Science & TechnologyS


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Light Speed Hurdle to Invisibility Cloak Overcome by Undergraduate

An undergraduate student has overcome a major hurdle in the development of invisibility cloaks by adding an optical device into their design that not only remains invisible itself, but also has the ability to slow down light.

The optical device, known as an 'invisible sphere', would slow down all of the light that approaches a potential cloak, meaning that the light rays would not need to be accelerated around the cloaked objects at great speeds ― a requirement that has limited invisibility cloaks to work only in a specified region of the visible spectrum.

This new research, published today, Tuesday 9 August, in the Institute of Physics and German Physical Society's New Journal of Physics, could open up the possibility for a potential invisibility cloak wearer to move around amongst ever-changing backgrounds of a variety of colours.

Under the guidance of Professor Ulf Leonhardt, Janos Perczel, originating from Hungary and reading Logic, Philosophy of Science and Physics at the University of St Andrews, acknowledged the huge potential of the invisible sphere and was able to fine-tune it so that it was a suitable background for cloaking.

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Invisibility Carpet Cloak Can Hide Objects from Visible Light

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© Majid Gharghi, et al./American Chemical SocietyWhen an input beam (black arrow) reflects off (a) a bump without a cloak, the bump causes a perturbation. When the beam reflects off (b) a bump covered by a cloak, the cloak masks the bump, and the reflected beam is reconstructed as if the bump did not exist. (c) Light after reflection from a flat mirror, a bump without a cloak, and a cloaked bump, at three different wavelengths.
Most of the invisibility cloaks that have been demonstrated to date conceal objects at frequencies that are not detectable by the human eye. Designing invisibility cloaks that can conceal objects from visible light has been more challenging due to the strict material requirements. But in a new study, researchers have fabricated a carpet cloak that can make objects undetectable in the full visible spectrum.

The researchers, led by Prof. Xiang Zhang at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, have published their study in a recent issue of Nano Letters.

As the researchers explain, most previous invisibility cloaks have used metallic metamaterials for cloaking at microwave frequencies. But at optical frequencies, the metal absorbs too much light and leads to significant metallic loss, and Berkeley and other groups have had to design dielectric cloaks at infrared frequencies. More recently, researchers at University of Birmingham (UK) have experimented with using uniaxial crystals as the cloak material, which can enable cloaking in visible frequencies, but only for a certain polarization of light.

In the current study, the researchers used a technique called quasi conformal mapping (QCM) to conceal an object with a height of 300 nm and a width of 6 µm underneath a reflective "carpet cloak." The carpet itself has the appearance of a smooth optical mirror, so that the object and the bump that the object makes underneath the carpet are undetectable by visible light.

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New Invisibility Cloak Hides Objects from Human View

For the first time, scientists have devised an invisibility cloak material that hides objects from detection using light that is visible to humans. The new device is a leap forward in cloaking materials, according to a report in the ACS journal Nano Letters.

Xiang Zhang and colleagues note that invisibility cloaks, which route electromagnetic waves around an object to make it undetectable, "are still in their infancy." Most cloaks are made of materials that can only hide things using microwave or infrared waves, which are just below the threshold of human vision. To remedy this, the researchers built a reflective "carpet cloak" out of layers of silicon oxide and silicon nitride etched in a special pattern. The carpet cloak works by concealing an object under the layers, and bending light waves away from the bump that the object makes, so that the cloak appears flat and smooth like a normal mirror.

Although the study cloaked a microscopic object roughly the diameter of a red blood cell, the device demonstrates that it may be "capable of cloaking any object underneath a reflective carpet layer. In contrast to the previous demonstrations that were limited to infrared light, this work makes actual invisibility for the light seen by the human eye possible," the scientists write.

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Physicists Demonstrate a Time Cloaking Device

Physicists Moti Fridman and colleagues at Cornell University have successfully demonstrated a so-called time cloaking device that is able to "hide" time for 15 trillionths of a second. In a paper published on arXiv, the researchers describe how they were able to cause light passing through a fiber optic cable to compress, than decompress, causing a hole or void to exist, long enough for there to be a lag between the two.

