A team at Boston College in Chestnut Hill, Mass., and Massachusetts Institute of Technology in Cambridge has discovered a class of materials that may offer an efficient and robust method of ultradense data storage based on multiphoton absorption. The technique may boost the storage capacity of a single disc to almost 20 times that of current DVDs.Using multiphoton absorption and fluorescent materials, researchers have developed a means to increase optical data storage capacity to 20 times that of a standard DVD. Courtesy of John T. Fourkas. The multiphoton absorption of laser light acts as a catalyst for photochemical and -physical processes that modify the optical properties of a suitable medium. The changes in these properties can be localized within the focal volume, the 3-D position of which can be maintained and controlled precisely. A number of materials and processes for multiphoton-absorption-based data storage have been investigated, but those that employ low laser powers often are associated with data degradation. This can be avoided by the use of amplified laser systems, but this increases the expense while decreasing the maximum rate of data storage."We were doing experiments to study single-molecule dynamics in deeply supercooled liquids using two-photon excitation," John T. Fourkas of Boston College's Eugene F. Merkert Chemistry Center explained. "One of the liquids we were using, cresolphthalein dimethyl ether, became fluorescent upon repeated exposure to the laser beam, so we had to abandon using this material for those studies. However, we later realized that the development of localized fluorescence at the laser focus could be useful for optical memory applications, and so we began to study this phenomenon and quickly realized that spots that we stored were highly robust to readout."Further investigation led to the discovery of a class of inexpensive materials that share a chemical structure with cresolphthalein dimethyl ether and that are easily processed into molecular glasses and cross-linked polymers. Because these materials fluoresce on multiphoton absorption, they lend themselves to high-density data storage. Currently, they are subjected to pulses of 800-nm radiation from a homebuilt Ti:sapphire laser pumped by 532-nm light from a Coherent Verdi solid-state laser.There are a number of possible uses of the approach, but also limitations to be overcome before it can be brought to market. "The likeliest early applications would be ones that don't require large numbers of copies of any data," Fourkas said. "For instance, this technology could be used to store many movies on a single disc for a video-on-demand system."Currently, data must be written into the medium one bit, or perhaps a few bits, at a time, resulting in a relatively slow storage rate. Moreover, compact and relatively inexpensive ultrafast lasers must be developed for the readout systems. Nonetheless, Fourkas said that the technology might be available commercially within five years.