Stanford makes ultrafast modulator

Oct. 28, 2005
October 28, 2005, Stanford, CA--The dream of all-optical computing using a silicon platform is now much closer to reality, thanks to new research at Stanford University being announced in the Oct. 27 issue of the journal Nature. The researchers have invented a tiny modulator made of silicon and germanium--a solid-state shutter--that can turn a beam of light into a stream of digital data by selectively absorbing the beam (a zero) or allowing it to continue on (a one).

October 28, 2005, Stanford, CA--The dream of all-optical computing using a silicon platform is now much closer to reality, thanks to new research at Stanford University being announced in the Oct. 27 issue of the journal Nature. The researchers have invented a tiny modulator made of silicon and germanium--a solid-state shutter--that can turn a beam of light into a stream of digital data by selectively absorbing the beam (a zero) or allowing it to continue on (a one).

The researchers estimate that the modulator, which could be about a millionth of a meter tall and about as long, could be made to operate at rates greater than 100 billion times a second, which is 50 times faster than the rate employed in computing hardware today and as fast as the highest rates being considered for optical communications.

The secret to the discovery was making the Stark effect work in materials compatible with chip manufacturing.

The Stark effect allows materials to act as shutters for particular wavelengths of light, absorbing one or another as engineers turn an electric field on or off. With atoms themselves, the fields required to produce the Stark effect are so large that they would require a voltage too high to use in chips. But in very thin layers of some materials, a strong and sensitive version of this process, known as the quantum-confined Stark effect, occurs at acceptable voltages. Much of today's high-end telecommunications equipment uses thin materials featuring this effect to transmit data along fiberoptic cables.

Silicon and germanium both belong to a group of materials where the electrons do not appear favorably arranged for the Stark effect. What Miller, Harris and their group discovered is that this commonly accepted unfavorable appearance in germanium was deceiving. In fact, energy levels in germanium that are essentially immune to this Stark effect were obscuring more promising energy levels. What they found is that when germanium layers are properly situated in a crystal with silicon, their electrons do not "leak" from useful levels into useless ones. The Stark effect could indeed work in germanium.

A key next step for the team is to show that they can make modulators for standard telecommunications wavelengths. They are confident that they can, and that their discovery can help usher in an "enlightened" age of computing and communications.

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