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07 Feb 2008
A holographic display that allows 3D images to be captured, stored and updated has a promising future in medical and defence applications.
US researchers have demonstrated for the first time a 3D holographic display that allows the image to be erased and updated within a matter of minutes. The device, which measures 4x4 inches and can be viewed without special eyewear, uses a light-sensitive polymer that can also store 3D images for several hours. (Nature 451 694)
"Our holographic displays are the first updateable, three-dimensional displays with memory ever to be developed," Nasser Peyghambarian, a researcher from the University of Arizona, told optics.org. "They are ideal tools for applications that require situational awareness, such as medical, industrial and military imaging and for advertisement and entertainment."
Although holographic 3D displays are now commercially available, uptake has been limited because they don't offer the ability to refresh the image. "Existing 3D holographic displays cost tens of thousands of dollars, and since they are permanent, the cost per image is very high," explained Peyghambarian. "Our displays can be refreshed with new images every few minutes so you can have an unlimited number of 3D images using one display."
The key to the new device is a light-sensitive plastic film made from a photorefractive polymer. While the photopolymers used in existing holographic displays exploit chemical reactions that can't be reversed, photorefractive materials are based on the movement and trapping of charges generated within the material by light - a process that is fully reversible.
"Images are written onto the polymer using two incident coherent laser beams and an externally applied electric field," explained Peyghambarian. "The interference pattern from the two laser beams creates bright and dark regions across the material. Charges are generated in the bright region as a result of absorption of light. Positive charges move to the dark regions and negative charges move to the bright regions."
The result is a space-charge field within the polymer that replicates the interference pattern created by the laser beams. This space-charge effect modifies the local refractive index, which enables the hologram to be encoded as a refractive index pattern. A 633 nm laser can then be used to diffract from this index hologram and create the 3D image. "So far we have recorded images of a human skull, airplanes, a molecular model and a car," commented Peyghambarian.
The ideal photorefractive polymer for this application should offer both rapid recording times and a low decay time, but most polymers with fast recording times also lose the image very quickly. The Arizona team overcame this problem by developing a composite based on a copolymer, which they say minimizes the phase separation between functional elements that compromises the performance photorefractive polymers.
The space-charge field can be erased within minutes by using a spatially uniform 532 nm laser beam to re-distribute the charges uniformly inside the polymer. "When such a beam is shined onto the polymer, it creates charges everywhere inside the material in a uniform fashion, and excites the holes trapped in the dark nodes of the original interference pattern, causing them to re-distribute uniformly inside the polymer," commented Peyghambarian. "This effect results in washing away the refractive index pattern."
The next step for the group before the display can be commercialized is to make full colour displays and to extend the screen size to 12x12 inches and beyond. "The ultimate goal in updateable holographic display research is 3D-television where the images are refreshed at a rate of at least 33 times a second," concluded Peyghambarian. "One cannot think of reaching this goal with a static hologram like that on a credit card."
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