Most of the technologies that create three-dimensional images-- from virtual-reality helmets to holograms to the cheap glasses handed out for watching 1950s 3-D movies--are grounded in deception. In one way or another, they all trick the eye into converting flat images to three- dimensional ones. Stanford engineer Elizabeth Downing has broken with that tradition of visual subterfuge. She recently developed a fundamentally new way of creating three-dimensional images. Her images--which can be made to move--actually have height, length, and width, all in an inch-high cube.
Downing downplays her efforts. A lot of problems that engineers have solved are complex, she says. This one, in many regards, is trivial. Most engineers would demur. Her cube is an elegant and subtle solution to a problem that has stumped many of her colleagues.
Downing’s cube exploits one of the most fundamental interactions between light and matter. When a particle of light, or photon, hits an atom, an electron inside the atom will suddenly jump to a higher energy level. That same energized electron can later tumble back to its original energy level, emitting a photon in the process.
An image arises in the cube as a result of the carefully controlled interplay of laser light with atoms in the cube. Downing uses a pair of infrared lasers to generate the image. Each infrared laser beam (which is invisible to human eyes) passing through the crystal by itself would excite electrons in its path to a higher energy level. When those electrons fell back to lower energy levels, they would all emit infrared photons, which we can’t see.
But if the two infrared laser beams intersect, the combined beam boosts electrons to an energy level such that when the electrons return to their original states, they emit photons of visible light. By sweeping the infrared laser beams through her cube, Downing generates the points of light that make up her images only where the beams cross.
Downing wasn’t the first to grasp this idea, but previous attempts by other researchers to use the scheme ran into problems: most ordinary materials convert infrared light into heat, making it impossible to use infrared lasers to generate visible light. Early efforts in the 1970s were so disappointing that most researchers gave up on the idea. Downing began toying with the method in the late 1980s. She burrowed through the literature until she uncovered three obscure elements-- praseodymium, erbium, and thulium--that produced red, green, and blue light, respectively, when stimulated with the right combination of two infrared laser wavelengths. Impregnated with the right amounts of these materials, Downing’s cube--a blend of heavy metals, fluoride, and glass-- could create almost any color. Unlike those in conventional glass, the electrons in Downing’s cube aren’t affected by laser light, so the cube makes a good noninterfering conduit.
Currently, the cube shows only a repeating geometric pattern. But in a lab at 3D Technology, a company in Mountain View, California, Downing is developing a version that will use hundreds of tiny lasers mounted in a two-dimensional array. As one part of the array fires pulses of infrared light through one side of the cube, another cluster of lasers parallel to the bottom of the cube will sweep the crystal, forming points of light wherever the beams intersect.
Downing believes this design could be scaled up to show mri scans in all three dimensions, and she envisions a cube a yard high in which air traffic data can be displayed in all their original dimensions instead of on flat radar screens. Air traffic control data are intrinsically three- dimensional, Downing says.
But don’t look for 3-D tvs anytime soon. In order to get a television-quality picture, you would need the broadcast capacity of 500 channels, each delivering a different slice of the picture. And unlike real objects, the images in Downing’s cube don’t block light, so a complex scene would look like a ghostly mishmash of surfaces--you’d see the backyard of a house shine through its wall. The technology is so new, she says, that we’re not exactly sure where the limit of visualization is.
