Now, Pendry and two engineering colleagues, David Schrig and David Smith, are predicting that both electrical and the magnetic properties can be varied in inhomogeneous composites with embedded nanoparticles. The result is a variable index-of-refraction material that is being touted for applications like an "invisibility cloak."

Such a cloak could theoretically be made from a metamaterial that adapts at the nanoscale level to conceal what's under it. It would work by bending the light impinging on one side to be emitted from the opposite side.

A split-ring structure etched on a copper circuit board along with copper wires yields negative permittivity and negative permeability. (Courtesy of David Smith and Shelly Schultz, UCSD)

Index of refraction is derived from Snell's Law, or the "right-hand rule," that states that magnetism curls around a wire in the direction of the fingers when the thumb is pointed in the direction of current flow. For light, the right-hand rule predicts that light will always bend in one direction (toward the normal) when passing through a material due to positive electrical permeability and magnetic permittivity.

However, metamaterials with negative permeability or permittivity can follow a left-hand rule that bends light away from the normal, thereby allowing a flat lens to focus light. By varying permeability or permittivity, any sort of lens could theoretically be produced, even for an invisibility cloak, the researchers postulated.

Pendry demonstrated that composite materials made from different configurations of dielectric and metal would respond to magnetic fields by following a left-hand rule. In 2001, researchers at Imperial College and Marconi Caswell (London) announced an MRI system that uses a magnetic metamaterial based on Pendry's design. Since then, dozens of metamaterials have been demonstrated over a variety of frequencies.

Pendry and his team predicted that future composites will allow permeability and permeability to vary independently and arbitrarily throughout a material, assuming positive or negative values as desired.

They also claim that recent demonstrations of metamaterials over a wide range of frequencies and for both electrical and magnetic fields promises perfect variable index-of-refraction materials that can accommodate all frequencies and wavelengths. By utilizing different nanoscale materials—inhomogeneous composites with embedded nanoparticles—Pendry postulated an invisibility cloak that forbids electromagnetic energy from entering.

The flexible nanoscale cloak would be concealed from view using adaptive algorithms that change the dielectric and magnetic properties of the cloak's surface, redirecting photons falling on one side over to the other, where they are emitted.

Pendry and his team have also showed how to mathematically generate the values of electrical permittivity and magnetic permeability for the material at a single frequency. However, they did not attempt to build a device.