A 'cloaking device' -- it's all done with mirrors

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Scanning electron microscope images of the cloaking device. Top: Light passes through silicon posts as it bounces off a deformed reflector. Varying density of the silicon posts bends light to compensate for the distortion in the reflector. Bottom: a close-up of the array of silicon posts, each about 50 billionths of a meter in diameter. Image: Nanophotonics Group
(PhysOrg.com) -- Somewhat the way Harry Potter can cover himself with a cloak and become invisible, Cornell researchers have developed a device that can make it seem that a bump in a carpet -- or, indeed, any flat surface -- isn't there.

So far the illusion works only at the nanoscale, but the researchers suggest that the basic principle might eventually be scaled up for military and communications applications, or perhaps used in reverse to concentrate solar energy.
Devices that bend microwaves around small objects have previously been demonstrated, but this is the first cloaking device to work at optical frequencies, the researchers said.
The experimental device was built by Michal Lipson, associate professor of electrical and computer engineering, and colleagues in her Nanophotonics Research Group, based on a design by British physicists. It bends light bouncing off a reflective surface in a way that corrects for the distortion caused by a bump in the surface. Imagine controlling the light in front of a funhouse mirror so that reflections look perfectly normal, and the mirror looks flat.
A similar device has been reported by University of California-Berkeley researchers.
On a silicon wafer, Lipson's group made a tiny reflector about 30 microns (millionths of a meter) long with a 5-micron-wide bump in the middle, then placed an array of vertical silicon posts, each 50 nanometers (billionths of a meter) in diameter, in front of it. Because the posts are much smaller than the wavelength of the light, the light behaves as if it were passing through a solid whose density varies with the density of the posts. As light passes between regions of high and low density it is refracted, or bent, in the same way light is refracted as it passes from air to glass. By designing smooth transitions of the density of posts, the researchers could control the path of the light to compensate for the distortion caused by the bump.
As a result, an observer looking at light reflected from the mirror sees a flat mirror, with no sign of the bump. The device is expected to work over a range of wavelengths from infrared into visible red light, the researchers said
Of course it's still a long way to cloaking tanks on a battlefield. For starters, the thing being hidden has to hide behind a mirror, and the presence of a mirror would be a giveaway. A practical cloaking device also would have to adjust in real time to changing configurations of the object behind it.
A variation of the method might be used to bend light around an object, the researchers suggested, and a light-bending device could be made much larger by using technology that stamps or molds nanoscale patterns onto a surface.
Such refraction control might also be used in reverse, they added, to concentrate light in a small area to efficiently collect solar energy.
"At the core is the fact that we're manipulating light, telling it where to go and how to behave," said Carl Poitras, a research associate on the Cornell team.
The device was manufactured at the Cornell Nanoscale Facility, which is supported by the National Science Foundation.
Provided by Cornell University (news : web)

