Scientists Create Cloak of Partial InvisibilityBy Ker Than, LiveScience Staff Writerposted: 19 October 2006 10:10 am ET
Live Science
Scientists have created a cloaking device that can reroute certain wavelengths of light, forcing them around objects like water flowing around boulders in a stream. To creatures or machines that see only in microwave light, the cloaked object would appear nearly invisible.
"The microwaves come in and are swept around the cloak and reconstructed on the other side while avoiding the interior region," said study team member David Smith at Duke University's Pratt School of Engineering. "So it looks as if they just passed through free space."
The device [image] only works in the microwave range of light, so cloaked objects are still visible to humans. Also, it only works in two dimensions and only for microwaves moving in a plane. A three-dimensional invisibility cloak would hide an object completely.The microwave cloak is also slightly reflective and casts a partial shadow.
Despite these shortcomings, however, the new device is "a very good achievement," said Ulf Leonhardt, a theorist at the University of St. Andrews in England who was not involved in the study.
"It's surprising that it's as simple as it is and that it works so well," Leonhardt said in a related news article about the work in the journal Science.
The achievement, reported online today by the journal, comes five months after the same team published a study detailing the precise mathematical specifications a needed to build such a cloaking device.
Metamaterials
The apparatus was made using "metamaterials," artificial materials engineered to have precisely patterned surfaces that interact with and manipulate light in novel ways.
Although called a cloak, the device is not something that can be worn. Rather, it consists of a series of concentric circles, made of copper rings and wires patterned onto sheets of fiberglass, and resembles a loosely coiled reel of film.
The patterns enable the manipulation of light, and the size of the patterns determines which wavelengths of light can be manipulated. Smaller patterns affect shorter the wavelengths. Microwaves have relatively long wavelengths and can be affected with metamaterials having relatively large patterns. Manipulating visible light, which has much shorter wavelengths, will require metamaterials with much finer patterns.
While making those finer patterns is possible with current nanomanufacturing technologies, the metals used to make the microwave cloak would behave differently with visible light, Smith said.
"They act very differently at optical wavelengths; they become very absorptive. A cloaked object would just become very opaque, rather than transparent," he told LiveScience.
But even if metamaterials are made that can deflect visible light, don't expect the kind of invisibility offered by Harry Potter's cloak or Star Trek cloaking devices any time soon.
Human eyes are sensitive to many different wavelengths of light, as evidenced by the rainbow of colors that we see, and it's still uncertain if metamaterials can deflect so many wavelengths simultaneously.
Still Useful
But even imperfect cloaking devices might be useful, the researchers say. Cloaks that deflect radio waves could render an object invisible to radar or improve cell phone receptions by rerouting signals around obstructions. They might also be used to protect people from penetrating and harmful radiation.
"If you knew that you had radiation of a certain bandwidth frequency, you could have it skirt around some region that you wanted shielded," Smith said.
The team says the next step is to create a cloak that works in three dimensions and to perfect the cloaking effect.
Cloaking Device Concept Moves Beyond Theory
By Diane Banegas, National Science Foundation
LiveScience
This Behind the Scenes article was provided to LiveScience in partnership with the National Science Foundation.
Applied mathematician Graeme Milton dreams up new materials, develops mathematical formulas to describe them and leaves it to others to construct and demonstrate their novelties and usefulness in a laboratory.
While many of his theoretical musings are published in peer-reviewed journals, his research on a superlens with the ability to hide or “cloak” an object is too similar to cloaking devices portrayed in Star Trek and Harry Potter to stay buried in the annals of academia.
The concept of a superlens came originally from Sir John Pendry in 2000 — although Milton and his colleagues Nicolae Nicorovici and Ross McPhedran conducted closely related studies back in 1994 — and the concept has been studied extensively. Yet no one had realized the cloaking properties until they were discovered through the research by Milton’s team.
The concept of a superlens cloak is a long way from a workable device, but the integrity of the mathematical concept has sent some experimentalists into the laboratory to try and turn the theory into reality. So far, the groups working in this area are not ready to publish papers, but they’ve accomplished enough to keep trying.
“We’re along way off from the Star Trek device but some of the experimental results achieved so far are surprising and exciting,” Milton noted.
Milton pursues his research at the University of Utah where the math department has some of the best scientists in the nation for studying the mathematics of new materials.
While colleagues consider Milton among “the best in the world” with the mathematics of materials, he was not always so highly ranked. In fact, Milton, who was born in Sydney, Australia, came close to failing his sophomore year at Sydney University because he spent so much time reading about advanced subjects such as quantum mechanics and general relativity he failed to attend his regular classes, and eventually dropped out of school to spend a year hitchhiking through New Zealand.
