Scientists in Canada have used photonic crystals to create a novel type of flexible electronic-paper display. Unlike other such devices, the photonic-crystal display is the first with pixels that can be individually tuned to any color.
“You get much brighter and more-intense colors,” says André Arsenault, a chemist at the University of Toronto and cofounder of Opalux, a Toronto-based company commercializing the photonic-crystal technology, called P-Ink.
Several companies, including MIT startup E Ink and French firm Nemoptic, have begun producing products with e-paper displays. E Ink’s technology uses a process in which images are created by electrically controlling the movement of black or white particles within tiny microcapsules. Nemoptic’s displays are based on twisting nematic liquid crystals. The benefit of such screens is that compared with traditional displays, they are much easier to view in bright sunlight and yet only use a fraction of the power.
While the quality and contrast of black-and-white e-paper displays were almost on par with real paper, color images were lacking because each pixel was limited to a single primary color. To display a range of colors, pixels must be grouped in trios. In each trio, one pixel is filtered red, another is filtered green, and the third is filtered blue. Varying the intensity of each pixel within the trio generates different colors. But Arsenault says that these old systems lack intensity. For example, if one wants to make the whole screen red, then only one-third of the pixels will actually be red.
With P-Ink, it’s a different story. “We can get 100 percent of the area to be red,” Arsenault says. This is because each pixel can be tuned to create any color in the visible spectrum. “That’s a three-times increase in the brightness of colors,” he says. “It makes a huge difference.”
P-Ink works by controlling the spacing between photonic crystals, which affects the wavelengths of light they reflect. Photonic crystals are the optical equivalent of semiconductor crystals. While semiconductor crystals influence the motion of electrons, photonic crystals affect the motion of photons.
Although recently there has been a lot of research looking at using photonic crystals for anything from optical fibers to quantum computers, it’s actually an ancient phenomenon. For example, photonic crystals are responsible for giving opals their iridescent appearance. “There are many organisms that have coloration that doesn’t come from a dye,” says Arsenault. “This is the basis of our technology.”
With P-Ink, each pixel in a display consists of hundreds of silica spheres. Each of these photonic crystals is about 200 nanometers in diameter and embedded in a spongelike electroactive polymer. These materials are sandwiched between a pair of electrodes along with an electrolyte fluid. When a voltage is applied to the electrodes, the electrolyte is drawn into the polymer, causing it to expand.
The swelling pushes the silica beads apart, changing their refractive index. “As the distance between them becomes greater, the wavelengths reflected increases,” says Arsenault. P-Ink is also bistable, meaning that once a pixel has been tuned to a color, it will hold that color for days without having to maintain a power source. “And the material itself is intrinsically flexible,” Arsenault says.