February 6, 2013
Improving On ‘Peacock’ Technology Could Lead To Better High-res Displays
April Flowers for redOrbit.com - Your Universe Online
Peacock feathers have a beautiful iridescence about them — a sheen that shifts colors depending on your perspective. Nature created this color shifting with relative ease, it seems, but it has been a nuisance for engineers trying to reproduce this unique color mechanism to make high-resolution, reflective color display screens.
A new study from the University of Michigan reveals researchers have found a way to lock in so-called structural color, which is created from texture rather than chemicals.
Precisely arranged hairline grooves reflect light of certain wavelengths in a peacock's mother-of-pearl tail, resulting in colors that appear different depending on the movement of the animal or the observer. Attempting to imitate this system without the rainbow effect has been a leading approach to next-generation reflective display development.
The findings of this study could lead to advanced color e-books and electronic paper. Advances could be made in other color reflective screens that don't need their own light to be readable as well. Backlit screens on laptops, tablet computers, smartphones and TVs require much more power than reflective displays. Other applications of this new technology could include data storage, cryptography and counterfeit-proof documents.
The research team harnessed the ability of light to funnel into nanoscale metallic grooves and become trapped inside. Using this approach, the team found the reflected hues stay true regardless of the viewer's angle.
"That's the magic part of the work," Jay Guo, professor of electrical engineering and computer science, said. "Light is funneled into the nanocavity, whose width is much, much smaller than the wavelength of the light. And that's how we can achieve color with resolution beyond the diffraction limit. Also counterintuitive is that longer wavelength light gets trapped in narrower grooves."
Scientists have long thought that the diffraction limit was the smallest point on which you could focus a beam of light. Other research groups have broken this limit as well, but according to Guo, the U-M team did so with a simpler technique that also produces stable and relatively easy-to-make color.
"Each individual groove–much smaller than the light wavelength–is sufficient to do this function. In a sense, only the green light can fit into the nanogroove of a certain size," Guo said.
Their process involves determining what size slit would catch what color light. Using the standard print industry framework of cyan, magenta and yellow, the team determined that at a groove depth of 170 nanometers and spacing of 180 nanometers, a slit 40 nanometers wide can trap red light and reflect a cyan color. Likewise, a slit 60 nanometers wide can trap green and make magenta; and one 90 nanometers wide traps blue and produces yellow. The entire visible spectrum spans from about 400 nanometers (violet) to 700 nanometers (red).
"With this reflective color, you could view the display in sunlight. It's very similar to color print," Guo said.
White paper is a reflective surface as well, and to create color, printers arrange pixels of cyan, magenta and yellow in such a way that they appear to our eyes as the colors of the spectrum. Using Guo's approach, an electronic display would work in a similar manner.
The team etched nanoscale grooves in a plate of glass with the technique commonly used to make integrated circuits, or computer chips to demonstrate their device. The plate was then coated with a thin layer of silver. Light is a combination of electric and magnetic field components. When it hits the grooved surface, its electric component creates a polarization charge at the metal slit surface. This boosts the local electric field near the slit, which pulls a particular wavelength of light in.
Currently, the device can only create static pictures, but the research team hopes to develop a moving picture version in the near future.
The findings of this study have been published online in Scientific Reports.
Image 2 (below): University of Michigan researchers created the color in these tiny Olympic rings using precisely-sized nanoscale slits in a glass plate coated with silver. Each ring is about 20 microns, smaller than the width of a human hair. They can produce different colors with different widths of the slits. Yellow is produced with slits that are each 90 nanometers wide. The technique takes advantage of a phenomenon called light funneling that can catch and trap particular wavelengths of light, and it could lead to reflective display screens with colors that stay true regardless of the viewer's angle. Image credit: Jay Guo, College of Engineering