Science Daily (Dec. 24, 2007)

Last year researchers from Duke University stunned the world when they announced a cloaking device for the microwave range. This device made use of metamaterials that had a negative refractive index for electromagnetic radiation. The metamaterials were carefully designed split-ring resonators with a structure size much smaller than the wavelength. Only 10 stacked layers of metamaterials were necessary to achieve the desired invisibility effect.

Now, researchers from the group of Harald Giessen at the University of Stuttgart have succeeded in manufacturing a stacked split-ring metamaterial for the optical wavelength range. This layer-by-layer stacking procedure, which can be repeated as often as desired, is capable of producing well aligned three-dimensional metamaterial structures.

The key to this achievement was a planarization method for the rough nanolithography surface in combination with robust alignment marks which survived the dry etching processes during nanofabrication. This resulted in perfect lateral alignment as well as in perfectly flat layers. The method is suited to produce arbitrary shapes in each layer as well. Thus, more complex structures such as twisted or chiral structures are possible.

The present 3D structures consist of horseshoe-shaped gold nanowires that are arranged in a square pattern and perfectly stacked above each other. Due to the strong vertical coupling, broadband optical features arise. Also, the vertical coupling leads to negative permeability of the structure, which is a prerequisite for a negative refractive index.

Possible applications in the future include perfect lenses that beat the diffraction limit, and optical cloaking devices which provide some invisibility for macroscopic objects.

Na Liu et al., Nature Materials Jan. 2008 issue, Advance Online Publication.

Adapted from materials provided by University of Stuttgart, via AlphaGalileo.

http://www.sciencedaily.com/releases/2007/12/071221231539.htm

These two images (Cloak off, top. Cloak on, bottom) were taken from corresponding videos depicting scientific simulations performed at Purdue to show how objects might be "cloaked" to render them invisible. The new findings demonstrate how to cloak objects for any single wavelength, not for the entire frequency range of the visible spectrum. But the research represent a step toward creating an optical cloaking device that might work one day for all wavelengths of visible light. The videos show how light interacts with an uncloaked and cloaked object. When uncloaked, as depicted in the first image, light waves strike the object and bounce backward. As depicted in the second image, a cloaking device designed using nanotechnology guides light around anything placed inside this cloak. (Credit: Birck Nanotechnology Center, Purdue University)

Science Daily (Apr. 2, 2007)

Researchers using nanotechnology have taken a step toward creating an "optical cloaking" device that could render objects invisible by guiding light around anything placed inside this "cloak."

The Purdue University engineers, following mathematical guidelines devised in 2006 by physicists in the United Kingdom, have created a theoretical design that uses an array of tiny needles radiating outward from a central spoke. The design, which resembles a round hairbrush, would bend light around the object being cloaked. Background objects would be visible but not the object surrounded by the cylindrical array of nano-needles, said Vladimir Shalaev, Purdue's Robert and Anne Burnett Professor of Electrical and Computer Engineering.

The design does, however, have a major limitation: It works only for any single wavelength, and not for the entire frequency range of the visible spectrum, Shalaev said.

"But this is a first design step toward creating an optical cloaking device that might work for all wavelengths of visible light," he said.

Research findings are detailed in a paper appearing this month in the journal Nature Photonics. The paper, which is appearing online this week, was co-authored by doctoral students Wenshan Cai and Uday K. Chettiar, research scientist Alexander V. Kildishev and Shalaev, all in Purdue's School of Electrical and Computer Engineering.

Calculations indicate the device would make an object invisible in a wavelength of 632.8 nanometers, which corresponds to the color red. The same design, however, could be used to create a cloak for any other single wavelength in the visible spectrum, Shalaev said.

"How to create a design that works for all colors of visible light at the same time will be a big technical challenge, but we believe it's possible," he said. "It is clearly doable. In principle, this cloak could be arbitrarily large, as large as a person or an aircraft."

The research is based at the Birck Nanotechnology Center at Purdue's Discovery Park.

Other researchers published findings in 2006 describing the mathematics generally required for the optical cloaking device. Those researchers include: John Pendry at the Imperial College in London, along with David Schurig and David R. Smith at Duke University, and simultaneously, Ulf Leonhardt at the University of St. Andrews in Scotland.

"These mathematical requirements were very general, and then we determined how to fulfill the requirements with a specific design," Shalaev said.

Leonhardt, a professor of theoretical physics, wrote a commentary piece about the Purdue paper appearing in the same issue of Nature Photonics. In the commentary, he compares the Purdue design to the Roman creation of "the first optical metamaterial," a type of glass containing nanometer-scale particles of gold. In ordinary daylight, a cup made of the glass appeared green, but then it glowed ruby when illuminated from the inside.

