Last year researchers from Duke University stunned the world when they announced a cloaking device for the microwave range. This device made use of metamaterials that had a negative refractive index for electromagnetic radiation. The metamaterials were carefully designed split-ring resonators with a structure size much smaller than the wavelength. Only 10 stacked layers of metamaterials were necessary to achieve the desired invisibility effect.

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Possible applications in the future include perfect lenses that beat the diffraction limit, and optical cloaking devices which provide some invisibility for macroscopic objects

The Purdue University engineers, following mathematical guidelines devised in 2006 by physicists in the United Kingdom, have created a theoretical design that uses an array of tiny needles radiating outward from a central spoke. The design, which resembles a round hairbrush, would bend light around the object being cloaked. Background objects would be visible but not the object surrounded by the cylindrical array of nano-needles, said Vladimir Shalaev, Purdue's Robert and Anne Burnett Professor of Electrical and Computer Engineering.

The design does, however, have a major limitation: It works only for any single wavelength, and not for the entire frequency range of the visible spectrum, Shalaev said.

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Calculations indicate the device would make an object invisible in a wavelength of 632.8 nanometers, which corresponds to the color red. The same design, however, could be used to create a cloak for any other single wavelength in the visible spectrum, Shalaev said.

"How to create a design that works for all colors of visible light at the same time will be a big technical challenge, but we believe it's possible," he said. "It is clearly doable. In principle, this cloak could be arbitrarily large, as large as a person or an aircraft."

Such a cloak could hide any object so well that observers would be totally unaware of its presence, according to the researchers. In principle, their invisibility cloak could be realized with exotic artificial composite materials called "metamaterials," they said.

"The cloak would act like you've opened up a hole in space," said David R. Smith, Augustine Scholar and professor of electrical and computer engineering at Duke's Pratt School. "All light or other electromagnetic waves are swept around the area, guided by the metamaterial to emerge on the other side as if they had passed through an empty volume of space."

Electromagnetic waves would flow around an object hidden inside the metamaterial cloak just as water in a river flows virtually undisturbed around a smooth rock, Smith said.

Scientists have already created an 'invisibility cloak' made out of 'metamaterial' which can bend electromagnetic radiation such as visible light, radar or microwaves -- around a spherical space, making an object within this region appear invisible.

Until now, scientists could only make objects appear invisible from far away. Liverpool mathematician Dr Sébastien Guenneau, together with Dr Frédéric Zolla and Professors André Nicolet from the University of Marseille, have proven - using a specially designed computer model called GETDP - that objects can also be made to appear invisible from close range when light travels in waves rather than beams

A research team at Maryland's A. James Clark School of Engineering comprised of Professor Christopher Davis, Research Scientist Igor Smolyaninov, and graduate student Yu-Ju Hung, has used plasmon technology to create the world's first invisibility cloak for visible light. The engineers have applied the same technology to build a revolutionary superlens microscope that allows scientists to see details of previously undetectable nanoscale objects.

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This manipulation causes the plasmon waves to appear to have moved in a straight line. In reality they have been guided around the cloak much as water in a stream flows around a rock, and released on the other side, concealing the cloak and the object inside from visible light. The invisibility that this phenomenon creates is not absolutely perfect because of energy loss in the gold film.

Abstract: The diffractive nature of light has limited optics and photonics to operate at scales much larger than the wavelength of light. The major challenge in scaling-down integrated photonics is how to mold the light flow below diffraction-limit in all three dimensions. A high index solid immersion lens can improve the spatial resolution by increasing the medium refractive index, but only to few times higher than in air. Photonic crystals can guide light in three dimensions, however, the guided beam width is around a wavelength. Surface plasmons has a potential to reach the sub-wavelength scales; nevertheless, it is confined in the two-dimensional interface between metals and dielectrics. Here, we present a new approach for molding the light flow at the deep sub-wavelength scale, using metamaterials with uniquely designed dispersion. We develop a design methodology for realizing sub-wavelength ray optics, and demonstrate lambda/10 width light beams flow through three-dimensional space.