Home' Technology Review : May 2005 Contents 45
of command. "There is a terrestrial backbone of hardwired con-
nections, and there will be a space backbone between satellites.
What we are talking about adding, for aircraft, is an equivalent
third backbone in the sky," says Dave Kenyon, division chief of the
Technical Architectures Division at the U.S. Air Force Electronic
Systems Center in Bedford, MA.
The U.S. Air Force is beginning to de ne the architecture of
an airborne network and hopes to begin actively developing and
testing the network itself between 2008 and 2012, Kenyon says.
Taken together, the military research and the related air tra c
control research into airborne communications networks could
change how we travel in the decades to come.
OPTOELECTRONICS Making the material of computer
chips emit light could speed data flow. By Neil Savage
The Internet lives on beams of light. One
hair-thin glass ber can carry as much data
as thousands of copper wires. But inside
your computer, copper still r ules. The ad-
vantages of light haven t translated from
long-distance connections on the Internet
to the short jump between computer
chips, in part because the lasers used in
optical communications are made from exotic semiconductors
incompatible with the standard processes for making silicon
computer chips. As computers get faster and faster, they re near-
ing the physical limit of copper s ability to carry more infor ma-
tion, and they ll need something like the ber-optic network in
order to keep improving at the rate we ve come to expect.
Getting silicon to emit light could be the solution. A light sig-
nal s frequency is much higher than an electrical signal s, so it can
carry thousands of times as much information. Light also over-
comes another problem with electrical signals; as transistors get
closer together, the electrical signals passing through them start
to interfere with each other, like radio stations broadcasting at
the same frequency. But turning silicon into a light emitter has
proved an extraordinarily di cult challenge. The problem is
rooted in an energy-level mismatch between silicon s electrons
and its positively charged "holes" (electron vacancies in its crys-
tal structure): when an electron meets a hole, it s more likely to
release its excess energy as vibration than as light.
But last fall, a team at the University of California, Los Ange-
les, became the rst to make a laser out of silicon. In February,
Intel scientists upped the ante, reporting a silicon laser that put
out a continuous instead of a pulsed beam, a necessity for data
communications. "Once you identify the right piece of physics,
everything falls into place," says UCLA electrical-engineering
professor Bahram Jalali, who made the rst silicon laser.
The right piece of physics is the Raman e ect. Some photons
of light that pass through a material pick up energy from the natu-
ral vibration of its atoms and change to another frequency. Jalali
res light from a nonsilicon laser into silicon. Because of the
Raman e ect, the photons emerge as a laser beam at a di erent
frequency. This Raman laser is "a fundamental scienti c break-
POWER TRANSMISSION Wires spun from
carbon nanotubes could carry electricity
farther and more efficiently. By Erika Jonietz
Richard Smalley toys with a clear plastic tube that holds a thin, dark
gray fiber. About 15 centimeters long, the fiber comprises billions
of carbon nanotubes, and according to the Rice University chemist,
it represents the first step toward a new type of wire that could
transform the electrical power grid.
Smalley s lab has embarked on a four-year project to create a
prototype of a nanotube-based "quantum wire." Cables made from
quantum wires should conduct much better than copper. The
wires lighter weight and greater strength would also allow existing
towers to carry fatter cables with a capacity ten times that of the
heavy and inefficient steel-reinforced aluminum cables used in
today s aging power grid.
The goal is to make a wire with so little electrical resistance that
it does not dissipate electricity as heat. Smalley says quantum
wires could perform at least as well as existing superconductors---
without the need for expensive cooling equipment. The reason: on
the nanometer scale, the weird properties of quantum physics take
over, and a wire can carry current without resistance. But until a
couple of years ago, no one knew whether this amazing property
would hold up when nanotubes were assembled into a macro-
scopic system. Then Jianping Lu, a physicist at the University of
North Carolina at Chapel Hill, calculated that electrons could travel
down a wire of perfectly aligned, overlapping carbon nanotubes
with almost no loss of energy.
Smalley s group has already produced 100-meter-long fibers
consisting of well-aligned nanotubes. But the fibers are mixtures of
150 different types of nanotubes, which limits their conductivity.
The best wire would consist of just one kind of nanotube---ideally
the so-called 5,5-armchair nanotube, named for the arrangement
of its carbon atoms. Existing production techniques generate
multiple types of nanotubes, indiscriminately. But Smalley believes
that adding tiny bits of a single carbon nanotube at the beginning of
the process could catalyze the production of huge numbers of
identical nanotubes---in essence, "cloning" the original tube.
wires" could be made
from nanotubes like these
produced at Oak Ridge
COURTESY OF YUHUANG WANG/RICE UNIVERSITY (WIRES) COURTESY OF INTEL (SILICON)
This silicon chip
emits laser light.
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