The Erasable Holographic Display
New three-dimensional holographic material can be written and rewritten indefinitely, paving the way toward 3-D movies
19 February 2008—The iconic image of three-dimensional holography—Princess Leia inserting Death Star blueprints into R2-D2 and intoning, ”Help me, Obi-Wan Kenobi. You’re my only hope”—may be just a few years away from reality, says a researcher who has developed a method to write, erase, and rewrite holographic images.
Holographic motion, as featured in Star Wars, has long been confined to the realm of science fiction. But now, according to Nasser Peyghambarian, a professor of optical sciences at the University of Arizona, ”we can see we are pretty close to that.”
Peyghambarian and his colleagues at Arizona have found a way to create holograms that can persist for hours but can also be erased and written over. The group worked with researchers from Nitto Denko Technical Corp., in Oceanside, Calif., the research arm of a Japanese company that makes semiconductor and optical products.
Conventional holograms are written using a laser beam split into two out-of-phase beams. One beam bounces off the object being imaged before it recombines with the other beam to create an interference pattern. When that pattern strikes the holographic medium—usually a photosensitive polymer—the material undergoes chemical changes that alter its index of refraction. If you shine a light on the finished hologram, the refraction pattern recreates a 3-D image of the original object. But because the chemical change is nonreversible, these standard materials can be written on only once.
The Arizona group created a different type of holographic material, called a photorefractive polymer composite, which they reported in the 7 February 2008 issue of the journal Nature. Part of the composite is a polymer that absorbs light at a particular wavelength—in this case, green light at 532 nanometers. When the interference pattern from the writing laser strikes it, the polymer generates positive and negative charge carriers—electrons and holes. The positive holes have a higher mobility than the electrons, so they tend to move away from the light areas of the image and get trapped in the dark areas. This leaves light areas with a net negative charge and dark areas with a net positive charge—an electrical field that changes the refractive index of the material. To view the image without erasing it, researchers shone a red laser at 632 nm on the material. In the future, Peyghambarian plans to create a full-color display viewable under white light.
The researchers found that the image persisted for up to 3 hours after they turned off the writing beam. But more noteworthy, Peyghambarian says, ”it’s updatable, so you can actually erase it and put a new picture in there.” Whereas a chemically induced change in the index of refraction is permanent, the electrically induced change lasts only as long as the charge carriers stay in place. To erase the image, the researchers merely flood the whole volume of the hologram with the green light so that the charges are spread evenly. Peyghambarian’s group rewrote their material hundreds of times over several months with no degradation.
”I think it’s a significant advance in terms of what people have been able to do with photorefractive materials in particular and holographic materials in general for changeable 3-D displays,” says Joseph Perry, a professor of chemistry at Georgia Tech.
Holograms are attractive for 3-D displays because they’re autostereoscopic. In other words, a viewer doesn’t need special glasses to see them in three dimensions, and they’re viewable from a variety of angles. But Peyghambarian’s work won’t lead directly to 3-D movies without overcoming a few hurdles. For one thing, his hologram measured only 10 by 10 centimeters. He says he wants to increase that to 30 by 30 cm and doesn’t think it will be too difficult to make his polymer composite at that size. But because the film is less than 100 micrometers thick, keeping it uniform over a larger area might prove challenging.
A bigger issue is the writing speed. It took about 2 minutes to write an image in Peyghambarian’s 30- by 30-cm material. For video rates, you’d need a writing speed of 30 to 60 times a second. That should be achievable, he says, by developing materials that are more photosensitive and by increasing the power of the writing laser.
Peyghambarian would also like to make a three-color display instead of the monochromatic display used in the current experiment. That will require finding materials that are sensitive to red, green, and blue. ”I think that is not as challenging as getting to video or movie rates,” he says.
Depending on the size, he says, it might be feasible to develop a commercial 3-D holographic display in three to five years, though the image would probably be refreshed only every few minutes. But that can be useful too, according to Peyghambarian. For instance, a doctor doing brain surgery might have a 3-D image of a brain based on an MRI or CT scan. The surgeon could perform an operation, update the image, and then examine it to see what the next step is. Similarly, military commanders might have an image of a battlefield that is updated every 5 minutes to allow them to monitor a battle. Not surprisingly, the U.S. Air Force Office of Scientific Research is one of the sponsors of Peyghambarian’s work.
As for 3-D movies, holography might face a challenge from the makers of flat-panel displays, who are working on ways to trick the eye into seeing 3-D, says Georgia Tech’s Perry. There are also researchers working on different types of volumetric displays that produce 3-D without holography. There’s been an upswing in research into 3-D movie technology, Perry says: ”Right now, they all have their advantages and disadvantages, and I think it’s a bit early to pick the winner.”
About the Author
Neil Savage writes from Lowell, Mass., about lasers, LEDs, optoelectronics, and other technology. For IEEE Spectrum Online, he most recently wrote about chips that can self-correct timing errors.
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