Anaheim

Fresh angle: metamaterials such as this copper-based structure have strange optical properties. Credit: W. J. PADILLA

A research initiative is being launched by the Pentagon to investigate negatively refractive materials — structures that seem to defy basic physics by bending light the ‘wrong’ way.

A chunk of negatively refracting material about the size of a cheese wedge was on display at a technical meeting held last week by the Defense Advanced Research Projects Agency (DARPA) in Anaheim, California. DARPA officials say that it could be used in a variety of military technologies, including radar, communications and optics for lasers and imaging.

“We're looking to identify applications,” says Valerie Browning, a programme manager at DARPA's Defense Sciences Office in Arlington, Virginia. The agency will unveil plans to study and apply these materials in military systems in the next month, Browning says.

Negative refraction was conceived in the 1960s, when theorists claimed that some materials should bend light in a strange way. When a beam of light passes from one material, such as air, to another, such as glass, it bends. This refraction is how glass lenses focus light. Negative refractive materials should bend light in the opposite direction to other substances (see diagram).

The idea remained speculative until 2001, when physicists at the University of California, San Diego, announced that they had made such a material. The structure was a ‘metamaterial’, an array of sheets of copper rings and wires, which could negatively refract microwaves.

Negative refraction bends light in an unusual way.

Although some physicists were sceptical about the results (see Nature 420, 119–120; 2002), subsequent experiments have led most to accept the finding, says John Pendry, a physicist at Imperial College in London who works on negatively refracting materials. For example, George Eleftheriades and Anthony Grbic, physicists at the University of Toronto, have recently created a microwave lens from a negatively refracting structure (G. Eleftheriades and A. Grbic Phys. Rev. Lett. in the press).

Browning says that such advances convince her that the effect is real. But she still has questions about how the materials work. For example, some predict that the structures will amplify light waves that are normally dissipated by glass optics. If true, the material could make lenses that are more powerful than any possible with conventional optics. “But we're still unsure of the physics,” Browning adds.

The new initiative will explore the physics and investigate the uses of negatively refracting materials in real systems. Researchers at the Boeing Corporation in Seattle are already investigating negatively refracting materials as optics for steering radar and radio communications, Browning says. And if new materials can be developed to bend infrared or even visible light, the applications could be even broader.