Drilling holes to keep photons in the dark.

Researchers have taken an important step toward fabricating a material that, in effect, excludes photons of certain wavelengths. Such a structure, known as a photonic crystal, would prevent atoms embedded within it from spontaneously absorbing and reemitting light at wavelengths that fall within the excluded range, or band gap.

"What we're trying to do is make a semiconductor for light waves," says Eli Yablonovitch of Bell Communications Research (Bellcore) in Red Bank, N.J., who in 1987 first suggested building photonic crystals. Such materials could one day play key roles in the development of highly efficient lasers and solar cells.

To find a structure with a photonic band gap, Yablonovitch and his co-workers initially focused on arrays of spherical air pockets carved into an electrical insulator. They created such structures by drilling into the surfaces of flat plates, which they stacked and bolted together (see photo). In each case, the spherical air pockets lay in a face-centered cubic arrangement, a pattern resembling the way grocers stack oranges to make an orderly closely packed pile.

This structure looked promising, but we got into trouble," Yablonovitch says. Its band gap was extremely narrow and difficult to pick up experimentally

Subsequent theoretical calculations by K. Ming Leung of Polytechnic University in Brooklyn, N.Y., showed that a slightly different geometry would produce better results (SN: 9/29/90, p.196). Guided by these findings and the theoretical work of Kai-ming Ho and his colleagues at Iowa State University in Ames, the Bellcore team constructed and tested a new face-centered cubic structure in which the air pockets were no longer spherical.

As described in the Oct. 21 PHYSICAL REVIEW LETTERS, fabrication of this novel structure requires drilling three sets of holes, slanted at specific angles, into the top of a solid slab (see diagram). The holes crisscross below the slab's surface to produce an array of distorted, non-spherical cavities.

By studying what happens to microwaves traveling through the array, the researchers confirmed that this structure does prevent a certain range of microwaves from penetrating the material.

This new photonic crystal solves two outstanding problems, the Bellcore team says. it shows that a full, three-dimensional "forbidden" gap can exist in an electrically insulating material and that manufacturing the requisite structure is practical.

Although the researchers worked with microwaves and with structures fashioned by conventional drilling, they see no reason why the same effect shouldn't occur for visible light in an appropriately doctored material. It appears that the application of photonic band gaps to semiconductor, optical and atomic physics may soon be practical," they conclude.