gem channels light
Technology Research News
The fire within opals is caused by light
diffracting off tiny spheres of silica trapped inside the stones. NEC
Research Institute and Princeton University researchers have turned the
natural process of opal formation on its head to produce a photonic crystal
that traps light.
The material has the potential to manipulate photons of light in the same
way that electrical components like semiconductors
control electrons. This ability could change the face of computer
circuitry as we know it, fostering faster computers and cheaper telecommunications
Although data and voice signals are transmitted using pulses of light
carried over fiber-optic cables, these pulses can only be directed in
a fairly coarse way compared to how computer circuits control electrical
This is because semiconductors have electron bandgaps that channel the
flow of electrons. It has been more difficult to manufacture materials
that have photonic bandgaps, however. "All of modern-day electronics revolves
around semiconductors. The dream for photonic bandgap materials is that
manipulation of the photonic bandgap will lead to a similar revolution
in photonics," said David Norris, a researcher at NEC research.
The researchers' inside-out opal method allowed them to form a crystal
with a photonic bandgap, which blocks part of the light spectrum but is
otherwise transparent. If a crystal had a blue photonic bandgap, for example,
it would allow all colors of light except blue to pass through.
This is useful because if photons of the blocked wavelength of light were
placed inside a small void, or hole, in the middle of a material with
a photonic bandgap, the light would be indefinitely trapped.
Being able to trap and channel light opens the door to simpler and cheaper
devices for controlling signals in telecommunications networks, said Norris.
Today's networks use computers to route light signals, but each time a
light signal is redirected, it must be converted to an electrical signal
that the computer can examine and redirect, then back to a light signal
to flow across the fiber-optic line.
Photonic bandgap crystals could also eventually be used to make all the
components of a computer chip, which would make them much faster than
today's electronic versions.
In nature, opals form spontaneously when millions of minute silica spheres
stack up. The laboratory process and ingredients for artificial gemstone
formation are the same. But to make a photonic bandgap opal, the researchers
inverted the gemstone process, replacing the spheres with space and the
gaps between the spheres with silicon. "By filling the space between the
spheres with a semiconductor and then selectively removing the spheres,
[we made] an extremely porous material," said Norris.
The researchers formed the inverted opals directly on a silicon wafer;
they resemble honeycombs with thousands of round, rather than hexagonal,
The process is tricky because the spherical spaces must be precise in
order to form a photonic bandgap. In order to trap the light used in telecommunications
networks, the inverted opals had to contain spherical spaces about one
micron in diameter, which is one fifth the size of a red blood cell. A
micron is one thousandth of a millimeter.
Though the particles used to make the spaces are microscopic, smaller
particles are actually easier to work with. Previous attempts at making
inverted photonic bandgap opals have been unsuccessful because it is difficult
to position these 1-micron spheres precisely enough. The researchers solved
the problem by making the spheres flow in a temperature-induced current,
which kept the spheres mobile as they settled into their crystal formation.
The work is a clever demonstration that self-assembled crystals of spherical
particles can be ordered into nearly defect-free structures, said Vincent
Crespi, an associate professor of physics at Pennsylvania State University.
In order to allow optical materials to channel and guide light like the
guiding of electrical signals in regular circuits, they require a high
degree of perfection, Crespi said. "Through a clever induction of convective
motion in the fluid that supports the assembling particles, the [researchers]
keep these relatively large particles mobile enough to find their optimal
positions in the growing crystal," he said.
Because the material is self-assembled, it is potentially much cheaper
to make than today's computer chips, which require "extremely expensive
lithographic equipment," said Crespi. Lithography uses a combination of
chemicals and light to etch lines into silicon.
The crystal films seem to be of high quality, said Kai-Ming Ho, a professor
of physics at Iowa State University. "Many groups have tried to make these
inverse-opal photonic crystals, but usually the crystals have so many
defects that only [a] pseudogap appears," he said. The researchers crystals
contain a true, three-dimensional bandgap, he said.
The fabrication process is inexpensive and manufacturers should be able
to use it to make the devices in volume, said Norris. The researchers'
next step is to use the photonic bandgap crystals in a practical device.
"We still have a long way to go with our approach," he said. It will be
five or six years before the field of photonic bandgap materials in general
has a practical impact, he added.
Norris's research colleagues were Yurii Vlasov at NEC and James Sturm
and Xiang-Zheng Bo at Princeton University. They published the research
in the November 15, 2001 issue of the journal Nature. The research was
funded by NEC Corporation.
Timeline: 5-6 years
TRN Categories: Materials Science and Engineering; Optical
Computing; Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "On-Chip Natural Assembly
of Silicon Photonic Bandgap Crystals," Nature, November 15, 2001.
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