In collaboration with Nitto Denko Technical, the California-based research arm of a Japanese company, Peyghambarian has been working to improve the sophistication and refresh rate of holographic displays. The new displays refresh significantly faster than previous systems and are the first to be combined with a real-time camera system to show live images rather than ones recorded in advance. The new displays are based on a composite materials system developed by Nitto Denko Technical. In 2008, the groups produced a four-inch-by-four-inch red holographic display that could be rewritten every four minutes. By improving the materials used to make the display and the optical system used to encode the images, they have now demonstrated a full-color holographic display that refreshes every two seconds. This work is described today in the journal Nature.
The key to the technology is a light-responsive polymer composite layered on a 12-inch-by-12-inch substrate and sandwiched between transparent electrodes. The composite is arranged in regions called “hogels” that are the holographic equivalent of pixels. Writing data to the hogels is complex, and many different compounds in the composite play a role. When a hogel is illuminated by an interference pattern produced by two green laser beams, a compound called a sensitizer absorbs light, and positive and negative charges in the sensitizer are separated. A polymer in the composite that’s much more conductive to positive charges than negative ones pulls the positive charges away.
This charge separation generates an electrical field that in turn changes the orientation of red, green, and blue dye molecules in the composite. This change in orientation changes the way these molecules scatter light. It’s this scattering that generates a 3-D effect. When the hogel is illuminated with light from an LED, it will scatter the light to make up one visual point in the hologram.
Writing the data to the holographic display used to take several minutes. Part of the way the Nikko Denko researchers sped up the process was to decrease the viscosity of the dye materials so that they can change position more rapidly. The movement of the dye molecules inside the composite is analogous to the movement of liquid crystals in a conventional display, says Joseph Perry, professor of chemistry at Georgia Tech. A path to further increasing the speed of the display might be to make these materials more like liquid crystals, which can switch not just at video rates but faster than the human eye is capable of detecting.
Another boost in speed came from using a faster laser to write the data. For this to work, the researchers also had to pair the laser with polymers in the display that could respond to these faster pulses, separating charges to generate the electric fields with less delay time. In another advance over previous work, the company has developed a full set of dye molecules for red, green, and blue.
To demonstrate the relative speed of the system, the group used it as a “telepresence” system similar to the holographic communications used in sci-fi movies like Star Wars—but much choppier. Multiple cameras recorded images of an employee at Nitto Denko; these images were processed to create the data to write each hogel, and sent to the group in Arizona, where the holographic display showed a 3-D projection of their California collaborator. “Now what we can display is like a slow movie,” says Peyghambarian. To make a holographic video system, they’ll need to increase the display’s refresh speed to at least 30 frames per second.
The university and Nitto Denko groups are working with Michael Bove at MIT on improving the fidelity of the images. “What they’re reporting works beautifully, without a lot of computation,” Bove says. In hopes of making the imagery clearer, Bove has developed a system to render holographic video very rapidly on an ordinary computer graphics chip.