LED shines the right light
Technology Research News
Most Internet users connect to the Internet
through dial-up modems, which are several orders of magnitude too slow
to accommodate applications like downloading movies or videoconferencing.
Even digital subscriber line (DSL) or cable connections, which increase
download speeds from 56 thousand bits per second to 1.5 million or so,
would require six hours to download two hours of the type of compressed
full-motion video found on a DVD. Full motion video uses 30 frames, or
separate pictures per second, which adds up to about 4 gigabytes of data
for that two-hour movie.
One way to deliver on the full-speed hype of the Internet is to extend
the global network's fiber-optic backbones all the way to individual homes.
Fiber lines can carry much more information than telephone and cable wires
because light can be pulsed much more quickly than electricity. The ones
and zeros of digital information are represented by the presence or absence
of a pulse of light or electrical current.
Before speedy fiber optic lines reach the average house, the technology
will have to get considerably cheaper. "If these components will have
to be at every house, the price of each component will have to be very
low," said Nir Tessler, a senior lecturer of electrical engineering at
the Israel Institute of Technology.
Tessler is one of a team of researchers at the Israel Institute of Technology
and Hebrew University in Israel that has found a way to mix plastic and
microscopic semiconducting crystals to make material that could lower
the cost of fiber-optic communications. The material forms a light emitting
diode (LED) that emits wavelengths of light in the infrared range used
by existing fiber lines.
Most LEDs are made from small chips of semiconductor crystal, which is
also the type of material used to make computer chips.
Organic LEDs, which are made from polymers, or plastics, are cheaper than
those made from semiconductor materials, but emit only visible light waves,
which range from 0.4 to 0.7 microns. Telecommunications over fiber optics
uses 1.3 micron waves of near-infrared light.
Attempts to make near-infrared organic LEDs have run up against an efficiency
problem: they use a lot of power because the devices are not efficient
at converting electricity into light. The researchers got around this
problem by mixing their polymer with more efficient microscopic semiconductor
crystals. The nanocrystals have a core of indium arsenide surrounded by
Key to the new material's potential utility is that the nanocrystal-laced
plastic can be spread into a thin film to make LEDs, which is a cheaper
manufacturing process than etching layers of semiconductor crystals with
light and chemicals to produce today's LEDs.
The researchers' device converts electricity to light with an efficiency
of between 1.5 and 3 percent, which is much lower than the 90 percent
efficiency possible with semiconductor LEDs, but it has the potential
to improve to usable levels.
"Their claim of about three percent internal efficiency is certainly impressive
for a first attempt using an organic technology," said William Gillin,
a lecturer in experimental physics at the University of London. "A cheap
device that operates in the 1.3 micron band would certainly be useful
for the fabrication of optical amplifiers for [telecommunications]," he
The researchers are working on a second generation of more efficient devices
and are extending the wavelength range, said Tessler. The LEDs could be
used in practical applications in four to eight years, he said.
Tessler's research colleagues were Vlad Medvedev of Technion and Miri
Kazes, ShiHai Kan and Uri Banin of Hebrew University. They published the
research in the February 22, 2002 issue of the journal Science. The research
was funded by Israel Institute of Technology, Hebrew University and the
Israel Science Foundation.
Timeline: 4-8 years
Funding: University, Government
TRN Categories: Optical Computing, Optoelectronics and
Photonics; Materials Science and Engineering; Telecommunications
Story Type: News
Related Elements: Technical paper, "Efficient Near-Infrared
Polymer Nanocrystal Light-Emitting Diodes," Science, February 22, 2002
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