For most technophiles, a hologram evokes grainy images in Star Wars of Princess Leia pleading for Ben "Obi-Wan" Kenobi's help. More than three decades later, holograms have evolved little beyond providing authentication features on driver's licenses and credit cards. But that all could change soon with the development of a new photorefractive polymer that paves the way for 3-D images that will one day help doctors study X-rays, engineers design buildings, military officers plan battles—as well as be used in movies, video games and other multidimensional entertainment. No paper or plastic eyewear needed.
Researchers at the University of Arizona's College of Optical Sciences (OSC) in Tucson, and engineers from Nitto Denko Technical Corporation, in Oceanside, Calif., recently unveiled a prototype of a photorefractive polymer film on which 3-D images can be recorded, erased and replaced with new images. When carried out swiftly enough, this process leads to a series of images on the film that deliver three-dimensional action that can be picked up by the naked eye.
Conventional holograms—such as the silver bird emblazoned on credit cards to verify their authenticity—are static and have no memory. But "imagine a hologram that is dynamic, where the image is changed frequently," says Nasser Peyghambarian, chair of photonics and lasers at the OSC.
The University of Arizona photorefractive polymer is significant for several reasons, says Joseph Perry, a professor of chemistry and biochemistry at the Georgia Institute of Technology in Atlanta and associate director for photonics at the school's Center for Organic Photonics and Electronics. First of all, he says, it's "updateable"—images can be written, erased and rewritten onto the polymer in much the same way music and video is burned onto CDs or DVDs. "Equally important," he says, is that the researchers were "able to build the display using very basic materials." Dynamic holography has been possible using lithium niobate crystals, but the process of growing these crystals into large display screens is far more difficult and less practical than creating a polymer film.
The polymer is a complex composite of copolymers (which acts as a photosensitizer and absorbs light), a plasticizer (an additive used in plastics to provide strength and flexibility) and other materials formed into a film and melted between four-inch (100-millimeter) indium tin oxidecoated glass electrodes. Prototypes of the polymer so far offer small—foursquare inch (25square centimeter)—monochrome images of a car, human skull and molecule that can be viewed from different angles in front of the flat display.
Images are recorded onto the polymer using a green laser with 532-nanometer (billionths of a meter) wavelengths, whose light gets absorbed, creating a charge distribution across the material that modifies the film's refractive index and creates a 3-D image that can be viewed when the film is illuminated.
The key to creating what amounts to three-dimensional video using the polymer is the ability to change images in within about 30 milliseconds (thousandths of a second), quickly enough so that the eye doesn't notice. Not an easy thing to do with data-heavy 3-D images—today it takes more than two minutes to write an image across a piece of polymer. "Take a glass of water," Peyghambarian says. "How much information from every angle of the glass would you need to make a 3-D image of it? You have to put all of those coordinates in the memory and, when the scenery moves, that adds even more data." With a sensitive enough polymer or more laser power, an entire image could be written in a one flash, Perry adds.
Once researchers can figure out how to replace images more quickly (a pulsed laser is being considered), these images will look better as they are enlarged. Peyghambarian says that the new polymer must be made in pieces at least one square foot (929 square centimeters) before most people will take notice. The next step would be making a polymer that can display multicolored 3-D images.
Taken together, in the near future these developments could allow doctors to use the film to render three-dimensional images of the body (in green, for example) and highlight (in red) tumors and other serious medical conditions for analysis prior to surgery. Likewise, military leaders will be able to consult portable 3-D holographic maps of battlefields that automatically update as new intelligence becomes available.
Once the challenges of image replacement speed and resolution are met, home electronics makers will have what they need to create displays that can store an entire 3-D movie or video game—maybe even several—thus banishing today's much-coveted flat-panel TVs to the guest room or basement.
Holograms will also be used increasingly as sensor devices that change image and color when introduced to different stimuli. Take the hydrogel-based holograms made by Smart Holograms, a company spun out from the Institute of Biotechnology at the University of Cambridge in England. These holograms are part of the company's hand-held syringe system that measures water content in aviation fuel—necessary because excess moisture of more than 30 parts water to fuel can block aircraft fuel supply systems and stall engines during flight. The holograms react automatically when exposed to different stimuli—a gelatin-based substance in the hologram either swells or contracts in the presence of liquid, depending upon the concentration of water. Smart Holograms is also developing sensors that use holograms to alert diabetic patients and their doctors to dangerous glucose levels.
Emerging holographic and hologramlike technologies also promise to help physicians more precisely guide cancer treatments within their patients. In particular, Actuality Medical, Inc., in Bedford, Mass., is developing volumetric 3-D display devices that use data from computed tomography (CT) scans and other three-dimensional data to create lifelike computer renderings of portions of the body that doctors can study from different angles as though it were the real thing. These include the PerspectaRAD, a device that combines cancer-treatment software, a volumetric 3-D display and a haptic (or touch-based) interface that enables health care workers to visualize the location of a tumor and map out a treatment that minimizes damage to healthy tissue as well as ongoing development of the PerspectaSeed 4-D for treating prostate cancer and the MammoSolve for breast lumpectomy. "Think of this," says company CEO Michael Goldstein, "as GPS for the surgeon."