Liquid crystal coating controls light.

A material that combines silicon with a liquid crystal could one day serve as the foundation for microchips used in light-based computers, according to a new theoretical study.

Researchers at the University of Toronto have demonstrated that liquid crystal molecules provide a way to tune the optical properties of a silicon photonic crystal, a structure designed to exclude light within a chosen range of wavelengths (SN: 11/16/96, p. 309). By engineering switches and waveguides into a photonic crystal, researchers hope to create optical computer chips that rely on light signals instead of electrical currents.

Toronto's Sajeev John and his colleague Kurt Busch, now at the University of Karlsruhe in Germany, calculate that applying an electric field to the composite material will shrink its forbidden range of wavelengths, or band gap.

The team's theoretical analysis begins with a type of photonic crystal known as an inverse opal, which looks something like a hunk of Swiss cheese. Researchers make inverse-opal crystals by molding silicon around a stack of tiny glass balls, then etching away the glass with acid. What's left is a chunk of silicon containing an orderly arrangement of spherical, air-filled voids. These structures block light with wavelengths approximately equal to the diameter of the voids.

The researchers determined that by coating the inner surfaces of these hollow spheres with a common liquid crystal, they can control the size of the photonic-crystal band gap. Applying an electric field "rotates the axis of the liquid crystal materials, and that scatters light in a different way," John explains. By changing the orientation of the liquid crystal molecules, they can shrink the band gap down to zero. Light of all wavelengths can then pass through.

One way to design an optical computer chip is to build switches (SN: 11/15/97, p. 310) and waveguides (SN: 10/24/98, p. 271) into the photonic crystal to allow through particular wavelengths of light.

With the composite material, "you could change the architecture of your optical chip simply by applying an electric field," John says. "You could rewire the circuit at will on a millisecond time scale." The researchers describe their analysis in the Aug. 2 PHYSICAL REVIEW LETTERS.

"It's interesting that you can get enough tunability in the liquid crystal to get the job done," says Eli Yablonovitch of the University of California, Los Angeles. Researchers have always thought that liquid crystals could not change the material's index of refraction enough to close the band gap, he explains. "What [Busch and John] noticed is that you don't need that big a change, so that's quite significant," he says.

Yablonovitch, however, is reserving final judgment until the researchers fabricate the composite material. John says that his team hopes to have a sample constructed by the end of the year.

They have already made a simplified version, which has a forest of hollow cylinders etched through a silicon wafer and coated with liquid crystal molecules. These structures exclude only light propagating perpendicular to the cylinders, while the Swiss cheese photonic crystal can block light coming from any direction, John says.