Invisibility cloak one step closer, scientists say

[Dave Monroe]

Invisibility cloak one step closer, scientists say
Sun Aug 10, 2008 6:54pm EDT
By Maggie Fox, Health and Science Editor


WASHINGTON (Reuters) - Scientists have created two new types of
materials that can bend light the wrong way, creating the first step
toward an invisibility cloaking device.

One approach uses a type of fishnet of metal layers to reverse the
direction of light, while another uses tiny silver wires, both at the
nanoscale level.

Both are so-called metamaterials -- artificially engineered structures
that have properties not seen in nature, such as negative refractive
index.

The two teams were working separately under the direction of Xiang
Zhang of the Nanoscale Science and Engineering Center at the
University of California, Berkeley with U.S. government funding. One
team reported its findings in the journal Science and the other in the
journal Nature.

Each new material works to reverse light in limited wavelengths, so no
one will be using them to hide buildings from satellites, said Jason
Valentine, who worked on one of the projects.

"We are not actually cloaking anything," Valentine said in a telephone
interview. "I don't think we have to worry about invisible people
walking around any time soon. To be honest, we are just at the
beginning of doing anything like that."

Valentine's team made a material that affects light near the visible
spectrum, in a region used in fiber optics.

"In naturally occurring material, the index of refraction, a measure
of how light bends in a medium, is positive," he said.

"When you see a fish in the water, the fish will appear to be in front
of the position it really is. Or if you put a stick in the water, the
stick seems to bend away from you."

These are illusions caused by the light bending when it moves between
water and air.

NEGATIVE REFRACTION

The negative refraction achieved by the teams at Berkeley would be different.

"Instead of the fish appearing to be slightly ahead of where it is in
the water, it would actually appear to be above the water's surface,"
Valentine said. "It's kind of weird."

For a metamaterial to produce negative refraction, it must have a
structural array smaller than the wavelength of the electromagnetic
radiation being used. This was done using microwaves in 2006 by David
Smith of Duke University in North Carolina and John Pendry of Imperial
College London.

Visible light is harder. Some groups managed it with very thin layers,
virtually only one atom thick, but these materials were not practical
to work with and absorbed a great deal of the light directed at it.

"What we have done is taken that material and made it much thicker,"
Valentine said.

His team, whose work is reported in Nature, used stacked silver and
metal dielectric layers stacked on top of each other and then punched
through with holes. "We call it a fishnet," Valentine said.

The other team, reporting in Science, used an oxide template and grew
silver nanowires inside porous aluminum oxide at tiny distances apart,
smaller than the wavelength of visible light. This material refracts
visible light.

Immediate applications might be superior optical devices, Valentine
said -- perhaps a microscope that could see a living virus.

"However, cloaking may be something that this material could be used
for in the future," he said. "You'd have to wrap whatever you wanted
to cloak in the material. It would just send light around. By sending
light around the object that is to be cloaked, you don't see it."

http://www.reuters.com/article/scienceNews/idUSN1029418920080810