The Purdue research, Leonhardt writes, represents " ... theoretical simulations that show that a modified Roman cup based on modern nanofabrication technology will act as an invisibility device ... Any object you put inside will disappear as if dissolved in air, provided it is viewed through polarizing tinted glasses of precisely that colour."

Other researchers have developed concepts for cloaking objects smaller than the wavelengths of visible light and for objects detected in the microwave range of the spectrum, which are much larger than the wavelengths of visible light. But the new design is the first for cloaking an arbitrary object in the range of light visible to humans.

"What we propose is the cloaking of objects of any shape and size," Shalaev said.

Two requirements are needed to render an object invisible: Light must not reflect off of the object, and the light must bend around the object so that people would see only the background and not the cloaked object itself.

"If you satisfied only the first requirement of preventing light from reflecting off of the object, you would still see the dark shadowlike shape of the object, so you would know something was there," Shalaev said. "The most difficult requirement is to bend light around the cloaked object so that the background is visible but not the object being cloaked. The viewer would, in effect, be seeing around, or through, the object."

The device would be made of so-called "non-magnetic metamaterials." Meta in Greek means beyond, so the term metamaterial means to create something that doesn't exist in nature. Unlike designs for invisibility in the microwave range, the new design has no magnetic properties. Having no magnetic properties makes it much easier to cloak objects in the visible range but also causes a small amount of light to reflect off of the cloaked object.

"But this could, in principle, be offset by other means, for example, with antireflective coatings," Shalaev said. "The big challenge is how to make rays bend around the object, which we have described how to do in this paper."

A key factor in the design is the ability to reduce the "index of refraction" to less than 1. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. Refraction causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material.

Natural materials typically have refractive indices greater than 1. The new design reduces a refractive index to values gradually varying from zero at the inner surface of the cloak, to 1 at the outer surface of the cloak, which is required to guide light around the cloaked object.

Creating the tiny needles would require the same sort of equipment already used to fabricate nanotech devices. The needles in the theoretical design are about as wide as 10 nanometers, or billionths of a meter, and as long as hundreds of nanometers. They would be arranged in layers emanating from a central spoke in a cylindrical shape. A single nanometer is roughly the size of 20 hydrogen atoms strung together.

Although the design would work only for one frequency, it still might have applications, such as producing a cloaking system to make soldiers invisible to night-vision goggles.

"Because night-imaging systems detect only a specific wavelength, you could, in theory, design something that cloaks in that narrow band of light," Shalaev said.

Another possible application is to cloak objects from "laser designators" used by the military to illuminate a target, he said.

Leonhardt says in his commentary that creating a cloak for rendering total invisibility in the entire visible spectrum would require "further advances in optical metamaterials, new combinations of nanotechnology with highly abstract ideas ..."

The optical cloaking research is an indirect spinoff of research in Shalaev's lab that has been funded by the U.S. Army Research Office to develop metamaterials. In previous work, Shalaev's team created a metamaterial that has a "negative index of refraction" in the wavelength of light used for telecommunications, a step that could lead to better communications and imaging technologies. More recently, the researchers moved the wavelength for a negative refractive index material to the visible range.

Adapted from materials provided by Purdue University.

http://www.sciencedaily.com/releases/2007/04/070402141206.htm

Science Daily (May 4, 2007)

A unique computer model designed by a mathematician at the University of Liverpool has shown that it is possible to make objects, such as aeroplanes and submarines, appear invisible at close range.

Scientists have already created an 'invisibility cloak' made out of 'metamaterial' which can bend electromagnetic radiation -- such as visible light, radar or microwaves -- around a spherical space, making an object within this region appear invisible.

Until now, scientists could only make objects appear invisible from far away. Liverpool mathematician Dr Sébastien Guenneau, together with Dr Frédéric Zolla and Professors André Nicolet from the University of Marseille, have proven - using a specially designed computer model called GETDP - that objects can also be made to appear invisible from close range when light travels in waves rather than beams.

Scientists predict that metamaterials could be of use in military technology, such as in the construction of fighter jets and submarines, but it will be some years before invisibility cloaks can be developed for human beings.

Dr Guenneau, at the University's Department of Mathematical Science, explains: "The shape and structure of aeroplanes make them ideal objects for cloaking, as they have a fixed structure and movement pattern. Human beings and animals are more difficult as their movement is very flexible, so the cloak - as it is designed at the moment - would easily be seen when the person or animal made any sudden movement.

"A cloak, such as the one worn by the Harry Potter character for example, is not yet possible but it is a good example of what we are trying to move towards. Using this new computer model we can prove that light can bend around an object under a cloak and is not diffracted by the object. This happens because the metamaterial that makes up the cloak stretches the metrics of space, in a similar way to what heavy planets and stars do for the metrics of space-time in Einstein's general relativity theory.

