Rewritable Holographic 3D Displays

Like lasers, holograms have played almost as prominent a role in science fiction as they have in real life and real science. Holography was the inspiration behind the holodeck on Star Trek—the simulated reality facility aboard the starship Enterprise—as well as the 3D holographic film of Princess Leia in Star Wars (“Help me, Obi-Wan Kenobi. You’re my only hope.”).

Of course, behind the science fiction glamour, holography is a well-established technique with a solid theoretical foundation. Pioneers such as Gabor, Leith, Upatnieks and Benton have developed an amazing array of applications that use holographic optical recording and processing. These include non-destructive testing and evaluation, holographic data storage, nano-fabrication, and the diffractive optical elements used in spectroscopy, laser beam combination and telecommunication. Unfortunately, the lack of suitable recording materials has limited the realization of dynamic 3D displays like that memorable one of Princess Leia.

We believe that photorefractive (PR) polymers may provide a solution. For nearly two decades, our research group at the University of Arizona has been studying these unique, high-performance, reversible holographic recording materials. In previous studies, we’ve improved the diffraction efficiency of this material from a fraction of a percent to nearly 100 percent; demonstrated a near-IR-sensitive PR polymer; created a security verification system using PR holograms; showed the PR effect by two photon absorption in a polymer; and operated a PR polymer at an optical communication wavelength.

Now, in our most recent research published in the February 7, 2008, Nature, we have demonstrated updateable holographic displays based on PR polymers. The holograms persist for several hours after they have been written and then can be erased and written again. A series of holograms—when captured quickly on a photorefractive material—might be used to create the effect of 3D movement.

Benefits of holography for 3D displays

Humans have been designed to see the world in three dimensions, thanks to the horizontal separation of our eyes and our brain’s extraordinary ability to process images. Three-dimensional images differ from two-dimensional ones in that they allow viewers to perceive depth through various cues, such as stereo parallax, accommodation and depth of field.

Two-dimensional displays such as televisions and computer screens are not capable of providing these depth cues. The result is a reduced sense of realism and the loss of a great amount of information related to the object or scene of interest.

Researchers have worked hard to develop 3D display technologies that overcome the limitations of 2D displays. Some of the most noteworthy approaches to 3D displays are stereoscopic displays based on special eyewear, volumetric displays that record bright spots in the bulk of a material and holographic displays.

Among these, holographic displays stand out because they can provide extremely realistic, high-resolution and full-color images that actually float in midair. Moreover, they are available in very large sizes—an important property for displays—and are capable of displaying images up to several meters across. Moreover, 3D images produced by holographic displays are autostereoscopic, which means they can be viewed directly on the recording material from multiple angles and do not require special goggles or other eyewear.

Limitations of recording materials

Most successful recording materials that have been used in holographic displays include photographic emulsions and photopolymers. These materials have several advantages: They are capable of extremely high spatial resolutions and they have the diffraction efficiencies needed to create high-quality images and large viewing angles.

However, because they are based on irreversible recording processes that result in permanent holograms, current commercial holographic displays are write-only systems—meaning that
the images displayed cannot be erased and updated with new ones. This lack of “rewritability” has limited the use of holographic displays to static appli-cations and increased the cost of 3D imaging considerably.

In previous attempts to create dynamic holographic displays, researchers used acousto-optic, liquid crystal and MEMS-based devices to project 3D images. Each of those devices are capable of very high refresh rates (up to several KHz) that allow them to create video-rate images (30 frames per second). But the fast response times come with a price: The recorded images do not persist.

The lack of memory requires that the entire display be refreshed at a rate higher than the video rate to avoid image flicker. As a result, dynamic holographic displays have been limited based on optoelectronic devices to small sizes and resolutions.

But there is an intermediate regime between static and real-time imaging, called near-real-time imaging, where images are updated every few minutes. For many applications of 3D displays, such as medical and military imaging, the 3D data cannot be generated in real time due to limitations in image acquisition or the sheer size of the data sets. Near-real-time displays are naturally suited for these scenarios. However, in order to realize practical applications of near-real-time holographic 3D displays, large holographic materials with reversible recording capability and memory are needed.