# Team posits 'cloaking' nanomaterial

Portland, Ore. -- The researcher who introduced the concept of negative-index-of-refraction metamaterials in 2000 is now positing that materials with a variable refractive index could enable such fantastic applications as a Harry Potteresque "invisibility cloak."

John Pendry, a physicist at Imperial College in London, predicted six years ago that metals could be engineered to make electrical fields behave oppositely to normal, yielding negative-index-of-refraction metamaterial composites. Since his prediction, such metamaterials have been created and demonstrated from gigahertz to optical frequencies.

Now Pendry has teamed with Duke University EEs David Schurig and David Smith to predict that both the electrical and the magnetic properties of an inhomogeneous composite with embedded nanoparticles could be altered to create a variable-index-of-refraction material. They postulate that such a material could adapt at the nanoscale to conceal what's under it by preventing electromagnetic energy from entering an area. Light hitting the material would "flow" around it and continue, undistorted, on the other side. The material thus would neither reflect light nor cast a shadow.

The index of refraction is derived from Snell's Law, or the "right-hand rule," which states that magnetism curls around a wire in the direction of your fingers when you point your thumb in the direction of current flow. The right-hand rule predicts that light will always bend in one direction (toward the normal) when passing through a material (because of positive electrical permeability and magnetic permittivity). Metamaterials with negative permeability or permittivity, however, can bend light away from the normal and thereby can allow even a flat lens to focus light to a point.

Pendry, Schurig and Smith claim that recent demonstrations of real metamaterials over a wide range of frequencies show that science will eventually perfect variable-index-of-refraction materials that can accommodate all frequencies and wavelengths. For the posited invisibility cloak, the flexible nanoscale material would be draped on an item to conceal it from view. By virtue of adaptive algorithms that would change the dielectric and magnetic properties of the cloak's surface, photons falling on one side of the material would be simply redirected around to the other side. The observer would therefore detect neither the cloak nor the item under it.

**Wrinkles accounted for**

The mathematical proof derived by Pendry and colleagues demonstrates all the mathematical steps required to transform the embedded-nanoparticle coordinates to cancel out the distortion from the wrinkles that would occur in the material as it was draped around the item to be cloaked. By canceling out the effects from wrinkles, light could theoretically be bent around the object so that it would arrive at the other side undistorted.

The scientists showed how to generate the values of electrical permittivity and magnetic permeability mathematically for such a material at a single frequency, buy they did not attempt to build practical devices.