"In order for the cloaking device to work in the first place light has to separate into two or more waves resulting in a new wave pattern. Within this pattern we get light and dark regions which are needed in order for an object to appear invisible.

"Until now, however, it was not clear whether photons -- particles that make up all forms of light -- can split and form new waves when the light source is close to the object. If we use ray optic techniques -- where light travels in beams - photons break down at close range and the object does not appear invisible. If we study light as it travels in waves however, invisibility is maintained."

Scientists predict that invisibility will be possible for objects of any shape and size within the next decade. The research findings are published in Optic Letters.

Adapted from materials provided by University of Liverpool, via EurekAlert!, a service of AAAS.

http://www.sciencedaily.com/releases/2007/05/070503100813.htm

ScienceDaily (Dec. 26, 2006)

The theorists who first created the mathematics that describe the behavior of the recently announced "invisibility cloak" have revealed a new analysis that may extend the current cloak's powers, enabling it to hide even actively radiating objects like a flashlight or cell phone.

Allan Greenleaf, professor of mathematics at the University of Rochester, working with colleagues around the globe, has announced a mathematical theory that predicts some strange goings on inside the cloak—and that what happens inside is crucial to the cloak's effectiveness.

In October, David R. Smith, associate professor of electrical and computer engineering at Duke University, led a team that used a circular cloaking device to successfully bend microwaves around a copper disk as if the disk were invisible. In 2003, however, Greenleaf and his colleagues had already developed the mathematics of invisibility.

"We were working on improving the mathematics behind tumor detection," says Greenleaf. "In the final section to one paper, we spelled out a worst-case scenario where a tumor could be undetectable. We then wrote a couple of additional articles describing when this could happen. At the time, we didn't think further about it because it seemed extremely unlikely that any tumor would be covered with the necessary material to be hidden that way."

This past summer, however, Greenleaf and his colleagues learned about a paper that researchers at Duke and Imperial College had published in the journal Science, which used nearly identical equations to give a theoretical proposal for a cloaking device. Once Greenleaf and his colleagues saw that their results could also be used to show how to "hide" an object, they decided to analyze and improve the proposed cloaking device, using the techniques they had developed in their earlier work. They knew that a crucial question would be: What was going on inside the cloaked region?

Smith, a physicist, gave a description of why the cloaking device should work. Greenleaf, as a mathematician, knew that to have any hope of extending and improving the cloaking, it was important to fully understand its mathematical underpinnings. Then, in October, Smith published another paper, describing how he and his team actually built a cloaking device. This made it even more crucial to carefully analyze the underlying structure.

Greenleaf and his collaborators used sophisticated mathematics to understand what must be happening inside the cloaked region. Everything seemed fine when they applied the Helmholtz equation, an equation widely used to solve problems involving the propagation of light. But when they used Maxwell's equations, which take the polarization of electromagnetic waves into account, difficulties came to light.

Maxwell's equations said that a simple copper disk like the one Smith used could be cloaked without a problem, but anything that emitted electromagnetic waves—a cell phone, a digital watch, or even a simple electric device like a flashlight—caused the behavior of the cloaking device to go seriously awry. The mathematics predicts that the size of the electromagnetic fields go to infinity at the surface of the cloaked region, possibly wrecking the invisibility.

Their analysis also revealed another surprise: a person trying to look out of the cloak would effectively be faced with a mirror in every direction. If you can imagine Harry Potter's own invisibility cloak working this way, and Harry turning on his flashlight to see, its light would shine right back at him, no matter where he pointed it.

Greenleaf's team determined that a more complicated phenomenon arises when using Maxwell's equations, leading to a "blow up" (an unexpected infinite behavior) of the electromagnetic fields. They determined that by inserting conductive linings, whose properties depend on the specific geometry of the cloak, this problem can be resolved. Alternatively, covering both the inside and outside surfaces of the cloaked region with carefully matched materials can also be used to bypass this problem.

"We should also keep in mind that, given the current technology, when we talk about invisibility, we're talking only about being invisible at just a narrow range of wavelengths," says Greenleaf. "For example, an object could be rendered invisible at just a specific wavelength of red; it would be visible in nearly every other color."

Smith's team at Duke is also working on improving their cloaking device. On Dec. 6, Smith and Greenleaf met for the first time and talked about Greenleaf's new math.

"Allan has been looking at the problem much more generally, and deriving the conditions for when true invisibility is or is not possible," says Smith. "We are very interested in what he and his colleagues come up with!"