Unlike other proven cloaking devices that work by bending light around objects, this cloaking device works by compressing the light passing through the optical cable by means of a special silicon lens that causes some of the light passing through it to speed up and some to slow down, which causes the waves to divide. Another lens, a little farther up the cable then causes the light to be put back together. The result is light emerging from the end of the cable that appears to be unaltered, which means, for the little bit of space between the lenses, things have or could have gone on, with no record of it occurring.

While it appears the new technology might be used for signal processing application, it appears equally likely it might be used for both good and bad purpose as well. For example, if coded messages could be hidden in a series of these cloaks, it would be mighty difficult to intercept them, making for very secure communications. On the other hand, it also seems logical to conclude that such a hidden time lag, if it could be made to pulse on and off, over and over as a data is passing through, could be used to intercept data without there being a record of it.

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Erasing History? Temporal Cloaks Adjust Light's Throttle to Hide an Event in Time

Researchers from Cornell University in Ithaca, N.Y., have demonstrated for the first time that it's possible to cloak a singular event in time, creating what has been described as a "history editor." In a feat of Einstein-inspired physics, Moti Fridman and his colleagues sent a beam of light traveling down an optical fiber and through a pair of so-called "time lenses." Between these two lenses, the researchers were able to briefly create a small bubble, or gap, in the flow of light. During that fleetingly brief moment, lasting only the tiniest fraction of a second, the gap functioned like a temporal hole, concealing the fact that a brief burst of light ever occurred.

The team will present their findings at the Optical Society's (OSA) Annual Meeting, Frontiers in Optics (FiO) 2011, taking place in San Jose, Calif. next week.

Their ingenious system, which is the first physical demonstration of a phenomenon originally described theoretically a year ago by Martin McCall and his colleagues at Imperial College London in the Journal of Optics, relies on the ability to use short intense pulses of light to alter the speed of light as it travels through optical materials, in this case an optical fiber. (In a vacuum, light maintains its predetermined speed limit of 180,000 miles per second.) As the beam passes through a split-time lens (a silicon device originally designed to speed up data transfer), it accelerates near the center and slows down along the edges, causing it to balloon out toward the edges, leaving a dead zone around which the light waves curve. A similar lens a little farther along the path produces the exact but opposite velocity adjustments, resetting the speeds and reproducing the original shape and appearance of the light rays.

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Miniature Invisibility "Carpet Cloak" Hides More than Its Small Size Implies

miniature carpet cloaking device
© Technical University of Denmark/Optics ExpressThe measured output image from a flat surface (left) and a cloaked protruded surface (right) at 1,480 nm (a), 1,550 nm (b), and 1,580 nm (c).
Invisibility cloaks are seemingly futuristic devices capable of concealing very small objects by bending and channeling light around them. Until now, however, cloaking techniques have come with a significant limitation - they need to be orders of magnitude larger than the object being cloaked.

This places serious constraints on practical applications, particularly for the optoelectronics industry, where size is a premium and any cloaking device would need to be both tiny and delicate.

An international team of physicists from the Technical University of Denmark (DTU), the University of Birmingham, UK, and Imperial College London, however, may have overcome this size limitation by using a technology known as a "carpet cloaks," which can conceal a much larger area than other cloaking techniques of comparable size. The researchers achieved their result by using metamaterials, artificial materials engineered to have optical properties not found in nature. They describe their approach in the Optical Society's (OSA) open-access journal Optics Express.

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Invisibility Cloaks May Be Just Around the Corner

microwave dielectric cloak
© Elena Semouchkina, ECE Department, Michigan Tech UniversityElena Semouchkina tests a microwave dielectric cloak in an anechoic chamber--a cave-like compartment lined with charcoal-gray foam cones to absorb sound.
In 1897, H.G. Wells created a fictional scientist who became invisible by changing his refractive index to that of air, so that his body could not absorb or reflect light. More recently, Harry Potter disappeared from sight after wrapping himself in a cloak spun from the pelts of magical herbivores.

Countless other fictional characters in books and films throughout history have discovered or devised ways to become invisible, a theme that long has been a staple of science fiction and a source of endless fascination for humans. Who among us has never imagined the possibilities? But, of course, it's not for real.

