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Science-Technology Title: Plasmons Create Beautiful Full-Color Holograms By harnessing the power of tiny waves dancing in an electron sea, Japanese physicists have developed a novel way to project holograms that don’t change color when you move your head. “In a conventional hologram, if you change the angle, the color changes,” said optical physicist Satoshi Kawata of Osaka University in Japan. “Our hologram shows natural color at any angle you observe.” The researchers’ machine takes advantage of how beams of light trigger waves of activity in free electrons, unattached to any atom, arrayed on a metal surface. Called surface plasmons, these waves could be used to blast cancer cells and build ultra-fast computer processors. They also show up in medieval stained glass windows, where plasmons on flecks of gold suspended in the glass make the window change color as the sun sets. Plasmons always emit colored light, Kawata says, but it’s usually only visible within a few nanometers of the metal’s surface. But if the light bounces off a ridged surface, it can project far enough from the metal to be seen by the naked eye. In a paper published April 8 in Science, Kawata and colleagues describe how they used surface plasmons to reconstruct a faithful, full-color holograph. First, the researchers used red, green and blue-colored lasers to etch a record of the way light scattered off an object (an apple, for instance) onto a thin sheet of light-sensitive material called a photoresist, and attached it to a plate of glass. Atop the photoresist they laid a corrugated layer of silver, with a layer of silicon dioxide on top of that. Silicon dioxide helps guide the holograph’s light waves in the right direction, the researchers say. The entire assemblage was 230 nanometers thick. A halogen lamp shining on the back of the plate excites different plasmons depending on the angle of the incoming light, Kawata explained. Each plasmon emits a specific wavelength, or color, of light. “So even if you’re given white light, only one color is chosen by the plasmon,” he said. The plasmon-emitted light reconstructs the hologram as a virtual image hovering above the plate. Kawata admits the device is far from ready for real-world applications; he’s mostly interested in the physics. “No one has thought to use plasmons for display applications, so it was fun for me,” he said. “I just wanted to demonstrate that this could be done. But I hope people would be interested in thinking seriously to use this technology for larger scale 3-D virtual display,” like for TV or movies. Other researchers are skeptical that the device will make it to the big screen. The image is static and very small, only about two centimeters across at present. “These issues would lower the chances that the technology would have commercial future,” said physicist Nasser Peyghambarian of the University of Arizona, whose group built a 3-D holograph that can be updated in real time. This is not the first device to produce 3-D, colored holographs under white light, notes Michael Bove of MIT’s Media Lab, whose research group also debuted an updatable 3-D holographic video earlier this year. “That said, the physics behind this approach is very interesting,” he said. “The technique looks as if it could offer some advantages in light efficiency and view angle for mass-produced holograms, provided they can figure out how to mass-produce their holograms cheaply.” For now, then, plasmonic holograms don’t look as if they’ll change the world — but they certainly are pretty.
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