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Electronic Imaging & Signal Processing

Multilayered memories

Eye on Technology - optical data storage

From oemagazine September 2002
31 September 2002, SPIE Newsroom. DOI: 10.1117/2.5200209.0002

An Osaka University (Osaka, Japan) research team comprising Takuo Tanaka and Sadahiko Yamamoto succeeded in recording and reading bit data on multiple layers inside a thick recording medium. "Recently, we've seen tremendous progress in increasing the storage capacity of CDs and DVDs," says Tanaka, "but the recording density of these optical systems is limited by the laser spot size, which is about half the wavelength. So we can see that recording density of these systems has about hit the upper limit." For further increases, engineers will have to consider multiple layer storage.

Indeed, Tanaka indicates that a number of researchers around the world have turned to 3-D multilayered optical memory recording as a possible way to overcome the limitations of optical disk memories. he and Yamamoto set out to develop a 3-D multilayered optical memory. First, they needed an optical pickup able to read the data from a particular layer without crosstalk from adjacent layers. They turned to the work of such researchers as Osamu Nakamura of Osaka University and Colin Sheppard of the University of Sydney (Sydney, Australia), whose work showed Tanaka and Yamamoto that confocal laser-scanning fluorescent microscopy has true 3-D resolving power and could be used for the pickup in 3-D optical storage.

"When we decided on the pickup system, that automatically set the kind of memory we had to shoot for," says Tanaka. "The only way we could use microscopy as pickup was if the medium recorded bit data as fluorescent patterns in three dimensions."

Tanaka and Yamamoto developed a recording medium from a solution of methyl-methacrylate monomer (MMA), poly-methyl-methacrylate (PMMA), rhodamine-B, and tetrachloroauric acid (HAuCl4). The MMA acted as a solvent and the PMMA as a matrix. The rhodamine-B is a fluorescent dye, and the HAuCl4 supplied Au(III) ions. "By vaporizing the MMA solvent, we were able to make a rhodamine-B-doped, Au(III)-doped solid PMMA block," Tanaka says (see figure).

Ordinarily, rhodamine-B molecules fluoresce. But when they are next to Au(III) ions, the energy of the optically excited rhodamine-B molecule is transferred to the Au(III) ions, which quenches the rhodamine-B fluorescence. If Au(III) ions are irradiated with violet or UV light, they are photochemically changed into Au particles. Turning an Au(III) ion into an Au particle unquenches the adjacent rhodamine-B molecule, and it once again becomes fluorescent. "Using this mechanism, we recorded bit data as a fluorescent pattern inside our medium. We used an objective lens to focus the violet laser inside the medium, and rhodamine-B molecules fluoresced only at the photoreduced points," Tanaka says. The system can only be used to record data, as optically reduced Au particles do not return to Au(III) ions.

By co-doping PMMA with rhodamine-B and Au(III), researchers have developed a medium (top) in which the Au(III) ions quench the rhodamine-B fluorescence until the Au(III) ions are converted into elemental Au by violet or UV light. At this point, the quenching halts, and the rhodamine-B molecules fluoresce. (Osaka University).

The team recorded the data with a helium cadmium laser (Λ=441.6 nm, 10 mW). They collimated the beam with a 5X beam expander and focused it with a 100X, oil-immersion lens with a numerical aperture of 1.25. A computer-controlled mechanical shutter modulated the intensity of the beam. The researchers placed the medium on a computer-controlled, motor-driven x-y-z stage and scanned it three-dimensionally to a precision of 0.5 µm, using an exposure time of 8 ms per data point for a 160 µW incident beam. "We recorded five layers inside the recording medium we developed, distancing the layers 10 mm apart," says Tanaka. "In each layer, the bit data were written every 5 mm x 5 mm."

After writing the data, Tanaka and Yamamoto read the data with a laser-scanning confocal fluorescent microscope, proving that each recorded dot had sufficient lateral separation. "Our experiment proved that bit data can be recorded three-dimensionally and that it can be read separately with a confocal fluorescent microscope pick-up," Tanaka says.

"We think Tanaka's work is extremely interesting," says Shigeru Tamura, Chief Information Officer for Mitsubishi Kagaku Media (Tokyo, Japan), a major producer of MO, CD, and DVD disks. "The technology seems complex; so many hurdles remain before it can be commercialized. Nevertheless, we will watch Tanaka's work with interest, as high-density recording media is our core business."