Greenleaf and his coauthors are now working to confirm the relationship between their work and experiments. Some of the equations do not have solutions, so they are looking at what the physical consequences are, and whether a cloak's effectiveness would be compromised. Since any physical construction is only an approximation of the mathematical ideal that Greenleaf's team analyzes, Greenleaf says it would also be very interesting to understand the extent to which small errors in the construction degrade the cloaking effect.

Greenleaf's colleagues on this research are Matti Lassas, professor of mathematics at the Helsinki University of Technology, Yaroslav Kurylev, professor of mathematics at of Loughborough University, and Gunther Uhlmann, professor of mathematics at the University of Washington.

Adapted from materials provided by University of Rochester.

http://www.sciencedaily.com/releases/2006/12/061226134711.htm

ScienceDaily (Sep. 6, 2007)

While we are a long way off from the lightweight, high-performance, magical cloak of Harry Potter, Muggle physicists have been busy designing ways to make invisibility possible.

A recent theoretical analysis of a column-shaped invisibility cloak, by a collaboration of researchers from Sweden and China, showed that a cloak made to ideal specifications could render an object (or wizard) hidden inside perfectly invisible. However, even slight deviations from these specifications will cause the invisibility to break down.

The researchers analyzed the properties of a simulated tube of special metamaterials (manmade materials with intricate, microscopic structures) that can force light to follow a specified path. With the ideal wall thickness, the tube would flawlessly guide light around the inner chamber, rendering the wizard inside invisible. You could walk about, unaware of the cloaked wizard's presence, unless you unceremoniously slam yourself into a pillar that looks like nothing but empty air.

The wizard, who would be unable to see anything outside of the invisibility cloak, could reveal himself by deconstructing the cloak a layer at a time. Imagine if he could wave his wand and remove a layer from the inside of the column, leaving it the same diameter on the outside but making that inner chamber a little larger. With the inner layer removed, the wizard would appear as a thin line, and the background would be slightly distorted. As more of the inside is removed, the wizard would become more apparent and the background would become more distorted. Physicists haven't yet worked out exactly how these distortions would appear to human eyes.

In any case, the collaboration's theoretical study affirms that the ideal column design will allow for perfect invisibility, if metamaterials can be made to the right specifications.

The  forthcoming article in Physical Review Letters was authored by Zhichao Ruan, Min Yan, Curtis W. Neff, and Min Qiu.

Adapted from materials provided by American Physical Society, via EurekAlert!, a service of AAAS.

http://www.sciencedaily.com/releases/2007/09/070905203843.htm

False color representation of the measured plasmon field scattering around the central area of the cloak. The flow of energy around the cloaked region is visualized. (Credit: Image courtesy of University of Maryland)

ScienceDaily (Dec. 19, 2007)

Harry Potter may not have talked much about plasmonics in J. K. Rowling's fantasy series, but University of Maryland researchers are using this emerging technology to develop an invisibility cloak that exists beyond the world of bespectacled teenage wizards.A research team at Maryland's A. James Clark School of Engineering comprised of Professor Christopher Davis, Research Scientist Igor Smolyaninov, and graduate student Yu-Ju Hung, has used plasmon technology to create the world's first invisibility cloak for visible light. The engineers have applied the same technology to build a revolutionary superlens microscope that allows scientists to see details of previously undetectable nanoscale objects.

Generally speaking, when we see an object, we see the visible light that strikes the object and is reflected. The Clark School team's invisibility cloak refracts (or bends) the light that strikes it, so that the light moves around and past the cloak, reflecting nothing, leaving the cloak and its contents "invisible."

The invisibility cloak device is a two-dimensional pattern of concentric rings created in a thin, transparent acrylic plastic layer on a gold film. The plastic and gold each have different refractive properties. The structured plastic on gold in different areas of the cloak creates "negative refraction" effects, which bend plasmons—electron waves generated when light strikes a metallic surface under precise circumstances—around the cloaked region.

This manipulation causes the plasmon waves to appear to have moved in a straight line. In reality they have been guided around the cloak much as water in a stream flows around a rock, and released on the other side, concealing the cloak and the object inside from visible light. The invisibility that this phenomenon creates is not absolutely perfect because of energy loss in the gold film.

The team achieved this invisibility under very specialized conditions. The researchers' cloak is just 10 micrometers in diameter; by comparison, a human hair is between 50 to 100 micrometers wide. Also, the cloak uses a limited range of the visible spectrum, in two dimensions. It would be a significant challenge to extend the cloak to three dimensions because researchers would need to control light waves both magnetically and electronically to steer them around the hidden object. The technology initially may work only for small objects of specific controlled shape.

The team also has used plasmonics to develop superlens microscopy technology, which can be integrated into a conventional optical microscope to view nanoscale details of objects that were previously undetectable.

