A novel architecture for excluding protons.

A stack of crisscrossed rods is the kind of structure one can readily imagine building out of pencils or soda straws. And that's precisely the point. It's easy to fabricate, even on a microscopic scale.

Discovered by C.M. Soukoulis and his collaborators at Iowa State University and the Energy Department's Ames Laboratory, this particular structure has just the right geometry to act as a photonic crystal. In other words, it prevents the absorption or emission of electromagnetic radiation at certain wavelengths that fall within an excluded range or band gap.

This discovery suggests a promsing route toward producing microscopic structures that exhibit band gaps at infrared or visible wavelengths. The fabrication of such photonic materials may one day lead to the development of highly efficient lasers and solar cells.

"We're very excited about this structure," Soukoulis says.

This research represents an outgrowth of earlier theoretical work done by Iowa State's Kai-Mung Ho and his colleagues. They predicted that an electrically insulating material having a repeating structural pattern resembling the arrangement of carbon atoms and bonds in diamonds would exhibit a photonic band gap.

This prediction was confirmed when Eli Yablonovitch of Bell Communications Research in Red Bank, N.J., and his co-workers created one version of this geometry by drilling three sets of holes, slanted at specific angles, into the top of a solid slab (SN: 11/2/91, p.277). The resulting structure excluded certain wavelengths of microwave radiation.

But scaling this drilled structure down to smaller dimensions to get band gas at visilbe wavelengths proved more difficult than expected. The Iowa State group decided to look for an alternative version of a diamond-like structure that would be easier to manufacture in a variety of sizes. They came up with a structure consisting of layers of parallel rods separated by a certain distance, with rods in adjacent layers at right angles to each other (see illustration).

The researchers then tested their idea by constructing this lattice out of rods of aluminum oxide (alumina). Measurements of microwave transmission through the model revealed a band gap at about 13 gigahertz. By scaling down the structure, they later produced band gaps at 24 gigahertz and 100 gigahertz.

"It's very robust," Soukoulis says. For example, varying the cross section of the rods from circular to elliptical or rectangular has little effect on the size of the band gap. "A graudate student could glue it together," he adds.

"I think it is a novel and interesting development," comments Fred M. Mueller of the Los Alamos (N.M.) National Laboratory, who has also worked on creating photonic materials. "It looks like it's going to have a lot of applications."

The Iowa State group and others are now exploring the possibility of crafting structures small enough to exhibit band gaps at visible wavelengths.