28 May 2009

Making the visible invisible …

A new type of invisibility cloak is now easier than previous designs and works for all colors of the visible spectrum, making it possible to cloak larger objects than before and possibly leading to practical applications in "transformation optics."

Whereas previous cloaking designs have used exotic “metamaterials,” which require complex nanofabrication, the new design is a far simpler device based on a “tapered optical waveguide,” said Vladimir Shalaev, Purdue University’s Robert and Anne Burnett Professor of Electrical and Computer Engineering.

This is a new type of invisibility cloak that is simpler than previous designs and works for all colors of the visible spectrum.

Waveguides represent established technology, including fiber optics, used in communications and other commercial applications.

The research team used their specially tapered waveguide to cloak an area 100 times larger than the wavelengths of light shined by a laser into the device, an unprecedented achievement. Previous experiments with metamaterials cloaked regions only a few times larger than the wavelengths of visible light.

Because the new method enabled the researchers to dramatically increase the cloaked area, the technology offers hope of cloaking larger objects, Shalaev said.

“All previous attempts at optical cloaking have involved very complicated nanofabrication of metamaterials containing many elements, which makes it very difficult to cloak large objects,” Shalaev said. “Here, we showed that if a waveguide is tapered properly it acts like a sophisticated nanostructured material.”

The waveguide is inherently broadband, meaning it could cloak the full range of the visible light spectrum. Unlike metamaterials, which contain many light-absorbing metal components, only a small portion of the new design contains metal.

Purdue did the theoretical design work, with BAE Systems doing the fabrication of the device, which they form by uniting two gold-coated surfaces, a curved lens and a flat sheet. The researchers cloaked an object about 50 microns in diameter, or roughly the width of a human hair, in the center of the waveguide.

“Instead of being reflected as normally would happen, the light flows around the object and shows up on the other side, like water flowing around a stone,” Shalaev said.

The research falls within a new field called transformation optics, which may usher in a host of radical advances, including cloaking; powerful “hyperlenses” resulting in microscopes 10 times more powerful than today’s and able to see objects as small as DNA; computers and consumer electronics that use light instead of electronic signals to process information; advanced sensors; and more efficient solar collectors.

Unlike natural materials, metamaterials are able to reduce the “index of refraction” to less than one or less than zero. Refraction occurs as electromagnetic waves, including light, bend when passing from one material into another. It causes the bent-stick-in-water effect, which occurs when a stick placed in a glass of water appears bent when viewed from the outside. Each material has its own refraction index, which describes how much light will bend in that particular material and defines how much the speed of light slows down while passing through a material.

Natural materials typically have refractive indices greater than one. Metamaterials, however, can make the index of refraction vary from zero to one, which is what they need for cloaking.

The precisely tapered shape of the new waveguide alters the refractive index in the same way as metamaterials, gradually increasing the index from zero to one along the curved surface of the lens, Shalaev said.

For related information, go to www.isa.org/manufacturing_automation.