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[link to www.eurekale] rt.org/pub_ releases/ 2006-10/du- fdo101206. php
Contact: Kendall Morgan
kendall.morgan@ duke.edu
919-660-8414
Duke University
First demonstration of a working invisibility cloak
DURHAM, N.C. -- 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
( [link to www.ee.] duke.edu/ ~drsmith/ neg_ref_home. htm).
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
( [link to www.pratt.] duke.edu/ news/?id= 433).
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
( [link to www.ece.] duke.edu/ ~dschurig/).
"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.
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