Magic material flips refractive index
Apr 5, 2001
An incredible characteristic of a radical new material has been verified by American physicists. Sheldon Schultz and colleagues at the University of California at San Diego have built a composite material that exploits a quirk in the fundamental rules of optics - it has a negative refractive index (R A Shelby et al 2001 Science 292 77). The special structure could overcome the diffraction limit to make a perfect lens - and revolutionise optoelectronics.
Shelby's team created the 'left-handed' material - so called because it reverses one of the well-known 'right-hand rules' of physics - from interlocking sections of copper coils and wires. When microwave radiation was shone into a prism-shaped chunk of this 'meta-material' - composed of repeating sub-units of coils and wires - they found that it bent towards the normal. In a conventional optical medium like glass, radiation bends away from the normal - this is why deep water looks shallow.
Such left-handed materials were first predicted to exist in 1967 by the Russian physicist Victor Veselago. He realised that one of Maxwell's famous equations - which describe the interplay of electromagnetic waves and matter - has a special solution when both the electric permittivity and magnetic permeability are negative. This solution corresponds to a material with a negative refractive index.
The confirmation that the left-handed material works follows the recent demonstration by John Pendry and co-workers at Imperial College, UK, that a lattice of copper wires could have negative electric permittivity for some wavelengths. His group went on to show that an array of copper coils could have negative magnetic permeability. "My colleague David Smith then had a brilliant insight", Schultz told PhysicsWeb. "If we combined the wires with the coils we could create a material in which both parameters are negative".
Schultz's group became interested in composite materials after experimenting with copper components in photonic bandgap materials. "We used Maxwell's equations in our simulations and experiments, so we realised the significance of the negative electric and magnetic parameters", says Schultz.
There are many practical applications for materials with a negative refractive index, including band-pass filters and lenses with sub-wavelength resolution. They may even have the striking ability to reverse the Doppler effect. It is unlikely, however, that the new material can be scaled down to operate at optical frequencies, according to Schultz and colleagues. "But we do think we can get much closer to the visible wavelengths than microwaves", says Schultz, "and we have other ideas for creating the effect in the optical range".
About the author
Katie Pennicott is Editor of PhysicsWeb