A new type of color filter could significantly increase the energy efficiency of liquid-crystal displays (LCDs), which dominate the market in everything from televisions to cell phones.
The best LCDs today only emit about 8 percent of the light produced by their backlights. This means that they drain batteries in portable electronics and ramp up electricity bills in homes (the California Energy Commission estimates that televisions consume 10 percent of the electricity in homes).
Color filter: The grating shown in the top image filters different colors of light depending on the width of its slits, and the distance between them. The bottom image shows the colors produced by the filter when it’s illuminated with white light.
Normally, LCDs use several layers of optical devices to colorize, polarize, and shutter light from a backlight, and inefficiencies emerge at every step. Now researchers at the University of Michigan have made an optical film that promises to boost the overall efficiency of LCDs by more than 400 percent--so that 36 percent of light makes it through. The new optical film was developed by researchers led by L. Jay Guo, professor of electrical engineering and computer sciences at the university. The film colors and polarizes the light that passes through an LCD more efficiently than conventional components can.
The color filter is a three-layer sandwich of an insulating material in between two layers of aluminum; the entire stack is less than 200 nanometers thick and is etched with periodic slits, like a grate. The distance between the slits and their width determines the color they'll produce when illuminated by a white backlight. This is because the grating patterns are on the same size scale as the wavelength of visible light. In a paper published online last week in the journal Nature Communications, the Michigan researchers show that they can make a rainbow of colors using such a filter. As well as being more efficient, the filter is also simpler to make than current LCDs, says Guo. It is possible to create red, blue, and green subpixels by patterning gratings of differing widths side by side in a single manufacturing step. Conventional LCDs use pigments to define the red, green, and blue subpixels that filter light from the backlight. Each of these colorants is deposited one at a time and then patterned to make a subpixel. The new gratings transmit more light than colorant-based filters; whereas a traditional green filter transmits about 40 percent of the light; a green, grating-based filter transmits 60 percent. Other colors have similar efficiency advantages.
The gratings also polarize the light very efficiently. This is vital, because the liquid-crystal shutters that open and close to let light from each pixel through only work with polarized light. Conventional displays use polarizing filters that absorb half the light--the portion with the wrong polarization. Guo's gratings let light of the right polarization pass though, but they don't absorb the other 50 percent--instead, this light is reflected back in the other direction. A mirror reflects this light and flips some of its polarization, allowing more light to pass back through the gratings.
Other researchers have made similar grating structures for focusing light or getting more light out of displays or solar cells. "It's a very easy structure to build and pattern," says Peter Catrysse, a researcher at Stanford University. He adds that Guo's color filter and polarizer shows versatility and tunability, a sign that the field is "getting closer to building components that can be used for practical purposes."
Nicholas Fang, a professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign, sees other directions for the work. He notes that Qualcomm is working on even lower-power reflective displays. Combining this type of display with the new filter could eliminate the backlight altogether. "There's a possibility of building this [grating] as a reflective-based color filter," he says.
The Michigan researchers are now focused on making the filters "production worthy," or compatible with the machinery used to mass-produce displays, says Guo. Last year, his group demonstrated a way to use the same nanopatterning techniques over large areas at high speed on roll-to-roll printers. "We have used continuous roll-to-roll manufacturing to make very similar structures," he says. "The individual elements are there, and now it's a matter of integration."
By Katherine Bourzac
From Technology Review
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