An optical chip built by an MIT graduate student at a cost of $10 could be a “game changer” for holography, enhancing the resolution of conventional 2-D displays and enabling color holographic videos suitable for 3-D television. In holograms, light beams pass through a so-called diffraction fringe, bending the light so that they emerge as a host of different angles. To produce a holographic video, diffraction fringes must be created from patterns displayed on an otherwise transparent screen. The problem with this approach, however, is that the pixels of the diffraction pattern must be as small as the wavelength of the light they are bending — a feat most display technologies cannot achieve. Stephen Benton, an MIT Media Lab professor who died in 2003, created one of the first holographic-video displays using an acousto-optic modulation technique, which sent sound waves through a piece of transparent metal. A hologram of a butterfly, displayed on both a conventional, monochromatic holographic-video monitor (left) and the MIT researchers’ new color monitor, which used an optical chip built by Daniel Smalley for roughly $10. Images courtesy of Daniel Smalley. “These waves basically squeeze and stretch the material, and they change its index of refraction,” said V. Michael Bove Jr., a principal research scientist at the Media Lab and head of its Object-Based Media Group. “So if you shine a laser through it, [the waves] diffract it.” Benton’s sophisticated display, called Mark-II, was built with the help of Bove’s group, which applied acousto-optic modulation to a crystal of an expensive material called tellurium dioxide. The crystal was not TV resolution and was difficult to scale down, Bove said. The new hologram-generating approach, developed by graduate student Daniel Smalley under the advisement of Bove, generates microscopic channels — or waveguides — under small crystals of lithium niobate that confine the light traveling through them. Metal electrodes were deposited onto each waveguide to produce acoustic waves. Using the new technique, Smalley is building a prototype color holographic-video display whose resolution is roughly that of a standard-definition TV and which can update video images 30 times a second — fast enough to produce the illusion of motion. At the heart of the display is an optical chip, resembling a microscope slide, that Smalley built for about $10 and using only MIT facilities. “Everything else in there costs more than the chip,” Bove said. “The power supplies in there cost more than the chip. The plastic costs more than the chip.” Each waveguide in the device corresponds to one row of pixels in the final image. Whereas the crystals in the Mark-II had to be large enough for the acoustic waves producing the separate lines of holograms to be insulated from each other, Smalley’s chip enables the waveguides with their individual electrodes to be packed mere micrometers apart. Beams of red, green and blue light are sent down each waveguide, and the frequencies of the acoustic wave passing through the crystal determine which colors pass through and which are filtered out. Combining, say, red and blue to produce purple doesn’t require a separate waveguide for each color, just a different acoustic-wave pattern. “Until now, if you wanted to make a light modulator for a video projector, or an LCD panel for a TV, or something like that, you had to deal with the red light, the green light and the blue light separately,” Bove said. This is inefficient because the filters throw away two-thirds of the light and reduce either the resolution or the speed at which the modulator operates, he said. Anisotropic leaky-mode modulator for creating holographic video images. The modulator is illuminated with red, green and blue laser light to create a full-color holograms using a novel frequency division technique. “What’s most exciting about [the new chip] is that it’s a waveguide-based platform, which is a major departure from every other type of spatial light modulator used for holographic video right now,” Smalley said. Waveguides are already a common feature in commercial optoelectronics, and techniques for manufacturing them are well established. “One of the big advantages here is that you get to use all the tools and techniques of integrated optics,” he said. “Any problem we’re going to meet now in holographic video displays, we can feel confident that there’s a suite of tools to attack it relatively simply.” “This has the potential to be a game changer, and I’m really serious about that,” said Pierre Blanche, an assistant research professor at the University of Arizona who is also researching holographic video. “It’s a huge achievement.” Blanche and colleague Nasser Peyghambarian, chair of photonics and lasers at Arizona, are developing a holographic video system that Blanche believes has some advantages over the MIT system. “Our images still have better quality,” he said, “but they achieve video rate, and we haven’t. This is a very exciting time for us.” The technique was detailed in Nature (doi: 10.1038/nature12217). For more information, visit: www.mit.edu