3D, Without the Glasses

3D model of an ethane molecule. Credit: Savas Tay, University of Arizona.

Friday, February 8th, 2008--

A new study may give three-dimensional movie junkies reason to celebrate. Researchers report that they can project a 3D hologram in a four-inch by four-inch piece of light-sensitive polymer.

Static holograms are common--your credit card may have one. But making a hologram that changes has been more difficult to achieve, researchers say. The new photorefractive polymer, described in the journal Nature by Nasser Peyghambarian and his team at University of Arizona in Tucson, allows for a dynamic display. "In our case, it's update-able," he says of the polymer. "You can look at one picture, erase it, and put a different image on the same polymer."

But don't toss your stereoscopic glasses yet, Peyghambarian's photorefractive polymer isn't quite ready for the silver screen. Although the 3D image can be updated, it takes a several minutes to erase an image and rewrite a new one.

If not for movies, Peyghambarian says this technology could be used for military simulations or for medical imaging--imagine a three-dimensional MRI, for example. The advantage over a photograph is that a hologram stores depth information.

Here's how the hologram works: one laser is shined at an object. The light bounces off the object and is directed by a lens into the polymer. A second laser beam--called a reference laser--shines directly at the polymer. The two beams start from the same place--the light beams are in synch originally, but get out of phase when the first beam bounces off the object.

Because the beams are out of synch, they interfere with each other in the polymer--creating a pattern of dark and light spots. (This interference pattern contains the information that codes for the 3D image.) The bright spots prompt certain light-sensitive molecules to get excited and lose their electrons. Other molecules in the polymer are electron-hungry--and grab those electrons. With the transfer of electrons, comes a transfer of charge. "The electron will essentially hop over to the partner and leave behind a positive charge on the original excited molecule," says Joe Perry, a chemist and optical scientist at Georgia Institute of Technology in Atlanta and author of a corresponding paper in Nature this week.

In this material, those charges are mobile--they hop from molecule to like molecule, Perry says. The positive charges end up migrating to the dark spots, while the negative charges huddle in the bright spots. "That leads to a pattern of charges in the material," Perry says. The boundaries between the positively-charged and negatively-charged regions make an electric field.

One effect of an electric field is that it changes the speed that light travels through the material (called the refractive index). The changes in the refractive index, prompted by the interference pattern, code for the image: "By creating an interference pattern that is three-dimensional, you have now saved in that material all the intensity and phase information that codes for the brightness and depth of all the rays that bounced off that object," Perry says.

To see the 3D projection, you shine light from another direction at the polymer and the object appears, Peyghambarian says.

Peyghambarian says he and his team spent over three years tweaking the polymer recipe to achieve a material that is not only update-able, but writes fast and holds the image for hours. The other advantage of this polymer is that it is relatively easy to fabricate at larger scales--differentiating it from crystal displays that have similar properties. To erase the image, get rid of the electrical fields by uniformly illuminating the polymer.

"The mechanism in these materials is rather elaborate. I think that's part of the mystique," says Perry.

--Flora Lichtman

Multimedia

Video: 3D hologram

Sources

Nasser Peyghambarian
Chair of Photonics and Lasers, Optical Sciences Professor of Optical Sciences Professor of Materials Science and Engineering University of Arizona Tucson, AZ