There's been a lot of talk about the convergence of computing and communications in recent years -- a lot more talk than actual convergence. One reason or stumbling block has been that modern communications systems are increasingly optical, with beams of light flowing through fiber-optic tubes, while computing platforms are based on electrons flowing through copper or silicon wires.

Before long, however, new developments may finally combine the two, providing an optical bus for silicon CPUs, or perhaps even using light itself to conduct computations. This is the idea of optical computing or silicon photonics, and it's moving ahead in both academic labs and chipmakers' fabs.

"Microelectronics is now facing the problem that the overall delay in the processor is not related to the gate speed but to the wiring," explains Lorenzo Pavesi, a professor of experimental physics at the University of Trento in the Italian Alps. This is the so-called "interconnect bottleneck." In addition, PC and processor designers are already struggling with the power dissipated by silicon chips in the form of heat -- power increasing at such a rate that, in a few years, CPUs could burn themselves up. Pavesi says, "Photonics will surely play a role in solving this bottleneck."

Light is inherently more efficient than electricity. It is faster and can be multiplexed (a single fiber can carry multiple channels at different frequencies). And as anyone who's put a Pentium 4 notebook on her lap can tell you, electrons moving through metal wires also generate a lot more heat than light moving through fiber optics.

The best kind of light for moving data is a laser. If your PC has a CD or DVD drive, there's a tiny laser in your system already. To fulfill the promise of photonics, however, chipmakers will need to put a laser not just inside your computer, but inside a silicon chip.

Building a Silicon Laser

Lasers are relatively easy to make and quite common in the networking and telecommunications industry these days, but they're far too big and expensive to fit on chips. To make photonics cost-effective on a chip level, manufacturers need to get the silicon itself to lase, or emit a coherent, focused beam of photons that can be switched on and off to transmit digital information. Unfortunately, silicon isn't very well-suited to being a laser: It doesn't emit light nearly as readily as currently popular materials such as gallium arsenide (GaAs).

"Silicon is an indirect-band-gap semiconductor -- that is, light is not generated efficiently [compared to direct-band-gap GaAs]," Pavesi explains. And combining silicon and GaAs or another natural emitter on a single chip is difficult and expensive.

Several different strategies have been applied in the lab to make silicon emit light, including the use of silicon nanocrystals to yield LED-style visible light (pioneered by Pavesi's team in 2000) and erbium-doped silicon --semiconductors doped with rare-earth ions -- for 1.5-nanometer emissions. Pavesi says his and other researchers' work proves that both these approaches are able not only to generate but to amplify light, or show optical gain, from silicon.