Quote:

    * 18:00 06 February 2008
    * NewScientist.com news service
    * Paul Marks

A material that can create rewritable holograms could bring 3D displays to the home, or provide dramatically high-capacity computer memory, US researchers say.

A layer of the material can record a holographic image, erase it, and replace it with another in a few minutes. While technological challenges remain, the researchers are confident they can advance the technology to refresh pictures at video frame rates of around 30 times a second.

The same material could store "pages" of rewritable data in layers through the depth of a hologram, they say. Today's emerging holographic disks are read-only.
Seeing the light

Seeing the light

Holographic images work by recreating the pattern of light that would be reflected by 3D objects. The viewer's eyes receive the same light they would if standing in front of the real scene, even as they move around in front of the hologram.

The highest resolution holograms are static, and are recorded by using a laser to make irreversible chemical changes in a material called a photopolymer. After that, the material interferes with light shone into it to create the light patterns of the holographic image.

Prototype holographic video displays have been made using liquid crystal filters like those in LCDs to scatter light instead. But they typically project cut-down holograms towards the viewer rather than creating one hologram across the whole panel. That means you cannot walk around the image, or have many people viewing at once as with static holograms.

The liquid crystal devices also have to be updated at very high speeds to show any image at all for a useful length of time, says optical engineer Nasser Peyghambarian of the University of Arizona, Tucson, US.

Persistent change

Peyghambarian and colleague Savas Tay have developed a holographic material that is rewritable and holds images for hours.

They modified a plastic used in optical communications systems that can have the way it bends, or refracts, light changed using laser beams. The researchers chemically tuned it to respond more to the lasers, and to reproduce colours better.

When two "writing" beams meet inside the material a build up of electric charge changes the refractive index in that spot. The effect can be reversed using a burst of uniform laser illumination.

The Arizona team built a rewritable hologram on a 10 centimetre square film of the material less than a millimetre thick.

"Our 3D display can be recorded in 3 minutes and can be viewed for up to 3 hours without fading," Peyghambarian told New Scientist. "Wee have reduced the writing time while maintaining hours of persistence."

Danger, high voltage

The trick to that performance lies in applying a high voltage of 9 kilovolts across the polymer. That supplies a pool of electric charge that magnifies the effect of the writing laser beams.

Once the hologram has been written, the voltage is reduced to four kilovolts – the level found to maintain the image for longest.

With more improvements to the material's optical and electronic properties, Peyghambarian is confident that refresh rates high enough for 3D video displays are possible "within the next few years".

In the meantime, he hopes the technology will prove useful in medical imaging, to instantly screen images of the body that remain viewable for some time.

Data risk

But other experts are sceptical of the Arizona team's approach. The polymers used in the new system are typically 1000 times less sensitive to lasers than those used in static holograms.

"Photorefractive polymers typically exhibit recording sensitivity about 1000 times lower than photopolymers," warns David Waldman, a scientist with DCE Aprilis, a US firm investigating holographic data storage. "They require expensive lasers with high output power."

InPhase Technologies, based in Colorado, US, is this year launching a holographic drive that squeezes 300 gigabytes onto a CD-sized disk. Chief technology officer Kevin Curtis says the Arizona pair's idea of using high voltages near critical data archives is risky.

"Photorefractive polymers could be promising for 3D imaging," he says.

"But nothing that requires kilovolts across thin media has any relevance to data storage at all – ever."