Simpler Structure for Negative-Refractive-Index Media Presented

Anne L. Fischer

Negative-refractive-index materials bend electromagnetic waves in a direction opposite that of ordinary media with positive refractive indexes, enabling a variety of novel applications, including flat lenses. A group of scientists at Iowa State University and at Ames Laboratory, both in Ames, and at the University of Crete in Iraklion, Greece, has demonstrated a simple way of developing negative-index materials using periodic arrays of pairs of H-shaped metallic wires.

When an electromagnetic wave encounters a slab of material with a negative refractive index, it bends in a direction opposite what it would be if the material had a positive refractive index. The effect can be used to fabricate flat lenses and other novel structures.

One type of negative-index materials investigated recently comprises split-ring resonators, as proposed by Sir John B. Pendry of Imperial College London. The wire-pair structure used by the researchers in Iowa and Greece is an alternative to such metamaterials. They chose the H-shaped wire-pair resonators because they increase the electric interaction between adjacent resonators such that the negative effective permittivity and permeability exist simultaneously in the same region, explained Costas M. Soukoulis, a scientist at the institutions. If they had used simple pairs of short wires, the electric resonance would have had a tendency to occur at a much higher frequency than the magnetic resonance.

In a demonstration of the new design, the investigators fabricated samples from printed circuit board stock and copper. Experiments with 14- to 18-GHz microwaves indicated that the material had a negative refractive index — as low as –2.66 — for radiation between 15.67 and 16.02 GHz.

The group will continue to work on alternative designs based on short-wire pairs or their reciprocal structures, with the goal of identifying reasonable low-loss metamaterials for use at optical frequencies, Soukoulis said. He expects that the work will generate new designs with lower losses and left-handed materials for use at terahertz frequencies. Potential applications include miniaturized antennas and resonators for radio waves.

Applied Physics Letters, May 29, 2006.