Opals On a Chip: Breakthrough in Next-Generation Photonics

Photonic band gap crystals are materials that are three-dimensionally periodic on a length scale comparable to the wavelength of light (about 0.5 micrometers, approximately 1/100th the width of a human hair). When light strikes these structures, it can be reflected in new directions. Thus, properly designed photonic band gap crystals allow light to be manipulated as it travels through the material.

Photonic band gap crystals have potential for the formation of all-optical integrated circuits -- ultracompact "chips" -- that can manipulate photonic signals. Since much of our telephone and internet traffic is transmitted as pulses of light along optical fibers, these photonic circuits are needed to replace the large and expensive devices that now control signals over optical networks. Such circuits have not been realized due to difficulties in making photonic band gap crystals.

While we now know many practical applications for photonic band gap crystals, the real challenge has been to fabricate them," said David Norris, research scientist and leader of the NECI research effort on photonic band gap crystals. "Previous approaches have succeeded in obtaining these materials, but they either have been too expensive or resulted in a material that was impractical for producing useful photonic devices. This is the problem we have tried to address."

Natural assembly approach

The research was conducted by Norris and Yurii Vlasov of the NEC Research Institute, in collaboration with James Sturm and Xiang-Zheng Bo of Princeton University. Their approach begins by emulating a natural process, the formation of gemstone opals. Under certain geological conditions, nature spontaneously forms extremely small silica spheres, like tiny glass marbles. When millions of these micrometer-scale marbles are stacked on top of each other, a natural opal is created. Prior work had shown that not only could such opals be grown in the laboratory, but that they could be used to make photonic band gap crystals. By filling the space between the spheres with a semiconductor and then selectively removing the spheres, an extremely porous material, referred to as an inverted opal, could be obtained. NECI used this "natural assembly" approach to form planar synthetic opals directly on a silicon wafer. They then used common silicon deposition equipment to fill this planar opal with silicon. Removing the opal template then yields silicon inverted opals.

"With our approach, we obtain silicon photonic band gap crystals that are integrated directly onto the wafer," said Vlasov. "Once on the wafer, it is possible to use many of the standard tools of the electronics industry to pattern the material into a photonic device. Therefore, while maintaining the low cost of natural assembly, we have fabricated a photonic band gap crystal in a technology friendly format."

Quality control

The NECI-Princeton team's research also addressed the existence of the photonic band gap in their structures. Researchers have been concerned that natural assembly could lead to too many defects in the structure, which could destroy the photonic band gap.

"To address this issue, we have performed careful optical measurements,"' said Norris. "While further work needs to be done, so far all of the results are consistent with the photonic band gap. This means that by using a very simple chemical approach, these complex, technologically relevant materials can be made."

The next challenge will be to use the materials that have been demonstrated to make an actual device. "We still have a long way to go, but this is an important demonstration that natural assembly has significant potential," said Dr. Norris. "In the near future we hope to push it even further."

The complete findings of the study are published in the current issue of the scientific journal Nature.