A Light Touch

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Magnetically encoded information is at the core of much modern technology, and researchers are always looking for better ways to manipulate it. In the 8 May Physical Review Letters, a German team shows that a surprisingly feeble light beam can flip zeros to ones and vice versa, in a special magnetic layer. Although currently limited to very low temperatures, the apparently new effect might one day be extended to improve data storage.
Most familiar magnets are metals. They contain atoms that host tiny magnetic bar magnets, or moments, that can point up or down, and the atoms are surrounded by a sea of electrons. But researchers have long been interested in a different type of magnet, one consisting of widely separated magnetic ions embedded in a semiconductor. Unlike a metal, the number of free electrons in a semiconductor changes when it's exposed to electric current or light, so these materials should provide new ways to influence the magnetic properties, via the electrons. Light can flip the magnetization--the total magnetic moment of atoms in a region--from up to down, for example. But until now, experimenters needed very bright light to weaken the magnetization enough to reorient it.
In the new research, a team led by Laurens Molenkamp of the University of Würzburg in Germany grew a thin, crystalline layer of the common semiconductor gallium arsenide but replaced about one percent of the gallium atoms with the magnetic atom manganese. At temperatures below about 25 Kelvin, this layer acts as a ferromagnet: the magnetic moments on different manganese atoms point in the same direction, either up from the surface or down into it. The magnetization direction persists even when the researchers apply an opposing magnetic field, as long as the field does not exceed a threshold called the coercive field.
But when the team focused the light from a standard red laser on the film, the magnetization in the illuminated spot changed direction to match the magnetic field-- opposite to the rest of the film. The light didn’t change the strength of the magnetization, the team found, but instead reduced the field strength needed to flip it.
A mundane explanation would be that the light simply heats the film, which decreases the coercive field. To check this possibility, the team monitored the size of the magnetically flipped spot over many seconds of illumination. "It grows rather slowly," notes Würzburg team member Georgy Astakhov. "Heat diffusion occurs much, much faster." Instead, the team proposes that electrons liberated by the light (and the "holes" they leave behind) affect the magnetization directly. These charge carriers, they suggest, are quickly trapped in regions with high or low manganese concentration. These trapped charges effectively grease the motion of the "domain wall" that separates regions of opposite magnetization, by smoothing out local variations that would otherwise impede its motion. When the domain wall can move smoothly, a region with magnetization pointing up can more easily spread at the expense of a neighboring region having oppositely-directed magnetization.
Theo Rasing, of Radboud University in Nijmegen, Netherlands, says that more work is needed to confirm this non-thermal explanation, including extending the experiment to more than one sample. He also notes that because the dim light is on for a long time, the energy needed to flip the magnetization is not so different from other experiments that use very bright but short light pulses. Nonetheless, Rasing says that seeing a non-thermal change in magnetism with such a dim beam expands such "opto-magnetic" effects to new materials and mechanisms and should inspire further experiments by others.--Don Monroe Don Monroe is a freelance science writer in Murray Hill, New Jersey.
Related Information:
Physics Viewpoint essay by Molenkamp: Convincing a Magnetic Semiconductor to Work at Room Temperature (December, 2008)
Focus story on another optical magnetization flipping technique: Flipping Atoms Fast (1999)
Nonthermal Photocoercivity Effect in a Low-Doped (Ga,Mn)As Ferromagnetic Semiconductor G. V. Astakhov, H. Hoffmann, V. L. Korenev, T. Kiessling, J. Schwittek, G. M. Schott, C. Gould, W. Ossau, K. Brunne, and L. W. Molenkamp Phys. Rev. Lett. 102, 187401 (issue of 8 May 2009)

Star Trek Gadget? 'Tractor Beam' For Cells Developed

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http://www.sciencedaily.com/releases/2007/10/071030095128.htm