The superlens microscope could one day image living cells, viruses, proteins, DNA molecules, and other samples, operating much like a point-and-shoot camera. This new technology could revolutionize the capability to view nanoscale objects at a crucial stage of their development. The team believes they can improve the resolution of their microscope images down to about 10 nanometers—one ten thousandth of the width of a human hair.

A large reason for the success of the group's innovations in both invisibility and microscopy is that surface plasmons have very short wave lengths, and can therefore move data around using much smaller-scale guiding structures than in existing devices. These small, rapid waves are generated at optical frequencies, and can transport large amounts of data. The group also has made use of the unique properties of metamaterials, artificially structured composites that help control electromagnetic waves in unusual ways using plasmonic phenomena.

The diverse applications the group has derived from their plasmonics research is an example of the ingenuity of researchers approaching new and dynamic technologies that offer broad and unprecedented capabilities. The research has attracted a great deal of attention within the scientific community, industry and government agencies. Related plasmonics research offers applications for military and computer chip technologies, which could benefit from the higher frequencies and rapid data transfer rates that plasmons offer.

The team's research has been funded by the National Science Foundation and Clark School Corporate Partner BAE Systems.

Smolyaninov and Davis have published an article in the journal Science about their superlens microscope technology, titled "Magnifying Superlens in the Visible Frequency Range." The group and their colleagues from Purdue University will also soon publish a paper about their invisibility cloak research.

Adapted from materials provided by University of Maryland.

http://www.sciencedaily.com/releases/2007/12/071218192009.htm

ScienceDaily (Mar. 23, 2007)

For the first time, physicists have devised a way to make visible light travel in the opposite direction that it normally bends when passing from one material to another, like from air through water or glass. The phenomenon is known as negative refraction and could in principle be used to construct optical microscopes for imaging things as small as molecules, and even to create cloaking devices for rendering objects invisible.

In the March 22 in the online publication  Science Express, California Institute of Technology applied physics researchers Henri Lezec, Jennifer Dionne, and Professor Harry Atwater, will report their success in constructing a nanofabricated photonic material that creates a negative index of refraction in the blue-green region of the visible spectrum. Lezec is a visiting associate in Atwater's Caltech lab, and Dionne is a graduate student in applied physics.

According to Lezec, the key to understanding the technology is first in understanding how light normally bends when it passes from one medium to another. If a pencil is placed in a glass of water at an angle, for example, it appears to bend upward and outward if we look into the water from a vantage point above the surface. This effect is due to the wave nature of light and the normal tendency of different materials to disperse light in different ways-in this case, the materials being the air outside the glass and the water inside it.

However, physicists have thought that, if new optical materials could be constructed at the nanoscale level in a certain way, it might be possible to make the light bend at the same angle, but in the opposite direction. In other words, the pencil angled into the water would appear to bend backward as we looked at it.

The details are complicated, but have to do with the speed of light through the material itself. Researchers in recent years have created materials with negative diffraction for microwave and infrared frequencies. These achievements have exploited the relatively long wavelengths at those frequencies--the wavelength of microwaves being a few centimeters, and that of infrared frequencies about the width of a human hair. Visible light, because its wavelength is at microscopic dimensions--about one-hundredth the width of a hair--has defeated this conventional approach.

Dionne, one of the lead authors, says that the breakthrough is made possible by the Atwater lab's work on plasmonics, an emerging field that "squeezes" light with specially designed materials to create a wave known as a plasmon. In this case, the plasmons act in a manner somewhat similar to a wave carrying ripples across the surface of a lake, carrying light along the silver-coated surface of a silicon-nitride material, and then across a nanoscale gold prism so that the light reenters the silicon-nitride layer with negative refraction.

Thus, the process is not the same as the one used for negative refraction of microwaves and infrared radiation, but it still works, says Dionne.  And this discovery is particularly exciting because visible light, as its name suggests, is the wavelength associated with the world of objects we see, provided they are not too small.

"Maybe you could create a superlens that can beat the diffraction limit," says Dionne. "You might be able to see DNA and protein molecules clearly just by looking at them, without having to use a more complicated method like X-ray crystallography."

Atwater, who is the Howard Hughes Professor and professor of applied physics and materials science at Caltech, says the plasmonic technique indeed has potential for a compact "perfect lens" that could have a huge number of biomedical and other technological applications. "Once the light coming from a nearby object passes through the negative-refraction material, it would be possible to recover all the spatial information," he says, adding that the loss of this information is why there is ordinarily a limit to the size of an object that can be seen in a microscope.

Even more tantalizing is the possibility of an optical "invisibility cloak" device that would surround an object and bend light in such a way that it would be perfectly refocused on the opposite side. This would provide perfect invisibility for the object inside the cloak, in a manner similar to the cloaks used by Harry Potter or the Klingons in the old Star Trek television series.

