Electrons Surfing on Silicon

Light is a great way to transmit information, but its speedy photons are difficult to slow down when signals must be delayed, for example, to be stored for brief times in optoelectronic circuits. Now a report to be published next week in Physical Review Letters describes a clever solution: translating light pulses into pairs of electric charges that slowly "surf" on a sound wave across a semiconductor chip.

The traditional way to delay an optical signal is to send it racing through loops of optical fiber several kilometers long--a bulky and expensive solution. A team of physicists at the University of Munich and the Technical University of Munich suspected they could do better. They began with a 10-nanometer-thick slice of an indium-gallium-arsenide semiconductor, a material that can translate light into electric charge and vice versa. The team sent an optical signal into one end of the chip by pulsing a laser onto its surface. The laser's photons created excitons: wandering pairs of electrons and the positively charged "holes" from which the electrons have been dislodged. Normally these excitons would recombine, giving off light again, within a nanosecond (a billionth of a second). But a second property of the material allowed the team to delay their reunion.

Indium-gallium-arsenide has piezoelectric properties, meaning that its electrical properties change if the material is stressed; inversely, the material extends or shrinks when an electric field is applied. By attaching a series of strips to the edge of the semiconductor to push and pull, the group set up compressional waves--sound waves--that swept across the chip. Along the way, the sound altered the electric field in the semiconductor, creating electric-field waves that trapped and preserved the excitons. "We can extend the lifetime of the excitons several orders of magnitude," says Achim Wixforth of the University of Munich.

The excitons survive until the migrating electrical fields drag them all the way across the chip. At the far end, they merge close to a nickel-chromium layer and emit a flash of light. In the experiment, the team detected a light pulse from recombining excitons 650 nanoseconds after they were created by a laser pulse. That may not sound like much, but an equivalent fiber-optic delay would require about 3 kilometers of cable, says Wixforth.

The device isn't ready for commercial use just yet. At present, the chip must be cooled to within a few degrees of absolute zero, but team member Carsten Rocke of the University of Munich says less chilly chips are on their way. After that hurdle is cleared, the team predicts that the chips could become an integral part of optical systems. Other physicists agree: "Whatever you can do with a delay line, you can do with this, too," says David Snoke of the University of Pittsburgh.