ScienceDaily (Oct. 31, 2007) — In a feat that seems like something out of a microscopic version of Star Trek, MIT researchers have found a way to use a “tractor beam” of light to pick up, hold, and move around individual cells and other objects on the surface of a microchip.
The new technology could become an important tool for both biological research and materials research, say Matthew J. Lang and David C. Appleyard, whose work is being published in the journal Lab on a Chip.
The idea of using light beams as tweezers to manipulate cells and tiny objects has been around for at least 30 years. But the MIT researchers have found a way to combine this powerful tool for moving, controlling and measuring objects with the highly versatile world of microchip design and manufacturing.
Optical tweezers, as the technology is known, represent “one of the world's smallest microtools,” says Lang. “Now, we're applying it to building [things] on a chip.”
Says Appleyard, “We've shown that you could merge everything people are doing with optical trapping with all the exciting things you can do on a silicon wafer…There could be lots of uses at the biology-and-electronics interface.”
For example, he said, many people are studying how neurons communicate by depositing them on microchips where electrical circuits etched into the chips monitor their electrical behavior. “They randomly put cells down on a surface, and hope one lands on [or near] a [sensor] so its activity can be measured. With [our technology], you can put the cell right down next to the sensors.” Not only can motions be precisely controlled with the device, but it can also provide very precise measurements of a cell's position.
Optical tweezers use the tiny force of a beam of light from a laser to push around and control tiny objects, from cells to plastic beads. They usually work on a glass surface mounted inside a microscope so that the effects can be observed.
But silicon chips are opaque to light, so applying this technique to them not an obvious move, the researchers say, since the optical tweezers use light beams that have to travel through the material to reach the working surface. The key to making it work in a chip is that silicon is transparent to infrared wavelengths of light - which can be easily produced by lasers, and used instead of the visible light beams.
To develop the system, Lang and Appleyard weren't sure what thickness and surface texture of wafers, the thin silicon slices used to manufacture microchips, would work best, and the devices are expensive and usually available only in quantity. “Being at MIT, where there is such a strength in microfabrication, I was able to get wafers that had been thrown out,” Appleyard says. “I posted signs saying, 'I'm looking for your broken wafers'.”
After testing different samples to determine which worked best, they were able to order a set that were just right for the work. They then tested the system with a variety of cells and tiny beads, including some that were large by the standards of optical tweezer work. They were able to manipulate a square with a hollow center that was 20 micrometers, or millionths of a meter, across - allowing them to demonstrate that even larger objects could be moved and rotated. Other test objects had dimensions of only a few nanometers, or billionths of a meter. Virtually all living cells come in sizes that fall within that nanometer-to-micrometers range and are thus subject to being manipulated by the system.
As a demonstration of the system's versatility, Appleyard says, they set it up to collect and hold 16 tiny living E. coli cells at once on a microchip, forming them into the letters MIT.
Lang is an assistant professor in the Department of Biological Engineering and the Department of Mechanical Engineering. Appleyard is a graduate student in Biological Engineering.
The work was supported by the Biotechnology Training Program of the National Institutes of Health, the W.M. Keck Foundation, and MIT's Lincoln Laboratory.
Adapted from materials provided by Massachusetts Institute Of Technology.

Fausto Intilla
www.oloscience.com

Quantum Cascade Laser Nanoantenna Created

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http://www.sciencedaily.com/releases/2007/10/071022122223.htm

ScienceDaily (Oct. 25, 2007) — In a major feat of nanotechnology engineering researchers from Harvard University have demonstrated a laser with a wide-range of potential applications in chemistry, biology and medicine. Called a quantum cascade (QC) laser nanoantenna, the device is capable of resolving the chemical composition of samples, such as the interior of a cell, with unprecedented detail.

Spearheaded by graduate students Nanfang Yu, Ertugrul Cubukcu, and Federico Capasso, Robert L. Wallace Professor of Applied Physics, all of Harvard's School of Engineering and Applied Sciences, the findings will be published as a cover feature of the October 22 issue of Applied Physics Letters. The researchers have also filed for U.S. patents covering this new class of photonic devices.
The laser's design consists of two gold rods separated by a nanometer gap (a device known as an optical antenna) built on the facet of a quantum cascade laser, which emits invisible light in the region of the spectrum where most molecules have their tell tale absorption fingerprints. The nanoantenna creates a light spot of nanometric size about fifty to hundred times smaller than the laser wavelength; the spot can be scanned across a specimen to provide chemical images of the surface with superior spatial resolution.
"There's currently a major push to develop powerful tabletop microscopes with spatial resolution much smaller than the wavelength that can provide images of materials, and in particular biological specimens, with chemical information on a nanometric scale," says Federico Capasso.
While infrared microscopes, based on the detection of molecular absorption fingerprints, are commercially available and widely used to map the chemical composition of materials, their spatial resolution is limited by the range of available light sources and optics to well above the wavelength. Likewise the so-called near field infrared microscopes, which rely on an ultra sharp metallic tip scanned across the sample surface at nanometric distances, can provide ultrahigh spatial resolution but applications are so far strongly limited by the use of bulky lasers with very limited tunability and wavelength coverage.
"By combining Quantum Cascade Lasers with optical antenna nanotechnology we have created for the first time an extremely compact device that will enable the realization of new ultrahigh spatial resolution microscopes for chemical imaging on a nanometric scale of a wide range of materials and biological specimens," says Capasso.
Quantum cascade (QC) lasers were invented and first demonstrated by Capasso and his group at Bell Labs in 1994. These compact millimeter length semiconductor lasers, which are now commercially available, are made by stacking nanometer thick layers of semiconductor materials on top of each other. By varying the thickness of the layers one can select the wavelength of the QC laser across essentially the entire infrared spectrum where molecules absorb, thus custom designing it for a specific application.
In addition by suitable design the wavelength of a particular QCL can be made widely tunable. The range of applications of QC laser based chemical sensors is very broad, including pollution monitoring, chemical sensing, medical diagnostics such as breath analysis, and homeland security.
The teams co-authors are Kenneth Crozier, Assistant Professor of Electrical Engineering, and research associates Mikhail Belkin and Laurent Diehl, all of Harvard's School of Engineering and Applied Sciences; David Bour, Scott Corzine, and Gloria Höfler, all formerly with Agilent Technologies. The research was supported by the Air Force Office of Scientific Research and the National Science Foundation. The authors also acknowledge the support of two Harvard-based centers, the Nanoscale Science and Engineering Center and the Center for Nanoscale Systems, a member of the National Nanotechnology Infrastructure Network.
Adapted from materials provided by Harvard University.

