Tiny pixels for wearable, bendy displays

A professor at the University of Michigan has revealed a revolutionary display technology that utilises a colour filter made of nano-thin sheets of metal. The pixels in Professor Jay Guo’s displays are about eight times smaller than the pixels on the iPhone4, which are about 78 microns. He envisions that this pixel size could position the technology for use in projection as well as wearable, bendable and compact displays.

The development is claimed to be a step toward more efficient, smaller and higher-definition display screens.

Gratings, precisely spaced on the sheets of metal, slice into metal-dielectric-metal stacks and act as resonators. They trap and transmit light of a particular colour, or wavelength, explains Guo, an associate professor in the Department of Electrical Engineering and Computer Science. A dielectric is a material that does not conduct electricity.

"Simply by changing the space between the slits, we can generate different colours," Guo said. "Through nanostructuring, we can render white light any colour."

A paper on the research is published in the August 24 edition of Nature Communications.

His team used this technique to make what they believe is the smallest colour University of Michigan logo. At about 12 by 9 microns, it's about 1/6 the width of a human hair.

The team argues that conventional LCDs are inefficient and manufacturing-intensive to produce. Only about five per cent of their back-light travels through them and reaches our eyes, Guo says. They contain two layers of polarisers, a colour filter sheet, and two layers of electrode-laced glass in addition to the liquid crystal layer. Chemical colorants for red, green and blue pixel components must be patterned in different regions on the screen in separate steps.

Guo's colour filter acts as a polariser simultaneously, eliminating the need for additional polariser layers. In Guo's displays, reflected light could be recycled to save much of the light that would otherwise be wasted.

Because these new displays contain fewer layers, they would be simpler to manufacture, Guo says. The new colour filters contain just three layers: two metal sheets sandwiching a dielectric. Red, green and blue pixel components could be made in one step by cutting arrays of slits in the stack. This structure is also more robust and can endure higher- powered light.

Red light emanates from slits set around 360 nanometres apart; green from those about 270 nanometres apart and blue from those approximately 225 nanometres apart. The differently spaced gratings essentially catch different wavelengths of light and resonantly transmit through the stacks.

"Amazingly, we found that even a few slits can already produce well-defined colour, which shows its potential for extremely high-resolution display and spectral imaging," Guo said.

The pixels in Guo's displays are about an order of magnitude smaller than those on a typical computer screen. They're about eight times smaller than the pixels on the iPhone 4, which are about 78 microns.

Guo has detailed his findings in a paper called Plasmonic nano-resonators for high resolution colour filtering and spectral imaging.