First demonstration of a working invisibility cloak

"You cannot easily warp space, but you can achieve the same effect on electromagnetic fields using materials with the right response," Schurig continued. "The required materials are quite complex, but can be implemented using metamaterial technology."

While the properties of natural materials are determined by their chemistry, the properties of metamaterials depend instead on their physical structure. In the case of the new cloak, that structure consists of copper rings and wires patterned onto sheets of fiberglass composite that are traditionally used in computer circuit boards.

To simplify design and fabrication in the current study, the team set out to develop a small cloak, less than five inches across, that would provide invisibility in two dimensions, rather than three. In essence, the cloak includes strips of metamaterial fashioned into concentric two-dimensional rings, a design that allows its use with a narrow beam of microwave radiation. The precise variations in the shape of copper elements patterned onto their surfaces determine their electromagnetic properties.

The cloak design is unique among metamaterials in its circular geometry and internal structural variation, the researchers said. All other metamaterials have been based on a cubic, or gridlike, design, and most of them have electromagnetic properties that are uniform throughout.

"Unlike other metamaterials, the cloak requires a gradual change in its properties as a function of position," Smith said. "Rather than its material properties being the same everywhere, the cloak's material properties vary from point to point and vary in a very specific way. Achieving that gradient in material properties was a fairly significant design effort."

To assess the cloak's performance, the researchers aimed a microwave beam at a cloak situated between two metal plates inside a test chamber, and used a specialized detecting apparatus to measure the electromagnetic fields that developed both inside and outside the cloak. By examining an animated representation of the data, they found that the wave fronts of the beam separate and flow around the center of the cloak.

"The waves' movement is similar to river water flowing around a smooth rock," Schurig said.

Moreover, the observed physical behavior of the cloak proved to be in "remarkable agreement" with that expected based on a simulated cloak, the researchers said.

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