Visible Light Bent the 'Wrong' Way

Another new trick magnifies fine details normally invisible to lenses
negative refraction experiment

Everyone has seen a prism bend light. Now researchers have constructed a material that bends visible light in the opposite way. The odd effect, known as negative refraction, is similar to what is needed in far-out proposals for creating a cloak of invisibility. For now, however, the device only works in two dimensions, so construction of invisible spaceships will have to wait.

In related news, another group used a similar trick to magnify light from objects too small to see with conventional lenses, which might someday prove useful in data storage. The new designs rely on an unusual property of gold and silver. Normally, light causes the electrons in a material (think water or glass) to slosh back and forth, which in turn nudges the light to bend in a way that makes, say, a straw in a glass of water look broken. But electrons in gold and silver can vibrate the opposite way, allowing researchers to circumvent light's ordinary rules. [continued below]

To produce negative refraction, California Institute of Technology researchers Henri Lezec, Jennifer Dionne and Harry Atwater sandwiched a 100-nanometer-thick layer of silver between silicon nitride and gold, with openings on either end to allow laser light to enter and exit the silver. As the light traveled through the silver (called a waveguide), it passed below a prism-shaped piece of gold, like going under a highway overpass. If the light was blue or green, then when it emerged, the team observed that it was bent back toward the direction from where it had entered [see image above].

"We just wanted to give a vivid demonstration of the effect itself," Lezec says. The device might help study cloaking or other effects, he notes, adding that the group wants to try to stack waveguides to bend light in three dimensions. The work, described in this week's Science, is "extraordinarily meticulous and extremely impressive," says engineer David Smith of Duke University.

Smith and his colleagues had previously observed negative refraction and a rudimentary form of cloaking by sending microwaves through layers of metal rings, which in principle can bend light in three dimensions. Similar "metamaterials" have not worked as well for visible light because they absorb the light so rapidly that it does not get a chance to bend. Smith says that similar problems would probably limit the new technique to two dimensions.

In a second Science paper, researchers took another step toward a "superlens" able to defy the usual limits on focusing light. They arranged concentric layers of silver and aluminum oxide into a half cylinder and carved tightly spaced lines on the inside. Normally visible light could not pick out the lines because the spacing between them—150 nanometers—is smaller than the light's wavelength.

The 35-nanometer-thick cylindrical layers, however, amplified faint light that carries extremely fine details about the object. Conventional materials cannot amplify these so-called evanescent waves, which fade intensely the farther they get from the object. But the metamaterial converted the waves into regular light, allowing the researchers to focus it onto a surface a meter away.

"The object [to be magnified] still has to be very close to the lens," notes study leader Xiang Zhang of the University of California, Berkeley. "That is a fundamental limitation," he says, adding that "we are working on a way to overcome this."

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