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First
Demonstration of
Working Invisibility Cloak
The cloak,
made with advanced 'metamaterials,'
deflects microwave beams and
may find a variety of
wireless communications or radar applications
DUKE
HUNIVERSITY, Durham, NC
: October 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, 2006 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.
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"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.
For more information, contact: Kendall Morgan,
Pratt School of Engineering | (919) 660-8414 | kendall.morgan@duke.edu
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