Friday, February 08, 2008

Large-Scale Rewritable Holograms

A new material allows researchers to write and erase 3-D images for displays.

By Kevin Bullis

Holographic car: A new material could make possible very large holographic displays that can be erased and rewritten. Eventually, the displays could be as big as cars. The car pictured here is on a prototype display that's 10 centimeters on a side. Credit: Savas Tay, University of Arizona

Multimedia •  See a video of a hologram. •  Listen to a description of the technology.

A holographic display based on a new material can be repeatedly written to and erased. Rewritable holograms have been possible at small sizes, such as for holographic memory devices. But it's been difficult to make these materials at a scale large enough for displays. The new material, developed by researchers at the University of Arizona and at Nitto Denko Technical Corporation, in Oceanside, CA, could eventually allow for life-sized displays of people and objects the size of cars that could be refreshed every few minutes.

Existing high-end holographic images can be full-color and extremely detailed, but they've been restricted to still images that can't be rewritten. Stereoscopic displays, in which a different two-dimensional image is shown to each eye, are the basis of 3-D movies, but they lack some of the realism of holograms. The new display can produce holographic images, which are easier to view than stereoscopic images and can be of higher quality. But the display is better than typical holographic images in that it can be updated.

The University of Arizona researchers developed a new polymer-based material that encodes information using electric fields. The material contains two components. When light strikes the film, one of these components, a polymer, absorbs photons and generates electrons and their positive counterparts, called holes. The polymer is also a good conductor of holes, but not of electrons. As a result, the holes can easily move away from the illuminated areas where they were generated, whereas the electrons stay put. This separation of charges creates patterns of tiny electric fields within the material. These electric fields change the way that light moves through the different parts of the film.

The second component of the material, a dye, responds to the electric fields in two ways. The dye molecules change their polarization and physically rotate depending on the nature of the fields in each part of the film. These changes locally affect the index of refraction, which has to do with how a material bends and reflects light. When the researchers shine a laser through the film, the dye alters the path of the light, projecting a pattern that the eye interprets as a three-dimensional image. "It comes out of thin air--you feel like you could touch it," says Nasser Peyghambarian, a professor of materials science and engineering at the University of Arizona, who led the work.

To erase the image, the researchers expose the film to uniform light, which redistributes the electrons and holes, removing the electric fields and the changes in the material that they had produced.