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Right on (Infra) Red

Photonics: Breakthrough in optical telecommunications

Can light turn sharp corners? Pierre Villeneuve and John Joannopoulos say yes. After nearly a decade of theoretical calculations, the two MIT physicists believe they-and collaborators at Sandia National Laboratories in New Mexico-have made light efficiently navigate a 90-degree bend in an optical material called a photonic crystal. This path-breaking research could lay the groundwork for an entire new generation of telecommunications devices, as well as integrated optical circuits.

Generally, light doesn’t like being tamed. Unlike electrons that can be easily routed (a fact that makes the world of electronic devices possible), photons are difficult to direct. It’s one thing to shoot light out in a straight line, but try and bend it around a sharp corner and light scatters wildly. As a result, existing photonics devices used to bend light at a 90-degree angle are bulky and inefficient.

The nature of light has set some strict limits on current optical devices. Light traveling through optical fibers has revolutionized telecommunications, but you still need electronics at each end in the twisty little circuits that process the signals. To make lightwaves do the twists and turns that would be required in optical circuits-and eventually in the optical computers envisioned by physicists-you need precise control.

Sometime within the next several months, researchers at Sandia, working with the pair of MIT theoretical physicists, expect to finish building and testing a tiny photonic crystal that will allow them to bend and twist light at the wavelengths used in telecommunications and many optical devices. It could give scientists control over light to an extent never before possible and could be the building block of everything from microlasers to optical integrated circuits. It would mean “that we can bend light like a roller coaster. The devices that then could be realized are up to your imagination,” says Shawn Lin, the Sandia electric engineer who is building the crystal.

“What’s exciting to me, and what has been right from the beginning,” says Joannopoulos, Francis Wright Davis professor of physics at MIT, “is that I saw photonic crystals as providing a new mechanism to control and manipulate light.” Photonic crystals are opaque to certain wavelengths of light. But photons can travel along defects in the crystal. By creating a linear defect in the crystal, Joannopoulos reasoned, it would be possible to route the light along that path.

That’s the theory. And, in fact, it works. Building such a device, however, is not easy. Last summer, Lin and his Sandia co-workers took a step forward by fabricating a 3-D photonic crystal that works with infrared light at wavelengths of 10 micrometers-it was the smallest photonic crystal ever made but still too large to work with the 1.5-micrometer wavelengths of light used in telecommunications.

Then, last fall, Lin and the MIT scientists collaborated to bend microwaves (which have wavelengths on the centimeter scale) at a 90-degree angle with nearly 100 percent efficiency. The results, published in Science, involved wavelengths much larger than the targeted 1.5 micrometers but proved Joannopoulos’ theory that photonic crystals could be used to guide light around corners.

Now it’s a matter of shrinking it all down to make a device that can bend light at around 1.5 micrometers-the really interesting commercial wavelengths. Lin points out that such a photonic crystal must have features at the scale of 100 nanometers (billionths of a meter) and must control the light in three dimensions. “It’s very difficult but it can be done. We’re on the verge of a breakthrough,” says Lin.

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