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Simpler Flexible Displays

A new approach avoids flexible electronics.

Flexible displays that are bigger, brighter, and cheaper could be made using a new approach that involves exciting fluorescent chemicals embedded in the screen with an infrared laser.

Flexible image: Stills from an animated cartoon, projected by a 635-nanometer scanning laser at eight kilohertz onto three different flexible plastic screens. The screens fluoresce blue, green, and yellow-red, and could eventually be combined to create a full-color display.

Researchers are exploring a range of flexible-screen technologies because they could have a range of applications, from electronic advertisements that can be pasted on a wall to laptops and electronic books that can be rolled up and tucked into a backpack. One approach is to use organic LEDs on top of a flexible substrate. Another is to use electronic “ink” consisting of tiny colored particles that can be controlled electrically. E-Ink, based in Cambridge, MA, has even created electronic “paper” that is used in a number of commercial products. However, both approaches require some form of flexible electronics to control the displays.

The new approach, developed by researchers in Germany–at Sony Deutschland Gmb, in Stuttgart, and the Max Planck Institute for Polymer Research, in Mainz–avoids the complications caused by flexible electronics. Their device consists of a chemical layer sealed between plastic sheets. Under normal light, the screen is transparent. But when exposed to infrared light, the chemicals in the screen fluoresce.

To create images, the researchers used a red or infrared laser to quickly scan across the screen, from either in front or behind, causing different parts to fluoresce in sequence to produce a fast-moving image. This is similar to the way that a cathode-ray tube uses an electron beam to make images. In a demonstration, the researchers made a cartoon image move around on their screen.

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Tzenka Miteva, a researcher at Sony who coauthored a paper on the technology, published today in the New Journal of Physics, says that the screens use specially-matched combinations of chemicals to “upconvert” light–that is, absorb light of longer wavelengths and emit light at shorter wavelengths. This means that the researchers were able to use a red or infrared laser to generate colors in the visible spectrum. “Red or infrared lasers are cheap and very much available on the market,” Miteva says. “And because it works at very low intensities, we can use them without problems with the viewers.”

The researchers used three different chemicals that fluoresced blue, green, and yellow. But Stanislav Baluschev, a researcher at Max Planck and coauthor of the New Journal of Physics paper, says that one next step will be finding a chemical that gives off a saturated red light, in order to produce a full palette of colors when combined with the others. Another issue is how to use the three colors to create full-color displays. So far, the researchers have created separate screens, each containing a different chemical, resulting in displays that give off only monochromatic images. The team is working to create multilayer and pixilated screens using all three colors.

But an important advantage of the process is that the screens are extremely simple to make. The chemical layer can be painted or screen-printed onto a layer of plastic, then sealed with another layer. The technology might be most practical for projected displays, such as advertisements or public information screens. And since the screens are transparent when not in use, they could perhaps be used for heads-up displays on car windshields, Baluschev says.

But using the screens on portable electronic devices would present complications since the scanning laser adds bulk and needs to be positioned far enough away from the screen that it can reach all parts evenly. Baluschev says that one way around the problem might be to use very fine fiber-optic cables to direct the light to each pixel on the screen. Nicholas Sheridon, a physicist who worked on flexible displays at the Xerox Palo Alto Research Center, says that the new technology is probably too bulky and power intensive to be useful in consumer electronics. But he agrees that the technology might be useful in projected displays, although it isn’t yet clear how it would compare with existing projection technologies.

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