"Of course, anyone inside the cloak would not be able to see out," Atwater says.

"But maybe you could have some small windows," Dionne adds.

Adapted from materials provided by California Institute of Technology.

http://www.sciencedaily.com/releases/2007/03/070322132145.htm

ScienceDaily (Jan. 11, 2008)

Contrary to earlier predictions, Duke University engineers have found that a three-dimensional sound cloak is possible, at least in theory.

Such an acoustic veil would do for sound what the "invisibility cloak" previously demonstrated by the research team does for microwaves--allowing sound waves to travel seamlessly around it and emerge on the other side without distortion.

"We've devised a recipe for an acoustic material that would essentially open up a hole in space and make something inside that hole disappear from sound waves," said Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke's Pratt School of Engineering. Such a cloak might hide submarines in the ocean from detection by sonar, he said, or improve the acoustics of a concert hall by effectively flattening a structural beam.

As in the case of the microwave cloak, the properties required for a sound cloak are not found among materials in nature and would require the development of artificial, composite metamaterials.

The engineering of acoustic metamaterials lags behind those that interact with electromagnetic waves (i.e. microwaves or light), but "the same ideas should apply," Cummer said.

In 2006, researchers at Duke and the Imperial College London used a new design theory to create a blueprint for an electromagnetic invisibility cloak. Only a few months later, the team demonstrated the first such cloak, designed to operate at microwave frequencies.

Cummer and David Schurig, a former research associate at Duke who is now at North Carolina State University, later reported in The New Journal of Physics a theory showing that an acoustic cloak could be built. But that theory relied on a "special equivalence" between electromagnetic and sound waves that is only true in two dimensions, Cummer said. A report by another team had also suggested that a 3-D acoustic cloak couldn't exist. It appeared they had reached a dead end.

Cummer wasn't convinced. "In my mind, waves are waves," he said. "It was hard for me to imagine that something you could do with electromagnetic waves would be completely undoable for sound waves."

This time, he started instead from a shell like the microwave cloak his team had already devised and attempted to derive the mathematical specifications required to prevent such a shell from reflecting sound waves, a key characteristic for achieving invisibility. On paper, at least, it worked.

"We've now shown that both 2-D and 3-D acoustic cloaks theoretically do exist," Cummer said. Although the theory used to design such acoustic devices so far isn't as general as the one used to devise the microwave cloak, the finding nonetheless paves the way for other acoustic devices, for instance, those meant to bend or concentrate sound. "It opens up the door to make the physical shape of an object different from its acoustic shape," he said.

The existence of an acoustic cloaking solution also indicates that cloaks might possibly be built for other wave systems, Cummer said, including seismic waves that travel through the earth and the waves at the surface of the ocean.

The report by Cummer's team is expected to appear in Physical Review Letters on Jan. 11.

Collaborators on the study included Bogdan-Ioan Popa, David R. Smith and Marco Rahm of Duke; David Schurig of N.C. State University; John Pendry of Imperial College London; and Anthony Starr of SensorMetrix, Inc. in San Diego, Calif.

Adapted from materials provided by Duke University.

http://www.sciencedaily.com/releases/2008/01/080109104244.htm

A method is presented to suppress the scattering of inorganic nanoparticle inclusions within organic embedding media by means of appropriate surface modification (using ATRP). (Credit: Image courtesy of Carnegie Mellon University)

ScienceDaily (Mar. 7, 2008)

Carnegie Mellon University's Michael Bockstaller and Krzysztof Matyjaszewski have created a version of Harry Potter's famed "invisibility cloak" for nanoparticles.

Through a collaborative effort, researchers from the departments of Materials Science and Engineering and Chemistry have developed a new design paradigm that makes particles invisible.

In a recent edition of Advanced Materials Magazine, the researchers demonstrate that controlling the structure of nanoparticles can "shrink" their visible size by a factor of thousands without affecting a particle's actual physical dimension.

"What we are doing is creating a novel technique to control the architecture of nanoparticles that will remedy many of the problems associated with the application of nanomaterials that are so essential to business sectors such as the aerospace and cosmetics industry," said Bockstaller, an assistant professor of materials science and engineering.

Colloidal particles are omnipresent as additives in current material technologies in order to enhance strength and wear resistance and other attributes. Light scattering that is associated with the presence of particles often results in an undesirable whitish, or milky, appearance of nanoparticles, which presents a tremendous challenge to current material technologies. Carnegie Mellon researchers have successfully created a way to prevent this problem by grafting polymers onto the particles' surface.

"Essentially, what we learned how to do was to control the density, composition and size of polymers attached to inorganic materials which in turn improves the optical transparency of polymer composites. In a sense, light can flow freely through the particle by putting 'grease' onto its surface," said Matyjaszewski, the J.C. Warner University Professor of Natural Sciences in the Department of Chemistry.

