[tt] technologyreview: photonic crystal-based displays

[Alejandro Dubrovsky]

(
http://www.technologyreview.com/printer_friendly_article.aspx?id=19337
)

Wednesday, September 05, 2007
E-paper with Photonic Ink
Photonic crystals are being used by a Toronto startup to create
commercial devices that offer better color and resolution than other
flexible displays.
By Duncan Graham-Rowe

Scientists in Canada have used photonic crystals to create a novel type
of flexible electronic-paper display. Unlike other such devices, the
photonic-crystal display is the first with pixels that can be
individually tuned to any color.

"You get much brighter and more-intense colors," says André Arsenault, a
chemist at the University of Toronto and cofounder of Opalux, a
Toronto-based company commercializing the photonic-crystal technology,
called P-Ink.

Several companies, including MIT startup E Ink and French firm Nemoptic,
have begun producing products with e-paper displays. E Ink's technology
uses a process in which images are created by electrically controlling
the movement of black or white particles within tiny microcapsules.
Nemoptic's displays are based on twisting nematic liquid crystals. The
benefit of such screens is that compared with traditional displays, they
are much easier to view in bright sunlight and yet only use a fraction
of the power.

While the quality and contrast of black-and-white e-paper displays were
almost on par with real paper, color images were lacking because each
pixel was limited to a single primary color. To display a range of
colors, pixels must be grouped in trios. In each trio, one pixel is
filtered red, another is filtered green, and the third is filtered blue.
Varying the intensity of each pixel within the trio generates different
colors. But Arsenault says that these old systems lack intensity. For
example, if one wants to make the whole screen red, then only one-third
of the pixels will actually be red.

With P-Ink, it's a different story. "We can get 100 percent of the area
to be red," Arsenault says. This is because each pixel can be tuned to
create any color in the visible spectrum. "That's a three-times increase
in the brightness of colors," he says. "It makes a huge difference."

P-Ink works by controlling the spacing between photonic crystals, which
affects the wavelengths of light they reflect. Photonic crystals are the
optical equivalent of semiconductor crystals. While semiconductor
crystals influence the motion of electrons, photonic crystals affect the
motion of photons.

Although recently there has been a lot of research looking at using
photonic crystals for anything from optical fibers to quantum computers,
it's actually an ancient phenomenon. For example, photonic crystals are
responsible for giving opals their iridescent appearance. "There are
many organisms that have coloration that doesn't come from a dye," says
Arsenault. "This is the basis of our technology."

With P-Ink, each pixel in a display consists of hundreds of silica
spheres. Each of these photonic crystals is about 200 nanometers in
diameter and embedded in a spongelike electroactive polymer. These
materials are sandwiched between a pair of electrodes along with an
electrolyte fluid. When a voltage is applied to the electrodes, the
electrolyte is drawn into the polymer, causing it to expand.

The swelling pushes the silica beads apart, changing their refractive
index. "As the distance between them becomes greater, the wavelengths
reflected increases," says Arsenault. P-Ink is also bistable, meaning
that once a pixel has been tuned to a color, it will hold that color for
days without having to maintain a power source. "And the material itself
is intrinsically flexible," Arsenault says.

The technology was developed with Geoffrey Ozin and Daniel Puzzo, among
others, at the University of Toronto and Ian Manners at the University
of Bristol, in the UK. The group demonstrated how 0.3-millimeter
pixels--about the same size as many LCD displays--can independently
generate a range of colors. Their results are published in the August
issue of the journal Nature Photonics. "One single material can give all
the necessary colors for a display without filters," says Arsenault.

In fact, by making the crystals slightly larger, it's also possible to
take them beyond the visible-light range and into infrared, says
Arsenault. The effects in this range would be invisible to the human eye
but could be used to make smart windows that control the amount of heat
that passes through them, he says.

This is a step forward, says Jacques Angele, a cofounder of Nemoptic.
"The aim of these color-display technologies is to be comparable with
paper. Unfortunately, the brightness of the [other technologies] today
is limited to about 30 percent of paper."

"It's a spectacular innovation," says Edzer Huitema, chief technology
officer of the Dutch firm Polymer Vision, based in Eindhoven. Even
traditional screens, such as cathode-ray tubes, LCDs, and plasma
displays, use three or even four differently colored pixels to generate
color. "It's a major limitation for all color-display technologies,"
Huitema says. When the color of each pixel is controlled, not only does
the color quality increase, but the resolution should also improve by a
factor of three.

There is one display technology, however, that can tune individual pixel
color, says Angele. Both Kent Displays, in Ohio, and Japanese
electronics firm Fujitsu have been taking this approach, which, in
essence, involves placing the three colored pixels on top of each other.
But besides being technically difficult and expensive, this approach
reduces the brightness of the colors, Angele says. "It's difficult to
have an optical stack without optical losses."

Arsenault predicts that Opalux will have the first products on the
market within two years, probably in the form of advertising displays.
But, he says, it will be a long while before P-Ink will be in a position
to completely replace traditional displays. "The caveat is that we are
not at video speeds," Arsenault says.

Currently, the P-Ink system can switch pixels in less than a second,
which is on par with other e-paper displays. "We're still early in our
development, and there's a lot of room for [improving] the material and
optimising its performance," says Arsenault.
Copyright Technology Review 2007.