Coming Attractions: Holographic Movies
Moving 3-D displays may no longer be science fiction, thanks to researchers from the University of Arizona in Tucson and from Nitto Denko Technical Corp. of Oceanside, Calif. They recently demonstrated an updatable holographic 3-D display based on a specially formulated photorefractive polymer. The technology could be used in medical, military and entertainment applications.
Holography captures 3-D images by recording the interaction of light beams, with the image retrieved by illuminating the recording material. Photorefractive polymers can store holographic information in a 3-D refractive index pattern. Such polymers have been around for decades but have lacked the blend of rapid recording, quick erasing and long image persistence required for an updatable holographic 3-D display.
A photorefractive polymer that can record or erase (top row) a hologram but that can store images for hours is the key to an updatable holographic display. The images are three-dimensional, as seen in pictures of the brain (middle row) and of a car (bottom row). Courtesy of Savaüs Tay, University of Arizona.
Team member Savaüs Tay, a postdoctoral researcher in Nasser Peyghambarian’s group at the University of Arizona, noted that other changeable holographic technologies have limited display sizes and resolutions. These restrictions don’t apply to the new technique, partly because the recording fades slowly, he said. “Our photorefractive holograms store 3-D images as long as three hours, once recorded.”
More importantly, he added, photorefractive polymers are inexpensive and can be processed to very large sizes with plastic casting and molding techniques.
The researchers developed a special photorefractive polymer composite, consisting of a copolymer that absorbs at 532 nm. They added a fluorinated dicyanostyrene chromophore to achieve the required nonlinear optical response.
They took the copolymer route, in part, to minimize the phase separation between functional components often seen in homopolymer composites. They assessed phase separation in 100-μm-thick thin-film devices constructed with their composite by performing accelerated-aging tests at elevated temperatures. The devices showed no phase separation, indicating that the problem had been solved.
The researchers found that the polymer had good image persistence but recorded too slowly to be very useful. The recording time was a function of the voltage applied while writing, with higher voltage translating to faster recording. However, a higher voltage degraded the film. So the researchers applied a high voltage — up to 10 kV across a 16 × 16-in. area — during recording but decreased it to 4 kV the rest of the time.
To demonstrate image recording and display with the polymer and the voltage kickoff technique, they constructed 4 × 4-in. devices out of their polymer and extracted holographic element data from 3-D images of an object. They used the data to drive a spatial light modulator from Holoeye Corp. of Lake Forest, Calif. They sent a 532-nm object beam from a Coherent laser through the modulator onto the photorefractive device, which was mounted on a stage that moved so that different elements could be written. After the recording, they illuminated the device with a 633-nm reading beam, and a 3-D image of the original object appeared.
The recording took a few minutes, and erasing took about the same time. The images persisted for hours, once recorded.
According to Tay, a commercial product based on the technology could appear within the next few years. The researchers are working to make displays that measure a foot or more on a side, in full color and with full parallax, an improvement over the horizontal-only parallax in the demonstration.
Nature, Feb. 7, 2008, pp. 694-698.
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