Sandia makes a major find in Photonics:
sciam.com
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Light Twisters
Photonic crystals promise advances in
communications and optical computers
Image: Randy Montoya,
Sandia
GLOWING GRID.
Photonic crystal
created at Sandia
National Laboratories
can trap photons in
its lattice and force
them to follow
intricate pathways.
MORE
EXPLORATIONS
Close up, it looks a lot like a stack of firewood waiting for a match,
but it's actually a synthetic crystal made of slivers of silicon. The
crystal has the unique ability to bend specific frequencies of light in
almost any direction, with almost no loss. This achievement, by
researchers at Sandia National Laboratories, is far more than a
laboratory curiosity: These devices hold the key to rapid advances in
communications and optical computing.
The lattice of interlocking bars, called a
photonic crystal, acts like a mirror to
prevent light of a particular frequency
caught in the cavities from escaping. By
selecting the proper width and distance
between the bars, researchers can select
the frequency of light that becomes
trapped in this photonic "band gap." Then,
by carefully introducing impurities or
variations in the lattice, they can create
pathways that take the trapped light on a
roller coaster ride through the crystal. No
matter how sharp the turns, light of a
frequency roughly in the middle of the
band gap cannot escape.
The nearly leakproof lattices guide
approximately 95 percent of the light
within them, compared to approximately
30 percent for conventional optical
waveguides. And they can turn the light on
a dime: they take up only one-tenth to
one-fifth the space required by conventional waveguides to bend the
light. The Sandia photonic lattice's turning radius is currently in the
one-wavelength range, rather than the traditional waveguide bend of
more than 10 wavelengths.
Researchers have attempted to build practical "photonic band gap
structures" since the idea was first proposed in 1987 by Eli
Yablonovitch, now a professor at the University of California at Los
Angeles. The first photonic crystal, which he built in 1990, was the
size of a baseball and could channel the microwaves useful in
antenna applications. In the mid-1990s, scientists at Iowa State
University and the nearby Department of Energy's Ames Laboratory
built crystals the size of Ping-Pong balls, also for microwaves. They
were assembled by hand from the common straight metal pins used
by tailors. Another group headed by J. D. Joannopoulos at the
Massachusetts Institute of Technology was also pursuing similar
goals.
Sandia's advancement was in taking the crystals down into the
nano-realm. The present device, which was constructed by Sandia
researchers Shawn Lin and Jim Fleming, functions in the infrared
range (wavelengths of approximately 10-micron) and can be used to
enhance or better transmit infrared images. "We had built the same
structure, but more than 100 times larger. It is quite remarkable that
Shawn Lin's group could do it at this size," says Rama Biswas, a
researcher at Ames Lab.
But Lin isn't stopping with the infrared. The next step, already
under-way, is a 1.5-micron crystal--the region in which almost all the
world's optically transmitted information is passed. Other photonics
scientists are confident that the Sandia team will achieve its goal.
"With the structure Lin is using now, he'll be able to hit the mark
within the next 12 months," predicts Pierre Villeneuve, a member of
MIT professor Joannopoulos's group who has theorized about uses for
photonic crystals.
The reason for the confidence of
Villeneuve and others is the fabrication
technique employed by the Sandia
group. Lin and Fleming were able to
draw on technology that Sandia has
perfected for making
micromachines--tiny gears and wheels
carved out of silicon using variations of
the techniques that produce computer
chips. A silicon wafer like those used in
semiconductor manufacturing was
coated with silicon dioxide. Then
trenches were etched into the silicon
dioxide and filled with polysilicon.
The chip was polished and another
layer added on top, this time with the
trenches at right angles to those on the
layer below. After repeating the process
a number of times, the silicon dioxide was etched away with
hydrofluoric acid, leaving a lattice of polysilicon bars that were 1.2
microns wide and 1.5 microns high, with a pitch of 4.8
microns--identical to a structure predicted by Ames Laboratory
researchers necessary to make the photonic equivalent of a band gap
for electrons. Tens of thousands of these devices can be fabricated
on a single, six-inch silicon wafer.
If the Sandia workers make their 1.5-micron-frequency mark, the
development will pave the way to tinier, cheaper, more effective
optical waveguides, sensors, lasers--and even make optical
computers a reality. Because little light is lost in the
three-dimensional mirroring that sends light back at itself, a new
type of microlaser requiring little start-up energy is theoretically
achievable.
In addition, photonic crystals will be a boon to researchers trying to
develop computers that utilize photons instead of electrons. Photons
are faster and cooler than electrons, but no one has been able to
bend useful frequencies of light around the tight corners needed to
navigate the million turns on a computer chip the size of a postage
stamp. In communications the devices will make it easier to separate
data carried on various frequencies on a combined stream of white
light that passes through optical fibers.
The Sandia discovery was first revealed in the July 16, 1998, issue of
Nature. Laboratory officials say they have applied for a patent on the
new devices and have already been contacted by at least one venture
capitalist eager to commercialize the technology.
Will this new way of manipulating light rival the transistor?
--By Alan Hall
RELATED LINKS:
Photonic crystal research at MIT
Photonic crystal research at Iowa State University
Background paper by J. D. Joannopoulos
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This photonics field LUMM is entering is
interesting, fascinating, and hopefully
very profitable for all concerned.
Major advances such as that outlined above
will speed the introduction of photonic
devices into various data handling and
transmission applications IMO.
Cheers,
Don
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