Most of us are familiar with the cliché about tradeoffs: fast, cheap, or good, choose any two. When it comes to holograms—two-dimensional media that can be used to create a three-dimensional image—there has been a well-defined series of tradeoffs among fast rewritability, image quality, image persistence, and image size. A paper that will appear in today's issue of Nature describes the production of a polymer that makes huge strides on all of these problems. As a result, rewritable holograms have become faster and better (although there was no mention of cost).





Image: Savas Tay, University of Arizona

The new material is comprised of photorefractive polymers. These chemicals have photoelectric properties that make them well-suited to storing the optical interference patterns used to produce holograms. When a photorefractive polymer is exposed to a pattern of bright and dark areas, electrons are released from the areas exposed to high-intensity light and migrate to areas that are darker. Once in place, the electron-rich areas diffract light differently from the electron-poor ones, allowing the original interference pattern to be reproduced when the material is exposed to light.

This process is normally slow, and the electrons will gradually return to their starting points, erasing the image. The paper indicates that previous work with photorefractive polymers had used materials where the image only lasted about as long as it took to record it. The new material makes a 10,000-fold improvement on that figure: data written in half a second can persist for up to three hours, provided an electric field (4 kiloVolts) is applied to the holographic device. These devices are also reasonably large (100 square centimeters in this case) and durable, having survived several months of repeated tests.

The researchers used a number of tricks to get this improved performance. The first was the use of a carefully chosen co-polymer that both helps evenly distribute the photorefractive material and serves as an electron donor, which speeds up the process of creating areas with different charges. They further sped the process by what they termed "voltage kick-off." This simply involves exposing the material to more than double the storage voltage (9kV in this case) during the writing process, which helps push the electrons to new locations.

The paper describes using the new device for two different types of holograms. One is simply the storage of a three dimensional image produced by traditional holographic methods. The second, holographic stereography, reveals the technique's full potential. In this case, stacks of two-dimensional image data, such as those produced by MRIs, CAT scans, and topographical data, can be encoded into the photorefractive material in such a way that the human visual system can interpret the resulting hologram as a full, three dimensional reconstruction.

The authors suggest that there are clear medical and military uses for these displays as things currently stand. If the material is as robust as it appears to be, refinements in its manufacturing and properties may eventually produce the first mass-market holographic display.

Nature, 2008. DOI: 10.1038/nature06596