August 11, 2008
Scientists Report New Developments For Invisibility Cloak
Two recently designed 3-D materials are opening new possibilities for scientists looking to develop an invisibility cloak.
The so-called metamaterials are artificially engineered structures that have properties not seen in nature.The materials were developed by two separate teams under the direction of Xiang Zhang of Nanoscale Science and Engineering Center at the University of California, Berkeley with U.S. government funding.
These metamaterials work to bend light in the wrong direction. One approach uses a fishnet of metal layers to reverse the direction of light, while the other uses tiny silver wires at the nanoscale level.
Immediate applications for these materials may include optical microscopes to discern individual living viruses or DNA molecules. As for the development of an invisibility cloak, the material would need to curve light waves completely around the object.
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. "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."
He said his team's material 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.
People can see objects because they scatter the light that strikes them, reflecting some of it back to the eye. Cloaking uses materials, known as metamaterials, to deflect radar, light or other waves around an object, like water flowing around a smooth rock in a stream.
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.
"What makes both these materials stand out is that they are able to function in a broad spectrum of optical wavelengths with lower energy loss," said Zhang.
"We've also opened up a new approach to developing metamaterials by moving away from previous designs that were based upon the physics of resonance. Previous metamaterials in the optical range would need to vibrate at certain frequencies to achieve negative refraction, leading to strong energy absorption. Resonance is not a factor in both the nanowire and fishnet metamaterials."
Image Caption: The fine structure of the material gives it light-bending abilities
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