Portland, Ore. — Researchers at IBM Corp. say they have succeeded in slowing the speed of light. A group at the T.J. Watson Research Center (Yorktown Heights, N.Y.) has demonstrated an optical silicon chip that can electrically alter the effective index of refraction of an integrated photonic-crystal waveguide — the world's first optical chip to electrically control the speed of light.

The experimental component could one day enable tunable optical delay-line chips, optical buffers, high-extinction optical switches and highly efficient wavelength converters, IBM said. Together with other optical components, such devices could also eliminate the telecommunications industry's reliance on bulky and costly optical-to-electrical and electrical-to-optical converters.

"What makes optical communications expensive is at the end of the line, where you have racks of equipment to convert the optical signals to electrical," said Tom Theis, director of physical sciences at IBM's research center. "The components have been miniaturized to some extent, so what used to take several racks of equipment now takes up a tiny fraction of a rack." But the lab's breakthrough, he said, opens the possibility of reducing these components to truly microscopic size. Some portions of the test chip "you can't see without an electron microscope," Theis said.

"Optical communications has been encroaching on electronic communications for decades now," he said. "Once you have the components down to that size, you can make hundreds of thousands of them on a single chip." Doing so would put optical chips on the same cost curve as electronic chips, he said.

Just as sound waves travel more slowly under water, light is less speedy in denser media. The refractive index indicates how much a material slows light compared with its behavior in a vacuum. Researchers have previously demonstrated exotic or supercooled materials that can be controlled to crank down the speed of light, but IBM did the job in silicon.

The optical chip can variably slow light by a factor of 300x, under active control by a low-power (under 2-milliwatt), fast-changing (less than 100-nanosecond) electrical signal. It was constructed using normal silicon-on-insulator CMOS fabrication techniques, the team said.

The active element is a 250-micron-long photonic-crystal waveguide formed with a nanoscale version of micromachining that perforated a 223-nm-thick membrane with holes 109 nm in diameter, spaced at a 437-nm pitch. As a result, the waveguide slows light passing through it in a 20-nm bandwidth at the communications wavelength of 1,620 nm.

"We have been working on this project for over four years, and the idea behind it is to build a photonic tool kit of devices that perform several functions for an all-optical communications system," said project leader Yurii Vlasov, a research staff member at the Watson lab. "This is just one piece of the puzzle — an optical buffer."

To measure the resulting slowdown in the speed of light, the IBM researchers also integrated a 40-micron-long, all-optical Mack Zehnder interferometer. As is common among well-performing interferometers, the resulting device was able to compare the interference fringes, with phase-sensitive accuracy, of delayed and undelayed signal paths either through the photonic-crystal waveguide or bypassing it, respectively.

The size and spacing of the holes in the perforated waveguide account for its high index of refraction, and thus the slower speed of light. But to vary the speed, the researchers installed a microheater beneath the waveguide. By controlling the current to the integrated waveguide heater, the index of refraction — and thus the speed of light — could be changed over a 300 percent range.

The current demonstration chip, according to Vlasov, uses several components that IBM has previously reported, now integrated into a higher-level device. For the future, the team plans to continue adding new components to the photonics tool kit and, as the components become available, to integrate them into optical chips of ever-higher levels of integration. Eventually, IBM wants to replace computer system backplanes, wires between chips and even on-chip communications with electrically controlled all-optical components.

"We think that optics is coming inside the computer box," said Theis. "We can see a day when communications among the various boards in a system and, eventually, between the chips in a system, use optics. And maybe someday, in the distant future, even to take a signal from one side of a chip to another — we could imagine using optics."

IBM's work was performed with some funding from the Defense Advanced Research Projects Agency's "slow light" program, aimed at the storage and subsequent retrieval of a light pulse after several hundred milliseconds.