by Anton Shilov
02/07/2013 | 11:59 PM
Researchers from the University of Michigan are developing a display technology based on iridescence that could lead to high-resolution, reflective, color display screens. What the developers are trying to do is to mimic pecock’s unique iridescent color shifting mechanism using electronics.
The new U-M research could lead to advanced color e-books and electronic paper, as well as other color reflective screens that do not need their own light to be readable. Reflective displays consume much less power than their backlit cousins in laptops, tablet computers, smartphones and TVs. The technology could also enable leaps in data storage and cryptography, for example, documents could be marked invisibly to prevent counterfeiting.
In a peacock's mother-of-pearl tail, precisely arranged hairline grooves reflect light of certain wavelengths. That's why the resulting colors appear different depending on the movement of the animal or the observer. Imitating this system—minus the rainbow effect—has been a leading approach to developing next-generation reflective displays.
Led by Jay Guo, professor of electrical engineering and computer science, the researchers harnessed the ability of light to funnel into nanoscale metallic grooves and get trapped inside. With this approach, they found the reflected hues stay true regardless of the viewer's angle.
"That's the magic part of the work. 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," said Mr. Guo.
The diffraction limit was long thought to be the smallest point you could focus a beam of light to. Others have broken the limit as well, but 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.
The U-M team determined what size slit would catch what color light. Within the framework of the print industry standard cyan, magenta and yellow color model, the team found that at groove depths of 170nm and spacing of 180nm, a slit 40nm wide can trap red light and reflect a cyan color. A slit 60nm wide can trap green and make magenta. One 90nm wide traps blue and produces yellow. The visible spectrum spans from about 400nm for violet to 700nm for red.
"With this reflective color, you could view the display in sunlight. It's very similar to color print," said Mr. Guo.
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 90nm 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.
To demonstrate their device, the researchers etched nanoscale grooves in a plate of glass with the technique commonly used to make integrated circuits, or computer chips. Then they coated the grooved glass plate with a thin layer of silver. When light – which is a combination of electric and magnetic field components – hits the grooved surface, its electric component creates what's called a polarization charge at the metal slit surface, boosting the local electric field near the slit. That electric field pulls a particular wavelength of light in.
Right now, the new device can make static pictures, and the researchers hope to develop a moving picture version in the near future.
Detail images of the Olympic rings, as viewed through a scanning electron microscope. The magenta color in the bow is produced with several short lines, as small as 100nm long and 60nm wide, demonstrating ultra-high color resolution. The purple color in the gymnast's rope is produced by just two nanoscale grooves.