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Memory Cell Seizes the Light

Photonics Spectra
Apr 1999
Aaron J. Hand

As useful as light is, it's not the easiest thing to keep hold of. Storing light for any substantial amount of time for later use is difficult, and current methods are less than optimal. Solutions such as bouncing the light back and forth in a resonator, running it along a coil of optical fiber or converting it to and from an electronic signal present problems including bulkiness, complexity and a lack of control.
A research collaboration has come up with a photonic memory cell that is relatively small and simple. The cell has stored light energy for as long as 35 µs -- a number that is limited only by the lab's experimental setup, noted Achim Wixforth, a researcher at the University of Munich. He and other scientists there and at the Munich Technical University in Garching expect to reach storage times of more than 100 µs.
As reported in the Feb. 26 issue of Science, the memory cell is based on a field-effect tunable lateral potential modulation in the plane of a semiconductor quantum well. The device stores the light energy by converting it to spatially separated electron-hole pairs, then releases it by switching off the lateral potential modulation, which causes the electrons and holes to approach each other and create a flash of light.
To achieve both large absorption and long storage times, the device combines direct and indirect bandgap semiconductors in a superlattice structure. A direct bandgap GaAs-based quantum well is sandwiched between two short-period AlAs-GaAs superlattice barriers. The researchers have been experimenting with different layer sequences and material combinations, Wixforth said, that could be varied according to the application.
Eventually, an optical chip would consist of many of these memory cells. An optical dynamic random access memory chip, for example, could store a whole "image" -- much like in a charge-coupled device camera, Wixforth noted, except the image could be read in the form of light.
"Now imagine that two of such devices 'talk' to each other, exchanging photons," he said, so that stored information could be compared with actual information. "This could have a large impact for pattern recognition."
The team has more work to do to reach this level of commercial feasibility. Besides realizing longer storage times and a smaller cell size, the device must also achieve room-temperature operation (the current memory cell operates at 100 K). Although the device works at room temperature in principle, Wixforth said, the otherwise sharp photoluminescent lines become spectrally broad.
"However, this does not hamper the basic operation; one simply has to detect the outcoming light over a broader spectral range," he said. "Based on this, we have already built a fast and sensitive 'switchable' detector operating at room temperature."

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