Or is it?

While no one yet has the power to put on a garment and disappear, Elena Semouchkina, an associate professor of electrical and computer engineering at Michigan Technological University, has found ways to use magnetic resonance to capture rays of visible light and route them around objects, rendering those objects invisible to the human eye. Her work is based on the transformation optics approaches, developed and applied to the solution of invisibility problems by British scientists John B. Pendry and Ulf Leonhardt in 2006.

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New Approach to Invisibility Cloaking Gets Much Closer to the Science-Fiction Version

calcite crystal
© PhysOrgThis image shows a calcite crystal laid upon a paper, causing all the letters to show double refraction.
The idea of being able to become invisible, especially by simply covering up a person or an object with a special cloak, has a perennial appeal in science-fiction and fantasy literature. In recent years, researchers have found ways to make very exotic "metamaterials" that can perform a very crude version of this trick, keeping an object from being detected by a certain specific frequency of radiation, such as microwaves, and only working at microscopic scales. But a system that works in ordinary visible light and for objects big enough to be seen with the naked eye has remained elusive.

Now, a team of researchers in the Singapore-MIT Alliance for Research and Technology (SMART) Centre has found a relatively simple, inexpensive system that can hide an object as big as a peppercorn from view in ordinary visible light. The team's discovery has been published online in Physical Review Letters and will appear soon in the print version of the journal.

Unlike the other attempts to produce invisibility by constructing synthetic layered materials, the new method uses an ordinary, common mineral called calcite - a crystalline form of calcium carbonate, the main ingredient in seashells. "Very often, the obvious solution is just sitting there," says MIT mechanical-engineering professor George Barbastathis, one of the new report's co-authors.

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Next Generation Cloaking Device Demonstrated

invisibility cloak
© Duke UniversityPictured is the new cloak with bump, left, and the prototype, right.
A device that can bestow invisibility to an object by "cloaking" it from visual light is closer to reality. After being the first to demonstrate the feasibility of such a device by constructing a prototype in 2006, a team of Duke University engineers has produced a new type of cloaking device, which is significantly more sophisticated at cloaking in a broad range of frequencies.

The latest advance was made possible by the development of a new series of complex mathematical commands, known as algorithms, to guide the design and fabrication of exotic composite materials known as metamaterials. These materials can be engineered to have properties not easily found in natural materials, and can be used to form a variety of "cloaking" structures. These structures can guide electromagnetic waves around an object, only to have them emerge on the other side as if they had passed through an empty volume of space.

The results of the latest Duke experiments were published Jan. 16 in the journal Science. First authors of the paper were Duke's Ruopeng Liu, who developed the algorithm, and Chunlin Li. David R. Smith, William Bevan Professor of electrical and computer engineering at Duke, is the senior member of the research team.

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Broadband Invisibility in the Microwave Range

klingon cloaking
© Paramount Pictures
In the series Star Trek, Klingons and Romulans have spaceships outfitted with cloaking devices that hide their presence from sight, as well as from the sensors of their rivals' spaceships. Unlike current invisibility cloaks, which are mostly effective only over a narrow range, these fictional devices provide a broadband type of invisibility that so far has eluded modern scientists.

Now, though, a team at the Helsinki University of Technology in Finland has demonstrated broadband cloaking of cylindrical objects in the microwave regime. "This really is a helpful development," Sergei Tretyakov tells PhysOrg.com. "Most other invisibility devices are very narrow in the range they affect." Additionally, many invisibility cloaks are produced with the use of materials with extreme electromagnetic properties. "These are exotic materials that often have a great deal of loss and are dispersive," Tretyakov continues.

Along with Pekka Alitalo, Olli Luukkonen and Constantin Simovski, Tretyakov believes that invisibility can be obtained over a broader range of frequencies without the need for electromagnetic mumbo-jumbo. Instead, the Helsinki team makes use of a simple metallic structure made from layers. The results of their experiment can be seen in Physical Review Letters: "Broadband electromagnetic cloaking of long cylindrical objects."