Closer to Vanishing: Bending light as a step toward invisibility cloaks

By Davide Castelvecchi
From Science News, March 24th, 2007; Vol.171 #12 (p. 180)

Harry Potter fans, do not despair! Invisibility cloaks may be a long shot, but last year physicists demonstrated technology that might someday hide you from radar. Now, two groups of researchers have taken steps toward performing the same trick with visible light.

Light rays passing from one transparent material to another generally refract, or change direction. That’s why a pencil looks broken when partially submerged in water. Recently, physicists have begun to explore materials with a characteristic called negative refraction. If water had that property, the underwater half of the pencil would appear to stick out above the surface.

The first demonstration of negative refraction used materials that displayed the effect at one particular microwave wavelength. Last year, researchers showed that a suitably shaped piece of negatively refracting material could hide an object by guiding microwaves around it (SN: 7/15/06, p. 42: http://sciencenews.org/articles/20060715/bob9.asp).

A team led by Henri Lezec of the California Institute of Technology in Pasadena has now achieved negative refraction for visible light over a spectrum of blues and greens.

Lezec’s team built a micrometer-size prism of layered metals perforated by a maze of nanoscale channels. Light striking the prism transforms into plasmons, two-dimensional waves in which electromagnetic fields displace electrons along the metal surfaces. Guided by the nanochannels, the plasmons travel through the prism, turning back into light when they emerge on the other side.

Made from a certain combination of metals and maze structures, the prism acted as if it were made of negatively refracting material. The researchers describe their results in a paper published online by Science on March 22.

So far, their device works only when light rays striking the prism are in a particular plane. “We’re really doing optics in Flatland,” Lezec says, referring to an 1884 novella whose characters lived in a two-dimensional world.

“This is an ingenious and beautiful experiment,” says Vladimir Shalaev of Purdue University in West Lafayette, Ind. He says that turning light into plasmons may be the only way to negatively refract a broad spectrum of visible light.

Shalaev and his colleagues have recently come close to achieving direct negative refraction of light. At a meeting of the American Physical Society in Denver earlier this month, he described a negatively refracting material that works at infrared wavelengths barely beyond the visible spectrum.

John Pendry of the Imperial College London doesn’t view this material as an advance toward the invisibility so useful in books. “You could do only cloaking in 2-D, which wouldn’t be very useful to Harry,” he says.

But microscopic components such as Lezec’s could be useful for integrating optical circuits into computer chips, he adds. That might increase computer speed because optical components don’t overheat.

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