"Metamaterial" goes against the norm

17 Jun 2002

Physicists in the US have built a composite material that has enabled negative refraction to be realised.

Physicists Richard Shelby, David Smith and Sheldon Schultz from San Diego's University of California, US, have achieved a feat that their peers, past and present, have only dreamed about - they have bent electromagnetic radiation the "wrong" way (Science (292) 77).

In the 1960s, Russian theoreticist Victor Veselago suggested that materials with both a negative electrical permittivity and magnetic permeability would force light to defy convention, that is bend away from rather than towards the normal. Described as having negative refractive indices, these materials did not exist.

Shelby and colleagues realised that combining two different components, each with just one of these properties, would change this. The team placed copper split rings, known to have negative permeabilities, and copper wires, which exhibit negative permittivities, on opposite sides of a circuit board. The boards were sectioned and pieced together in an interlocking lattice to form a "metamaterial". On removing a prism-shaped wedge from the metamaterial and passing a microwave beam through it, the beam bent away from the prism's normal. The team had demonstrated negative refraction.

The scientists now want to take the work further. "An immediate question is whether the negative index-of-refraction property can be implemented at optical frequencies," they said. They believe that using conductors to extend negative refraction to infrared frequencies is simple, but beyond this to visible light is unlikely. However, the team thinks that negative refraction at optical wavelengths could be possible with photonic crystals as long as difficult design issues surrounding these materials are tackled.

"The possibility of using photonic crystals is intriguing," they commented. "Any material that exhibits a negative refractive index will have a variety of practical applications, including beam steerers, band-pass filters and lenses permitting sub-wavelength point-source focusing."