Silicon microchips, the thumbnail-sized microprocessors that constitute the brains of a PC, are heading for a disaster created by their own remarkable success. As chips get faster, the electrons that carry messages through the tiny metal wires within the integrated circuit are having a hard time keeping up.

One place where this looming problem is particularly acute is in the ultrafast clocks used to pace computation. Roughly speaking, faster clocks mean faster computing; microprocessors now run at clock rates over one gigahertz (a billion pulses per second) and are getting faster all the time. Soon, says Lionel Kimerling, director of MIT's Microphotonics Center, electrons moving through metal wires will simply be too slow to keep pace. "Assume that somewhere in the future is a 10-gigahertz clock. It's impossible to distribute that kind of signal electrically," he explains. The solution, says Kimerling, is tiny pulsed lasers that can distribute the clock signals through the processor chip. "Intel thinks that three gigahertz is a big problem," says Kimerling, "and that is about two years away."

Over a dozen research groups are racing to develop miniature optical devices capable of being integrated right into the silicon chip. It would be a kind of optical network to ferry data around the microprocessor, boosting its capabilities in the same way fiber optics have transformed telecommunications. But there's a problem: silicon is a lousy light emitter.

Silicon's curse is that it is, in the jargon of physicists, an "indirect-bandgap" material. Other semiconductor materials are good light emitters because when their electrons are kicked up to a higher energy by a current, the electrons can drop right down again and fire off a photon in the process. Pump a lot of electrons rapidly into a higher energy state, and you can make a laser. This is how the semiconductor laser used in a DVD player works, for example. But the laws of physics say that the electrons in silicon cannot travel directly back to a lower state. As a result, the electron usually gives up its energy as heat rather than as light.