Aug 14, 2008

Tech - Closer to making the invisible Cloak

Using tiny wires and fishnet structures, researchers at the University of California, Berkeley, have found new ways to bend light backward, something that never occurs in nature.
This technology could lead to microscopes able to peer more deeply and clearly into living cells. And the same kind of structures might one day be adapted to bend light in other unnatural ways, creating a Harry Potter-like invisibility cloak. "This is definitely a big step toward that idea," said Jason Valentine, a graduate student and a lead author of a paper being published online on Wednesday by the journal Nature. But scientists are still far from designing and manufacturing such a cloak.
The work involves materials that have a property known as negative refraction, which means that they essentially bend light backward.
Once thought to be pure fantasies, these substances, called metamaterials, have been constructed in recent years, and scientists have shown they can bend long-wavelength microwaves.
Negative refractive materials can in principle lead to fantastical illusions; someone looking down at a fish in a pool of negative refractive liquid would see the fish swimming in the air above.
Two separate advances are described in two scientific papers, both out of the research laboratory of Xiang Zhang, a professor at the Nanoscale Science and Engineering Center in Berkeley.
When a ray of light crosses the boundary from air to water, glass or other transparent material, it bends, and the degree of bending is determined by a property known as the index of refraction. Transparent materials like glass, water and diamonds all have an index of 1 or higher for visible light, meaning that when the light enters, its path bends toward an imaginary line perpendicular to the surface.
With the engineered metamaterials, scientists can create refractive indices less than 1 or even negative. Light entering a material with a negative index of refraction would take a sharp turn, almost as if it had bounced off the imaginary perpendicular line.
In the Nature paper, the Berkeley researchers created a fishnet structure with 21 layers, alternating between a metal and magnesium fluoride, resulting in a metamaterial with a negative index of refraction for infrared light. The researchers said that by making the fishnet structure even smaller, they should be able to do the same with visible light.
In the second paper, to be published in the issue this Friday of the journal Science, a different group of scientists in Zhang's laboratory used a different approach, building an array of minuscule upright wires, which changed the electric fields of passing light waves. That structure was able to bend visible red light.
One application of negative index materials could be a "superlens." Light is usually thought of as having undulating waves.
But much closer up, light is a much more jumbled mess, with the waves mixed in with more complicated "evanescent waves."
The evanescent waves quickly dissipate as they travel, and thus are usually not seen. A negative refraction lens actually amplifies the evanescent waves, preserving detail lost in conventional optics, and the hope is to eventually build an optical microscope that could make out tiny biological structures like individual viruses.

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