A LIGHT-ACTIVATED OPTICAL SWITCH under development at the Georgia Institute of Technology could be the basis for a new type of rewritable three-dimensional data storage system. By utilizing a small number of "trigger molecules" to induce a phase transition in liquid crystal materials, the system would write, read and erase information using different forms of polarized and unpolarized light.

Such an optical storage system would offer significant advantages over conventional computer floppy disks, magnetic tape and compact disks, which use two-dimensional media to store data. The optical switch materials could also be used in spatial light modulators, and in active coatings for optical fibers.

A photomicrograph of liquid crystals suspended in glycerol helps provide information about the properties of the crystals.

"The idea is that you would write to the liquid crystal with circularly-polarized light, read it with linearly-polarized light, and erase it with unpolarized light," explains Dr. Gary B. Schuster, professor of chemistry at Georgia Tech. "You can read, write and erase information in liquid crystal materials using this system."

Information about the new optical switch was presented March 25 at the 211th national meeting of the American Chemical Society in New Orleans. The research is sponsored by the National Science Foundation.

Operation of the new optical switch is based on "chiral" molecules that Schuster and his co-workers use to trigger changes in the liquid crystal. Chiral molecules exist in right- handed and left-handed forms. Each form is affected differently by circularly-polarized light -- which also exists in right-handed and left-handed versions.

When right-handed trigger molecules are struck by left-handed light, for example, they may be converted preferentially to left-handed molecules. If the chiral molecules are dissolved in a liquid crystal material, this structural change can be used to prompt a phase transition in the crystal.

The phase transition alters the optical properties of the liquid crystal material, and one way that change can be detected is by passing linearly-polarized light through the crystals. Because the linearly-polarized light can be at a wavelength that does not affect the chiral trigger molecules, reading the stored information would not alter it.

Multiple phase transition "switches" could therefore be used together to store digital information.

When the information was no longer needed, it would be erased from the liquid crystal by shining unpolarized light through it, reversing the phase changes originally made by the circularly-polarized light. Returning the storage material to its original state would make the system truly rewritable -- and of significant potential value as a computer data storage media, for example.

Schuster believes the system is an improvement over earlier optical switches not only because it is rewritable, but also because a small number of photons can trigger the phase transition. This makes the liquid crystals amplifiers for the photonic signal.

Schuster and graduate student Jennifer Galvin are developing trigger molecules made up of bicyclic ketones substituted with a styrene group. These materials appear to satisfy most criteria needed for a two-dimensional switch and have set a record for the magnitude of their response to circularly-polarized light.

Before the switches become useful for optical data storage, they must be converted from two-dimensional layers to a true three-dimensional system.

"There are a lot of people who have thought about ways of using light for memory applications," Schuster explains. "I think the reality is that it is only going to be useful if you can do it in three dimensions. "Furthermore, that would be useful only if you don't have to write information a bit at a time, which means writing it holographically. If you could write three-dimensional holograms optically and read them, you would really have something worthwhile."

To make a three-dimensional system, the liquid crystals and their dissolved trigger molecules would need to be dispersed in a polymeric material. Microdroplets of the liquid crystal, perhaps less than a micron in size, could then be addressed individually or as groups.

Because the system would be optically-based, multiple beams of light could be used to simultaneously write information to the data storage media without interference. That optical advantage would permit large amounts of information to be packed into a relatively small space, facilitating the further miniaturization of computer systems.

Development of practical optical switches has long been frustrated by the lack of suitable materials. Optical materials studied earlier used irreversible photochemical changes to store information. That meant data could be written to them only once, limiting their practical value for computer information storage.

Though the Georgia Tech system shows promise, Schuster emphasizes that much work remains to be done before it could reach practical application.

"From a scientific point of view, looking into the future, this is what we hope to do with these materials," he says. "Our grandchildren might see the first computers based on this system."

Further information is available from Dr. Gary Schuster, College of Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0365. (Telephone: 404/894-3300) (FAX: 404/894-7466).

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Last updated: 12 Sept. 1996