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  • Adding Muscle to Displays

Photonics Spectra
Oct 2006
Hank Hogan

For truly full-color displays, scientists at Swiss Federal Institute of Technology in Zurich have devised a technology that is based on tunable diffraction controlled by artificial “muscles.” The method uses pleated electroactive polymers that are plated with gold. When the muscles expand and contract under applied voltage, they change the diffraction pattern produced by a white light source, thereby enabling the selection of specific colors.


Using pleated, gold-plated polymers, researchers have constructed tunable diffraction gratings that broaden the color range of video display pixels. When the musclelike material expands and contracts under applied voltage, it changes the diffraction pattern, enabling the selection of specific colors. The round circles are images of the central spot of a pixel. Images courtesy of Manuel Aschwanden.

Standard displays are based on red-green-blue primary-color subpixels, with the intensity of each adjusted to create various colors. The eye, however, can see a wider range of color than this approach can reproduce.

To get around this problem, the investigators turned to diffraction. For a prototype pixel, they constructed tunable gratings of the pleated artificial muscles, which they coated with gold to increase reflectivity. They attached carbon electrodes to the muscles and shone white light on them. When they applied voltage, which in their first prototype was 4500 V, the muscles contracted in thickness and expanded in length and width. That altered the spacing between the membrane’s pleats, changing the diffraction angle and, thus, the location of specific colors.

In a composite of three images, the background is an image of the electrode array. At the intersection of the carbon black electrodes are individual tunable gratings. The color images show the difference in pixel presentation when the applied voltage changes from 0 to 350 V. The brown image (below left) is an atomic force microscope image of the polymeric diffraction grating.

In a fully realized system, there also would be a fixed hole through which the diffracted light would pass, thereby selecting a given color for that pixel. Three such tunable gratings at every location would enable the creation of all the colors the eye could see. Manuel Aschwanden, a nanotechnologist at the institute, said that the number of required gratings potentially could be reduced to two, if they could increase the tuning range enough.

He noted that the smallest prototype they have constructed thus far comprised 75-μm-wide polymers, and that the technology can compete with conventional LCDs in pixel size. “If the illumination can be integrated, then a flat panel display is feasible,” he said.

Before the prototype can be turned into a commercial product, however, a number of problems must be solved. One is the light source, which must produce true white light and not a composite. The researchers believe that LED technology could be used for this purpose.

In addition, the response time of the artificial muscles must be improved, although Aschwanden does not see this as the chief issue. “The biggest problem is the voltage,” he said.

The group now has prototypes that work with as little as 300 V. That figure must be cut by more than half — to 120 V — before a consumer product can be considered. Aschwanden estimated that, with the right amount of interest and financial backing, commercialization could take only eight years.

As a wavefront of light passes by an opaque edge or through an opening, secondary weaker wavefronts are generated, apparently originating at that edge. These secondary wavefronts will interfere with the primary wavefront as well as with each other to form various diffraction patterns.  
Contraction of "picture element." A small element of a scene, often the smallest resolvable area, in which an average brightness value is determined and used to represent that portion of the scene. Pixels are arranged in a rectangular array to form a complete image.
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