He later returned to the university to buckle down and earn his bachelor’s and master’s degrees in physics, followed by a doctorate in physics at Cornell University. His love of the outdoors led him to accept the job in Utah – he does some of his best thinking while skiing, mountain biking, swimming and road biking.
The superlens theorized by Milton and Nicorovici cloaks a nearby object by making light behave in an unusual way. Instead of having a positive refractive index that makes light bend in the same way as it does when passing from air into a medium like glass or water, the superlens has a negative refractive index that in essence causes light to reverse and travel backwards. When an object is placed next to the superlens, the light bouncing off the object is canceled out by the light reflecting off the superlens, rendering the object invisible. Milton said the phenomenon, theorized by Milton and Nicorovici and confirmed by computer, is somewhat analogous to the noise cancellation headphones passengers wear on airplanes.
“At this point, experiments would be considered successful if they worked with a single frequency of light and cloaked a few specks of dust, and the object cloaked would need to be much smaller than the superlens,” said Milton. In spite of the challenge, a group of scientists is currently working to demonstrate the proof of principle. “They’ve made some progress, but they want to do some more work before they put their results in a paper,” he added.
Potential early uses for a cloaking device are varied. While stealth military devices are an obvious option, one approach is in medicine where the concept would allow certain electronic instruments to be used despite the presence of strong electromagnetic fields, such as those produced by hospital brain scanners. Milton and other researchers have also carried out related work that might prove capable of guiding the elastic shock waves of earthquakes around buildings.
While practical applications are fun to speculate about, mathematicians may appreciate the new math presented by the concept just as much.
“The findings will lead to a better understanding of partial differential equations, which will lead to a better understanding the propagation of sound, light, fluid and turbulence,” Milton said. The new math will also help scientists better understand the new field of metamaterials – a family of new materials with properties not seen in naturally occurring materials. “Existing laws don’t adequately describe materials with exotic microstructures that are associated with some metamaterials.”
Devising new materials is how Milton and his colleagues came across the superlens in the first place. A metamaterial’s properties are dictated by its internal structure rather than by its composition. His job requires him to discover or design new geometric structures and then develop mathematical formulas to prove their properties. He was working in his home country of Australia on a composite material composed of arrays of coated cylinders when he realized the cylinders could focus light in an unconventional way. That discovery eventually led to his mathematical model for the superlens cloak.
Cloaking Device a Reality? Only if You're Very Small
Levi Beckerson (Blog) - December 20, 2007 11:19 AM
DailyTech
The cloaked circle measures a mere 10 micrometers, but try to spot it next to an uncloaked circle. (Source: UMD)
Scientists at the University of Maryland demonstrate the first working visible light cloaking device.
Cloaking devices and technology have long been the fodder of science fiction, but researchers at the University of Maryland's James Clark School of Engineering have created a material that seems to fit the bill – at least in 2D. The device uses the properties of plasmons in its functionality.
Plasmons are electron waves which are generated when light strikes a metallic surface under controlled conditions. Plasmonics is a relatively new field though it promises to provide many beneficial scientific achievements.
The cloak itself is quite small, a mere 10 micrometers in width (PDF). The structure of the device is a simple thin layer of acrylic plastic with a pattern of concentric, two-dimensional rings atop a gold film. The ring pattern creates a negative refraction effect on visible light striking it, bending the plasmons around the object. While the light appears to have passed straight through the material, it has in fact gone around it.
Far from a usable cloaking system, the device only functions under specialized conditions and only in two dimensions. It is also not perfect invisibility as it only works on a limited range of the visual spectrum and suffers energy loss in the gold film. Three dimensional use of the material would be difficult because visible light would need to be controlled both magnetically and electronically.
Of a more practical purpose, the team has also used the unique properties of plasmons to develop a superlens microscopy technology which could augment existing conventional microscopes. The light bending techniques could allow a real view into nanoscale objects like DNA, viruses and proteins. The group believes they can still improve the superlens technology, bringing the resolution to an impressive 10 nanometers.
Plasmons could one day be employed in a variety of technology due to their unique properties. Since plasmons have very short wavelengths, they can be controlled with impressively small guide structures, much smaller than systems currently in use. As the waves are generated at optical frequencies, they could be used to carry impressive amounts of data in future computing systems.
Not surprisingly, the research has garnered attention from not only the scientific community, but government agencies and industries. One can only dream of the possible applications the military could have in mind for such a technology, less long advances that could be made on the optical computing frontier.
The Mathematics of Cloaking
General Science / Mathematics
Published: 12:26 EST, December 26, 2006
PhysOrg.com
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.
For additional information, please see http://www.seas.rochester.edu/~gresh/math/math_113006.html
Source: University of Rochester
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