Technology: Making Invisibility Cloaks Real with Metamaterials

David L. Shenkenberg, david.shenkenberg@laurin.com

Harry Potter’s invisibility cloak could become a reality, thanks to some exotic man-made materials called negative-index metamaterials. Whereas materials found in nature have a positive refractive index and bend light forward, some metamaterials with a negative refractive index bend light backward.

In 2006, Sir John B. Pendry at Imperial College London predicted that negative-index metamaterials could be used to make an invisibility cloak. In the same year, Pendry and Duke University scientists David Schurig and David R. Smith announced that they had created the world’s first cloaking device made from negative-index metamaterials.

This fishnet structure of metamaterials with a negative refractive index may be the basis of an invisibility cloak or superlens someday. Courtesy of Jason Valentine and the University of California, Berkeley.

Their device uses concentric rings of metamaterials to channel microwaves around a copper cylinder and out the other side. Light is guided around the metamaterials similar to the way a stream moves around a rock.

Although the cloaking device was an important step, it was not really an invisibility cloak. It can prevent any detection device that uses microwaves from “seeing” through it, but, to conceal objects from human eyes, it must channel visible light around it.

The path to visibility

In early 2007, researchers from Karlsruhe University in Germany, the Ames Laboratory in Iowa and the Institute for Nanotechnology in Karlsruhe reported the creation of the first metamaterial with a negative index of refraction in the visible light range. However, the group reported that a great deal of the light dispersed or was lost as heat rather than being channeled by the metamaterials.

Most recently, Jason Valentine and his co-workers in Xiang Zhang’s laboratory at the University of California, Berkeley, reported in the Sept. 18, 2008, issue of Nature that they had solved the dispersion and loss problems by making the meta-materials thicker.

They use material similar to the Duke group, but each metamaterial unit is about 4000 times smaller. The material must be that small to operate in the visible range. To create each metamaterial unit, they etched into the material with a focused beam of ions. The biggest challenge to making these metamaterials into an invisibility cloak is that it is difficult to manufacture these very tiny units on a large scale, Valentine said.

At the same time that this work appeared in Nature, another group from the Zhang lab published in Science. In the latter publication, the researchers demonstrated that they could bend light backward without a negative refractive index, instead using an array of nanowires.

The scientists are much closer to developing a superlens from negative-index metamaterials than an invisibility cloak. A superlens could enable researchers to see molecules that are too tiny to observe with conventional lenses.