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.”