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Germanium laser fires up

Optical networking moves up a gear
Fri Feb 05 2010, 09:56

BOFFINS AT MIT have built the first laser built from germanium that can produce wavelengths of light useful for optical networks and chip design.

It's also the first germanium laser to operate at room temperature, which is handy as that is the temperature most networks have to operate in.

Germanium is a bit of a holy grail as it is easy to incorporate into existing processes for manufacturing silicon chips. Easily fabbed lasers could mean that computers conceivably could be powered by light rather than electricity.

All MIT is saying about its breakthough for now is that it has proved that a class of materials called indirect-band-gap semiconductors can yield practical lasers.

Materials used in today's lasers, such as gallium arsenide, are "all tough fits", according to the researchers, that have to be constructed separately and then grafted onto the chips, which is more expensive and time-consuming than building them directly on silicon would be. Moreover, gallium arsenide is much more expensive than silicon.

Integrating germanium into the manufacturing process, however, is something that almost all major chip manufacturers have already begun to do, since the addition of germanium increases the speed of silicon chips.

In a forthcoming paper in the journal Optics Letters, Jifeng Liu, the lead author of the paper, and grad students Xiaochen Sun and Rodolfo Camacho-Aguilera describe how they spent their evenings coaxing excited germanium electrons into the higher-energy, photon-emitting state.

They said they used doping, in which atoms of some other element are added to a semiconductor crystal. The group doped germanium with phosphorous, which has five outer electrons. Germanium has only four outer electrons, "so each phosphorous gives us an extra electron," they said. The extra electron fills up the lower-energy state in the conduction band, causing excited electrons to, effectively, spill over into the higher-energy, photon-emitting state.

They lowered the energy difference between the two conduction-band states so that the excited electrons would be more likely to spill over into the photon-emitting state.

Then the whole lot went like the clappers. µ


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