The new "particle invisibility cloak" will help create a vast array of new material technologies that combine unknown property combinations such as strength and durability with optical transparency.

Adapted from materials provided by Carnegie Mellon University.

http://www.sciencedaily.com/releases/2008/03/080306161934.htm

David R. Smith of Duke's Pratt School of Engineering is one of the invisibility cloak's technological tailors. (Image courtesy of Duke University)

ScienceDaily (Oct. 19, 2006) 

A team led by scientists at Duke University's Pratt School of Engineering has demonstrated the first working "invisibility cloak." The cloak deflects microwave beams so they flow around a "hidden" object inside with little distortion, making it appear almost as if nothing were there at all.

Cloaks that render objects essentially invisible to microwaves could have a variety of wireless communications or radar applications, according to the researchers.

The team reported its findings on Thursday, Oct. 19, in Science Express, the advance online publication of the journal Science. The research was funded by the Intelligence Community Postdoctoral Fellowship Program.

The researchers manufactured the cloak using "metamaterials" precisely arranged in a series of concentric circles that confer specific electromagnetic properties. Metamaterials are artificial composites that can be made to interact with electromagnetic waves in ways that natural materials cannot reproduce.

The cloak represents "one of the most elaborate metamaterial structures yet designed and produced," the scientists said. It also represents the most comprehensive approach to invisibility yet realized, with the potential to hide objects of any size or material property, they added.

Earlier scientific approaches to achieving "invisibility" often relied on limiting the reflection of electromagnetic waves. In other schemes, scientists attempted to create cloaks with electromagnetic properties that, in effect, cancel those of the object meant to be hidden. In the latter case, a given cloak would be suitable for hiding only objects with very specific properties.

"By incorporating complex material properties, our cloak allows a concealed volume, plus the cloak, to appear to have properties similar to free space when viewed externally," said David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke. "The cloak reduces both an object's reflection and its shadow, either of which would enable its detection."

The team produced the cloak according to electromagnetic specifications determined by a new design theory proposed by Sir John Pendry of Imperial College London, in collaboration with the Duke scientists. The scientists reported that theoretical work in Science earlier this year.

The principles behind the cloaking design, though mathematically rigorous, can be applied in a relatively straightforward way using metamaterials, said cloak designer David Schurig, a research associate in Duke's electrical and computer engineering department.

"One first imagines a distortion in space similar to what would occur when pushing a pointed object through a piece of cloth, distorting, but not breaking, any threads," Schurig said. "In such a space, light or other electromagnetic waves would be confined to the warped 'threads' and therefore could not interact with, or 'see,' objects placed inside the resulting hole."

The researchers used a mathematical description of that concept to develop a blueprint for a cloak that mimics the properties of the imagined, warped space, he said.

"You cannot easily warp space, but you can achieve the same effect on electromagnetic fields using materials with the right response," Schurig continued. "The required materials are quite complex, but can be implemented using metamaterial technology."

While the properties of natural materials are determined by their chemistry, the properties of metamaterials depend instead on their physical structure. In the case of the new cloak, that structure consists of copper rings and wires patterned onto sheets of fiberglass composite that are traditionally used in computer circuit boards.

To simplify design and fabrication in the current study, the team set out to develop a small cloak, less than five inches across, that would provide invisibility in two dimensions, rather than three. In essence, the cloak includes strips of metamaterial fashioned into concentric two-dimensional rings, a design that allows its use with a narrow beam of microwave radiation. The precise variations in the shape of copper elements patterned onto their surfaces determine their electromagnetic properties.

The cloak design is unique among metamaterials in its circular geometry and internal structural variation, the researchers said. All other metamaterials have been based on a cubic, or gridlike, design, and most of them have electromagnetic properties that are uniform throughout.

"Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position," Smith said. "Rather than its material properties being the same everywhere, the cloak's material properties vary from point to point and vary in a very specific way. Achieving that gradient in material properties was a fairly significant design effort."

To assess the cloak's performance, the researchers aimed a microwave beam at a cloak situated between two metal plates inside a test chamber, and used a specialized detecting apparatus to measure the electromagnetic fields that developed both inside and outside the cloak. By examining an animated representation of the data, they found that the wave fronts of the beam separate and flow around the center of the cloak.

"The waves' movement is similar to river water flowing around a smooth rock," Schurig said.

Moreover, the observed physical behavior of the cloak proved to be in "remarkable agreement" with that expected based on a simulated cloak, the researchers said.

Although the new cloak demonstrates the feasibility of the researchers' design, the findings nevertheless represent a "baby step" on the road to actual applications for invisibility, said team member Steven Cummer, a professor of electrical and computer engineering at Duke.

