Tuesday, June 24, 2008

Faster Optical Switching

Continued from page 1

By Karen Fox

By electrically controlling excitons, the California researchers can produce the rudiments of a digital logic gate. Here, the same chip causes a stream of excitons to turn left, to turn right, or to branch down two pathways simultaneously. Credit: Butov Lab

The new integrated chip, however, takes in light as is, operates on it as necessary, and spits light out the other side. Whenever a photon hits the chip, it forces a negatively charged electron out of a semiconductor atom, leaving behind a positively charged "hole." Without intervention, the electron and the hole simply recombine. But the UCSD team uses so-called quantum wells to keep the electron and hole separate yet close enough to remain bound into a single unit. This carefully bound "particle" is called an indirect exciton, and it has the odd property that it will move when placed in an electric field even though it is neutrally charged. Nudged by electric fields, the excitons scurry through the chip along a prescribed path until allowed to recombine. Then they release their energy in a flash of light that sends the communication signal off to its next destination.

The prototype chips have to be cooled to temperatures of less than -234 ºC. But the researchers are confident that they'll be able to re-create their delicate quantum wells in semiconductor materials that allow excitons to form at room temperature.

Building the chip such that the electric fields didn't rip apart the excitons was one of the main hurdles for the scientists. "If this field becomes too strong, it can tear apart the electron and hole. Designing the gates that define exciton circuits required new design ideas and extreme care in their implementation," says Leonid Levitov, a scientist at MIT. "I believe this is a fundamental achievement in exciton physics which may also have very practical consequences and lead to applications."

Butov and his colleagues agree. Their colorful pictures of their chips show streams of excitons that can be forced to veer either to the left or right, that can be split to go down both left and right paths, or--in reverse--that can travel in along two paths and combine into one stream. While the team would like to demonstrate additional tricks, such as amplification of a signal, the photos show that their circuits can be designed to perform any logical operation that a traditional electronic chip can do. And, the researchers say, their chip will do it better.