April 30, 2009
Scientists Develop New Invisibility Cloak Technology
Two teams of scientists have developed a cloak that renders objects invisible to near-infrared light, BBC News reported.
The new technology, however, does not contain metals unlike previous such "cloaks" that resulted in imperfect cloaking because of losses of light.Researchers say that since the approach can be scaled down further in size, the new technology is a major step towards a cloak that would work for visible light.
John Pendry from Imperial College London first theorized a cloak with a "carpet" design in 2008. One of the research teams describes its miniature "carpet cloak" in the journal Nature Materials.
A team at Cornell University, led by Michal Lipson, demonstrated a cloak based on Pendry's concept.
Xiang Zhang, professor of mechanical engineering at the University of California, Berkeley, led a separate team.
He explained that his team was essentially transforming a straight line of light into a curved line around the cloak, making it difficult to perceive any change in its pathway.
This is the first cloak built considered to be carpet-based, as it uses a dielectric - or insulating material - that absorbs far less light than previous invisibility cloaks designed using metals.
Zhang said metals introduce a lot of loss, or reduce the light intensity, which can leave a darkened spot in the place of the cloaked object.
He added that since the new design uses silicon, a material that absorbs very little light, it is a "big step forward" in the evolution of invisibility cloaking.
The cloak's design gives the illusion of a flattened surface by canceling out the distortion produced by the bulge of the object underneath. Therefore, light is bent around the object, like water around a rock.
The cloak changes the local density of the object it is covering, Zhang explained.
He told BBC News that when light passes from air into water it would be bent, because the optical density, or refraction index, of the glass is different to air.
"So by manipulating the optical density of an object, you can transform the light path from a straight line to any path you want," he said.
Through a series of minuscule holes strategically "drilled" into a sheet of silicon, the new material produces such an effect.
Zhang's team was able to "decide the profile" of the cloaked object by altering the optical density with the holes, thereby proving Pendry's theory.
He explained that the team drilled lots of very densely packed holes in some places and in others they were much more sparse.
"Where the holes are more dense, there is more air than silicon, so the optical density of the object is reduced," Zhang said.
"Each hole is much smaller than the wavelength of the light. So optical light doesn't see a hole - it just sees a sort of air-silicon mixture," he added.
He said this allows the density of the object to be adjusted as far as the light is concerned.
However, the current demonstration cloak was very tiny at just a few thousandths of a millimeter across. But he said there are applications even for such a minuscule cloak.
The electronics industry could use such technology to hide flaws on the intricate stencils or 'masks' that are used to cast processor chips.
Zhang said that alone could save the industry millions of dollars.
"It would allow them to fix flaws rather than produce an entirely new mask," he said.
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