'Invisibility cloak' material invented

Scientists have created two new types of materials that can bend light the wrong way, creating the first step toward an invisibility cloaking device.

One approach uses a type of fishnet of metal layers to reverse the direction of light, while another uses tiny silver wires, both at the nanoscale level.

Both are so-called metamaterials - artificially engineered structures that have properties not seen in nature, such as negative refractive index.

The two teams were working separately under the direction of Xiang Zhang of the Nanoscale Science and Engineering Center at the University of California, Berkeley with US government funding. One team reported its findings in the journal Science and the other in the journal Nature.

Fibre optics

Each new material works to reverse light in limited wavelengths, so no one will be using them to hide buildings from satellites, said Jason Valentine, who worked on one of the projects.

Valentine's team made a material that affects light near the visible spectrum, in a region used in fibre optics. "In naturally occurring material, the index of refraction, a measure of how light bends in a medium, is positive," he said.

These are illusions caused by the light bending when it moves between water and air. The negative refraction achieved by the teams at Berkeley would be different.

"What we have done is taken the material and made it much thicker," Valentine said.

Layers

His team, whose work is reported in Nature, used silver and metal dielectric layers stacked on top of each other and then punched through with holes.

The other team, reporting in Science, used an oxide template and grew silver nanowires inside porous aluminum oxide at tiny distances apart, smaller than the wavelength of visible light. This material refracts visible light.

Immediate applications might be superior optical devices, Valentine said - perhaps a microscope that could see a living virus.