Making Holographic Data Rock Hard

Storing data as a holograph can pack massive amounts of information into a tiny light-sensitive crystal, but so far there has been no simple way to retrieve the data without destroying it. Now researchers report in this week's Nature that they have found a one-two laser punch that annihilates this problem, opening the door to commercial holographic data storage devices.

The information in a holographic crystal is recorded by an interference pattern--regions of varying brightness and darkness--created by two laser beams of the same wavelength but varying in phase. Since some crystals, like lithium niobate, shuffle around their electrons in response to different intensities of light, an interference pattern can be branded onto a crystal. To read the information, the crystal must be illuminated with laser light of the same wavelength, which scatters off the electrons and interferes with itself to recreate the original interference pattern. But that moves the electrons, destroying the data.

Now, a trio of Caltech researchers--Karsten Buse, Ali Adibi, and Dimitri Psaltis--have figured out a nondestructive way to extract the information. First they doped a lithium niobate crystal with traces of iron and manganese, which created traps for the electrons by adding a new set of energy levels. Next they illuminated the crystal with ultraviolet light, together with red laser light--the red light containing the information to be stored. The ultraviolet light pushes the electrons into the iron trap--then the red light has enough energy to transfer them into the stronger manganese traps. The pattern of electrons creates a unique electric field that is jam-packed with data. To read out the information, only the red light is used, which doesn't have enough kick to dislodge the electrons, keeping the data intact. When light comes in, it responds to the electric field pattern inside the crystal, so as it comes out, it is changed to match the light that created that particular electric field pattern.

The work is a big step toward a practical holographic data storage device, says Hans Coufal, who works on such systems at the IBM Almaden Research Center in San Jose, California. "But it is by no means the last one," he says. Researchers still need to find a recording medium that is strong and durable and works over a wide temperature range, for instance. And, he says, they need to produce the devices at prices that are comparable to what's already available.