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Stanford Report, March 6, 2002

Photonic crystals may become the 'transistor of the 21st century', scholar predicts

BY CHRISTIAN HEUSS

Next-generation computers may run at the speed of light thanks to little "glass chunks" full of holes -- photonic crystals. Within these holey structures, photons, the particles of light, will carry and store data like electrons do on classical computer chips. Researchers predict that photonic crystals will invade every bit of electronics from cell phones to supercomputers and make them smaller, faster and more energy efficient.

"Photonic crystals are a revolution in optical technology," says Shanhui Fan, assistant professor of electrical engineering, who presented new photonic crystal designs at the annual meeting of the American Association for the Advancement of Science (AAAS) in Boston in February. "They will make wide ranges of optical devices at least a thousand times smaller."

In less than a decade, photonic crystals will begin augmenting and complementing electronic semiconductor technologies, and possibly later replace them completely, Fan predicts. Whereas the second half of the 20th century was transformed by the electronic transistor, the 21st may be powered by its photonic equivalent.

When data moves on silicon chips, electrons are routed through electronic gates, or transistors. But electrons are charged particles that interact with each other when brought into close contact, spawning excessive heat and limiting their movement. Photonic crystals promise to break this barrier through use of uncharged photons to carry data.

Like the regular pattern of a checkerboard, photonic crystals show a highly organized microstructure. Constructed of semi-transparent materials, such as silicon or glass, photonic crystals can look like a solid box filled with stacked hollow balls. Each ball is about a thousand times narrower than a human hair -- a width matching the wavelength of light. Light shone into the crystal gets trapped by its highly organized microstructure, like somebody getting lost in a mirror maze.

But the magic of photonic crystals really shines as soon as designers like Fan create "defects." By changing the size of a hollow ball or the chemical microstructure of the photonic crystal, they introduce irregularities. The defects act like efficient guides routing light through desired paths. The guided light travels like electrons in a chip, but far faster. By designing specific defects into the crystal, Fan and other researchers can build sophisticated optical elements, such as light switches, filters or even mini-lasers. Such systems are essential components for broadband networking over optical fibers, Fan says.

At AAAS, he presented new photonic crystal designs aimed at filtering data from the Internet. Existing comparable systems are bulky, filling up foot-long racks; plus, they're inefficient. Fan's photonic crystal design will do a better job at a fraction of the size of today's systems, he says.

Today, the field of optoelectronics is in a stage of development similar to where the field of electronics was 40 years ago. In the pre-transistor era, computers filled up whole decks. Likewise, Fan says, "with photonic crystals you get to a point where you can do every single thing on one chip."

Fan holds 12 key patents on photonic crystals. He is a theorist who uses the laws of optics to design and simulate photonic crystals on his computer. Based on his ideas, other researchers in academia, government and industry can create prototypes of photonic chips. Several companies already are working on their first photonic crystal applications. Although photonic crystals are theoretically fully understood, manufacturing perfectly consistent structures at these minuscule scales is still a major obstacle.

"It is still a challenge to build structures that show no roughness that scatters light off," Fan says. It is a question of finding the right materials and efficient processes. "The big thing right now is to get it to work," Fan says.

Shanhui Fan