Multiplying computer memories into 3-D.

Multiplying computer memories into 3-D

A flat, optical memory chip the size of a postage stamp could theoretically store 350 million bits of information, or more than seven years' worth of SCIENCE NEWS. Two chemists now propose a strategy for making a three-dimensional, sugar-cube-sized memory box that might store more than 6 trillion bits, or about 130,000 years' worth of the magazine.

Squeezing voluminous memory into tiny packages will be crucial for keeping future generations of supercomputers small and cheap enough for routine use, suggest Peter M. Rentzepis and Dimitri A. Parthenopoulos of the University of California, Irvine, in the Aug. 25 SCIENCE. They propose embedding so-called photochromic chemicals, which change color when briefly illuminated, inside transparent plastic cubes and using lasers to record data as tiny colored and uncolored dots in the cube, creating a binary code. Conventional memory devices store such codes electronically, magnetically or as physical bumps or grooves.

"We have shown that the idea works," says Rentzepis. The scientists have made their first experimental memory cubes and recorded a few bits of data, which appear as tiny blue dots. "We are far, far away from being able to write" trillions of data bits, Rentzepis stresses.

"Three-dimensional memory devices are the Holy Grail of information storage," says computer scientist John P. Riganati of Washington, D.C., who serves on the supercomputer committee of the Institute of electrical and Electronics Engineers. The highest-capacity experimental electronic memory chips, which are flat, store 16 million bits of data, he says.

To make the rudimentary memory cube and an easier-to-study strip of film, Rentzepis and Parthenopoulos embed photochromic molecules in a clear, rigid polymeric matrix. The chemists "write" data onto specific spots of the cube or film in as little as 20 trillionths of a second by beaming two perpendicularly arranged lasers into the material. To change into their blue, or written, form, the embedded molecules must simultaneously absorb a photon from each laser, a feat possible only at the tiny spot -- about 1 cubic micron -- where the beams intersect. "Reading" data from a spot involves a similar process but requires sensors to detect light emitted from the colored molecules several billionths of a second after stimulation by lasers tuned to longer wavelengths.

Rentzepis acknowledges that usable, three-dimensional, optical memory devices capable of storing trillions of data bits remain barely more than an idea at present. But he thinks the potential payoff makes the efforts worthwhile. At least a half-dozen other research groups are working on related projects, Riganati adds. "These devices are not available because it's a damn hard problem," Riganati says, "not because serious people haven't put the effort into them."