Fausto Intilla

www.oloscience.com

Nanowire Generates Its Own Electricity

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http://www.sciencedaily.com/releases/2007/10/071022161425.htm

ScienceDaily (Oct. 23, 2007) — Harvard chemists have built a new wire out of photosensitive materials that is hundreds of times smaller than a human hair. The wire not only carries electricity to be used in vanishingly small circuits, but generates power as well.
Charles M. Lieber, the Mark Hyman Jr. Professor of Chemistry, and colleagues created the nanowire out of three different kinds of silicon with different electrical properties. The silicon is wrapped in layers to create the wire. When light falls on the outer material, a process begins due to the interaction of the core with the shell layers, leading to the creation of electrical charges.
The idea of creating nanoscale photovoltaics is not new, Lieber said, but prior efforts used organic compounds in combination with semiconductor nanostructures that had lower efficiency and that degraded under concentrated sunlight. Lieber’s materials have several advantages, he said. The materials are more efficient, converting 3.4 percent of the sunlight into electricity; they can withstand concentrated light without deteriorating, gaining efficiency up to about 5 percent; and they’re as cheap to make as other related nanoscale photovoltaic devices.
“The real [question] is whether there’s a new geometry that will lead to better photovoltaic technology,” Lieber said. “We worked on coaxial geometry.”
The most recent development builds on Lieber’s considerable prior work on nanoscale devices. He has developed sensors with potential bioterrorism applications that can detect a single virus or other particle, nanowire arrays that can detect signals in individual neurons, and a cracker-sized detector for cancer.
A cheap nanoscale power source broadens the potential applications of such nanoscale devices. Though the tiny photovoltaic cells can generate enough electricity to power a similarly tiny circuit, Lieber said they’re not yet efficient enough to have applications on the scale of commercial power generation.
Commercial solar cells, he said, have efficiencies around 20 percent, compared with 3.4 percent for his nano-solar cells. One avenue of future research, Lieber said, will be to explore ways to boost efficiency of the nanowire photovoltaics. If they can reach 10 to 15 percent, he said, their lower cost of production — they can be made from relatively inexpensive materials and don’t require clean rooms to produce — may make them useful in larger-scale applications.
“There’s no physical reason it couldn’t be higher,” Lieber said. “I’m pretty optimistic that we’ll be able to track down the efficiency issue.”
Until then, Lieber sees a future for the nanowire photovoltaics in niche applications, such as multiple distributed sensors or durable, flexible devices, possibly sewn into clothing or worn as a patch.
“It will have to be unique to be an economically viable application, some place where you want durability and flexibility, where if it gets destroyed, people don’t care,” Lieber said.
The work was described in the Oct. 18 issue of the journal Nature.
Adapted from materials provided by Harvard 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