Memory cell: Change of the light, delayed

It's hard to store a pulse of light. The clumsy techniques available today include sending a light signal along a coiled kilometer of optical fiber. A compact optical memory chip would make telecommunications networks more efficient and optical computers more feasible, information technology experts say.

 German researchers report this week creating an optical-memory prototype that combines small size, speedy operation, and controllable release of signals. This sandwich of semiconductors stores light by transforming it into pairs of positive and negative charges and then stepping in like a referee at a fight to hold the opposite charges apart.

 The charges accumulate as light signals to be stored dislodge electrons from atoms in a thin intermediate semiconductor layer, known as a quantum well. The layer's properties enable it to confine charges.

 Each photon freeing an electron from the well's crystal structure also creates an electron vacancy, known as a hole, which can behave as a mobile positive charge. Voltages applied to electrodes steer the electrons and holes into separate spots in the well and hold them for potentially useful periods of up to tens of microseconds. An earlier version used sound waves to separate the charges. When the voltage is shut off, the electrons and holes combine, releasing a flash of light.

 Stefan Zimmermann of the University of Munich and his colleagues there and at the Munich Technical University in Garching describe their prototype device, which stores a single pixel of light, in the Feb. 26 Science.

 To improve the device's characteristics, the researchers say they are changing the materials from which it is made so that it can work at room temperature instead of the frigid 100 kelvins necessary now. They also anticipate being able to shrink it dramatically.

"We never thought it would work," says Jorg P. Kotthaus of the University of Munich. "We made it rather large to get a lot of signal out." Rather than its present 200 micrometers on a side, the circuitry to store one pixel could shrink to less than 2 mm on a side, he predicts.

 Storage times of many microseconds represent a valuable step, says Claude Weisbuch of the Ecole Polytechnique in Palaiseau, France. However, he suspects that "it will be tricky to make it work at room temperature" because more energetic electrons and holes will tend to leak past the voltage barriers.