The researchers said they plan to work toward developing a three-dimensional cloak and further perfecting the cloaking effect.

Although the same principles applied to the new microwave cloak might ultimately lead to the production of cloaks that confer invisibility within the visible frequency range, that eventuality remains uncertain, the researchers said.

"It's not yet clear that you're going to get the invisibility that everyone thinks about with Harry Potter's cloak or the Star Trek cloaking device," Smith said.

To make an object literally vanish before a person's eyes, a cloak would have to simultaneously interact with all of the wavelengths, or colors, that make up light, he said. That technology would require much more intricate and tiny metamaterial structures, which scientists have yet to devise.

Collaborators on the study included Jack Mock and Bryan Justice of Duke; John Pendry of Imperial College London; and Anthony Starr of SensorMetrix in San Diego, Calif. Pendry's research is supported by the United Kingdom's Engineering and Physical Sciences Research Council.

Adapted from materials provided by Duke University, via EurekAlert!, a service of AAAS

http://www.sciencedaily.com/releases/2006/10/061019100831.htm

ScienceDaily (May 25, 2006)

Using a new design theory, researchers at Duke University's Pratt School of Engineering and Imperial College London have developed the blueprint for an invisibility cloak. Once devised, the cloak could have numerous uses, from defense applications to wireless communications, the researchers said.

Such a cloak could hide any object so well that observers would be totally unaware of its presence, according to the researchers. In principle, their invisibility cloak could be realized with exotic artificial composite materials called "metamaterials," they said.

"The cloak would act like you've opened up a hole in space," said David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke's Pratt School. "All light or other electromagnetic waves are swept around the area, guided by the metamaterial to emerge on the other side as if they had passed through an empty volume of space."

Electromagnetic waves would flow around an object hidden inside the metamaterial cloak just as water in a river flows virtually undisturbed around a smooth rock, Smith said.

The research team, which also includes David Schurig of Duke's Pratt School and John Pendry of Imperial College London, reported its findings on May 25, 2006, in Science Express, the online advance publication of the journal Science.

The work was supported by the Defense Advanced Research Projects Agency.

First demonstrated by Smith and his colleagues in 2000, metamaterials can be made to interact with light or other electromagnetic waves in very precise ways. Although the theoretical cloak now reported has yet to be created, the Duke researchers are on their way to producing metamaterials with suitable properties, Smith said.

"There are several possible goals one may have for cloaking an object," said Schurig, a research associate in electrical and computer engineering. "One goal would be to conceal an object from discovery by agents using probing or environmental radiation."

"Another would be to allow electromagnetic fields to essentially pass through a potentially obstructing object," he said. "For example, you may wish to put a cloak over the refinery that is blocking your view of the bay."

By eliminating the effects of obstructions, such cloaking also could improve wireless communications, Schurig said. Along the same principles, an acoustic cloak could serve as a protective shield, preventing the penetration of vibrations, sound or seismic waves.

The group's design methodology also may find a variety of uses other than cloaking, the scientists said. With appropriately fine-tuned metamaterials, electromagnetic radiation at frequencies ranging from visible light to electricity could be redirected at will for virtually any application. For example, the theory could lead to the development of metamaterials that focus light to provide a more perfect lens.

"To exploit electromagnetism, engineers use materials to control and direct the field: a glass lens in a camera, a metal cage to screen sensitive equipment, 'black bodies' of various forms to prevent unwanted reflections," the researchers said in their article. "Using the previous generation of materials, design is largely a matter of choosing the interface between two materials." In the case of a camera, for example, this means optimizing the shape of the lens.

The recent advent of metamaterials opens up a new range of possibilities by providing electromagnetic properties that are "impossible to find in nature," the researchers said.

Their design theory provides the precise mathematical function describing a metamaterial with structural details that would allow its interaction with electromagnetic radiation in the manner desired. That function could then guide the fabrication of metamaterials with those precise characteristics, Smith explained.

The theory itself is simple, Smith said. "It's nothing that couldn't have been done 50 or even 100 years ago," he said.

"However, natural materials display only a limited palette of possible electromagnetic properties," he added. "The theory has only now become relevant because we can make metamaterials with the properties we are looking for."

"This new design paradigm, which can provide a recipe to fit virtually any electromagnetic application, leads to material specifications that could be implemented only with metamaterials," Schurig added.

The team's next major goal is an experimental verification of invisibility to electromagnetic waves at microwave frequencies, the scientists said. Such a cloak, they said, would have utility for wireless communications, among other applications.

Adapted from materials provided by Duke University.

http://www.sciencedaily.com/releases/2006/05/060525193729.htm