’s gravity. With the addition of several thousand photons to the cavity, the nanobeams are effectively suspended by the laser light.
Changes in the intensity and other properties of the light as it moves along the beams to the far end of the chip can be detected and recorded, just as with any large-scale interferometer.
The potential uses for this sort of optomechanical zipper cavity are myriad. It could be used as a sensor in biology by coating it with a solution that would bind to, say, a specific protein molecule that might be found in a sample. The binding of the protein molecule to the device would add mass to the nanobeams, and thus change the properties of the light traveling through them, signaling that such a molecule had been detected. Similarly, it could be used to detect other ultrasmall physical forces, Painter adds.
Zipper cavities could also be used in optical communications, where circuits route information via optical beams of different colors, i.e., wavelengths. “You could control and manipulate what the optical beams of light are doing,” notes Painter. “As the optical signals moved around in a circuit, their direction or color could be manipulated via other control light fields.” This would create tunable photonics, “optical circuits that can be tuned with light.”
Additionally, the zipper cavity could lead to applications in RF-over-optical communications and microwave photonics as well, where a laser source is modulated at microwave frequencies, allowing the signals to travel for kilometers along optical fibers. In such systems, the high-frequency mechanical vibrations of the zipper cavity could be used to filter and recover the RF or microwave signal riding on the optical wave.
Other authors on the Nature paper, “A picogram- and nanometre-scale photonic-crystal optomechanical cavity,” include graduate students Matt Eichenfield (the paper’s first author) and Jasper Chan, and postdoctoral scholar Ryan Camacho.
Their research was supported by a Defense Advanced Research Projects Agency seeding effort, and an Emerging Models and Technologies grant from the National Science Foundation.
Source: Science Daily