By J.R. Wilson
CHESTNUT HILL, Mass. — Researchers at Boston College in Chestnut Hill, Mass., have a new approach to 3-D optical storage that could lead to inexpensive memory disks that are the size of DVDs but hold as much as 100 times as much data.
The research, with funding from the U.S. Air Force Office of Scientific Research in Arlington, Va., is the serendipitous result of an unrelated experiment that failed, explains John Fourkas, a professor at the Boston College Eugene F. Merkert Chemistry Center.
Fourkas led the research team, which includes Boston College doctoral candidate Christopher Olson and Michael Previte, now a post-doctoral fellow at MIT.
Most surprising, perhaps, are the two materials Fourkas and his colleagues are using — a simple five-minute epoxy glue found at any hardware store, and a glass-like substance derived from the chemical phenolphthalein, which is common in high school chemistry labs to test for acidity.
"We were exploring these materials for use in a completely different application when they became florescent — started to glow when hit with a laser," Fourkas says.
The glowing material "was completely unacceptable for that project, but we later realized it might be a useful phenomenon for doing optical data storage," Fourkas continues. "So we began testing how well we could store data and read it back out. We realized the first class of materials worked quite well and were trying to understand the chemical transformation the laser caused to make the material florescent. We also looked for other materials containing similar chemical structures and epoxy actually was an obvious material to try."
Scientists found they could mold either material into DVD-size disks and use them as storage media, he adds; the materials do not need to be mixed together to work.
"Practical applications are still a good ways down the road. The materials work nicely, but a lot of work remains to be done to make it commercial," Fourkas adds. "In this scheme, you have to write the data bit by bit, so we have to come up with a method to speed the data storage. Nonetheless, I think there is a lot of potential here.
"It would require a different kind of laser (than current DVD drives), which is relatively expensive, but we are working on using a different, less expensive laser," Fourkas says. "You couldn't use it in a current DVD player, but it would be possible to build a reader with a different kind of laser that could read the current type of DVD as well as the new disk."
Team members first discovered the effect about three years ago. Once they realized its potential, Fourkas approached the Air Force, where officials began funding additional research in early 2001. The military's interest was on developing a super-high-density optical storage medium that is less vulnerable to the environmental impact than are existing magnetic media.
Fourkas admits the researchers still do not understand exactly how the process works.
"We have some decent ideas about what the chemical change that's going on may be and we're doing some experiments to understand that better. We know what the chemical unit is, but don't yet know the exact transformation that happens to it," he says. "We might be able to use that kind of information to design even better materials. As it is, we can still study the physical properties of the transformation without knowing the molecular details.
"With the storage density we have demonstrated so far, we have gotten 25 layers, but believe we can get well over 100. With 25 layers, you have 10 or 20 times the density of a standard DVD; with 100 layers, you could get up to 100 times the storage capacity of a DVD, perhaps even more, if we're clever about it," Fourkas says.
Details of the process are in a paper that team members authored for the December issue of Nature Materials. In that paper, they note, a major obstacle in developing optical data storage has been finding inexpensive, efficient and robust media. The common materials with which they are experimenting become highly fluorescent on multiphoton absorption of pulses of 800-nanometer light from a Ti:sapphire oscillator, making them excellent candidates for storage media.
"The materials are inexpensive, of high optical quality, can be processed readily, and can take a number of useful forms, including molecular glasses and highly crosslinked polymers. Three-dimensional data can be stored at high densities in these materials and are highly robust to readout," they wrote.
"One of the most important goals for any three-dimensional memory technology is to make it possible to retrieve data rapidly using compact and relatively inexpensive equipment (such as non-amplified laser systems and non-immersion optics) and materials; any technique that will also allow for data storage to be achieved under similar constraints will be all the more advantageous," they wrote. "To this end, there has been increasing interest in 3D optical data storage techniques based on multiphoton absorption (MPA). The premise of these technologies is that MPA of laser light can be used to initiate photochemical or photophysical processes that alter the local optical properties of a material."
Fourkas and his team members say the materials they are using meet that challenge and open the way to a new level of low-cost, stable, portable mass data storage.
"Our materials are inexpensive, can be molded to any shape, have high optical quality, and require a minimal amount of processing in preparation for data storage," they wrote. "These materials can also be readily chemically modified, which will be useful in the further optimization of their storage properties. The low intensities required should allow multiple spots to be stored or read simultaneously. Armed with a better understanding of the mechanism of the photoactivation process, it may be possible to use these materials in conjunction with the next generation of compact ultrafast lasers as the basis for relatively inexpensive, high-volume data storage and readout devices."