Will lefl anded materials yield the perfect lens? Physicists at the University of California, San Diego have experimentally verified that a new class of composite materials developed last year can bend electromagnetic waves in a direction opposite to that predicted by Shell's law, as previously suggested. "The experiments we report confirm earlier theoretical predictions that a new, unique class of materials can cause electromagnetic waves, such as radar and microwaves, to bend in a direction opposite to the way in which they travel through all other known materials," says Sheldon

Schultz. "If these effects turn out to be possible at optical frequencies, a material like ours could have the crazy property that a point light source shining on a flat slab could produce a focus point on the other side. There's no way you can do that with just a flat sheet of ordinary material." The composite material con- structed by Shuttz and his co- workers, David Smith and Richard Shelby, consists of a series of thin fiberglass sheets, superimposed with a geometric pattern of copper split ring resonators and wires, arranged into square like the interlocking

inserts in a wine-case. "This is the first demonstration of any material which has a negative index of refraction," says Smith. "Since no existing material has this property, we needed to demonstrate the effect using a 'metamaterial' - a composite material fabricated from repeated elements specifically engineered to produce a desired electromagnetic behavior." The results, reported in the April 6 issue of Science (292, 5514, 77-79], are nat just a physical curiosity, but could eventually lead to the development of a "perfect lens"

made from these new compos- ites which would not be bound by a diffraction limit. Other pos- sible applications include novel antennas and e l ~ m a g n e t i c devices. Researchers at Imperial College and Marconi Caswell in the UK showed improvements to a magnetic resonance imaging [MRI) system using a magnetic metamaterial based on a structure similar to that developed by the UCSD team.

Contact: David Smith,. UCSD e-mail: drs@sdss.ucsd.edu (See also John Pendry's paper in Physicat Review Letters, 2000.)

Researchers explain.the secrets of nanoporous gold Pre-Columbian goldsmiths may have known how to use selective dissolution of metal alloys to enhance the appearance of their artefacts, but until now, scientists could not explain the underlying mechanism. =Dealloying" harnesses selective dissolution to remove the moat e l e ~ h e m i c a l l y active element of an alloy to leave a nanoporous sponge of the more noble constituent. Erlabacher and co-workers, writing in the March 22 issue of Nature (410, 450-453), describe a numerical model which elucidates the mecha- nism and can predict pore size of deelloyed structures. Taking the example of a silver- gold alloy, Erlebacher explains how the dealloying mechanism works: "As the silver atoms in

the alloy are dissolved in acid, the remaining atoms of gold gather together in clusters to create a porous surface. With the gold, atoms condense into small clumps that form the basis of a porous surface." The researchers' numerical model will enable the production of nanoporous gold from silver-gold alloys with a pore size that can be tailored to suit the application. Nanoporous gold has many potential applications as catalysts or sensors, but most notably for biosensors. Instead of using polymers to immobilize biomoleculas such as enzymes, the pore size could perform the same task.

Contact: Jonah Erlebacher, John Hopkins University; tel: +1 410 516 7160

Chance discovery could cut the costs of thermoplastic elastomers dramatically Researchers from Cornell University told the annual meeting of the American Chemical Society in April how they chanced upon a new class of rubbery plastics [or thermoplastic elastomers). Geoffrey Coates described the "growth" of the new polymer from a feedstock of ethylene and polyethylene, using a titanium-based catalyst. As these monomers are among the least expensive on the market, =we anticipate that these new polyolefins will be dramatically cheaper," says Coates. =We simply stumbled across this, luckily with our eyes open." Until now it has not been possible to use these cheap monomers because there was

no suitable catalyst. Possible candidates tried over the last five years had failed to enable the required =living polymer- izstion process" that grows mol- ecules in connected monomer blocks continuously without breaking the chains. Coates' titanium catalyst; is able to facili- tate combination of a hard farm of propyiene - or "syndiomctic" - with a softer ethylene-propylene copolymer in alternate blocks, This method allows the pliability of the resulting elastomer to be tailored by altering the sizes of the blocks constituting the polymer strands. The plastic can be changed from tough rubber to a gum elaatomer. Contact: David Brand, e-mail-" deb27OcornelLedu

6 ~ Hay/June 2001