Researchers have been unable to build an ideal
“photonic crystal” to manipulate visible light,
impeding the dream of ultrafast optical computers. But now, University
of Utah chemists have discovered that nature already has
designed photonic crystals with the ideal, diamond-like structure: They
are found in the shimmering, iridescent green scales of a beetle from
Brazil.
 | | Researchers have been unable to build an ideal “photonic crystal” to manipulate visible light, impeding the dream of ultrafast optical computers. But now, University of Utah chemists have discovered that nature already has designed photonic crystals with the ideal, diamond-like structure: They are found in the shimmering, iridescent green scales of a beetle from Brazil |
“It appears that a simple creature like a beetle
provides us with one of the technologically most sought-after
structures for the next generation of computing,” says study
leader Michael Bartl, an assistant professor of chemistry and adjunct
assistant professor of physics at the University of Utah.
“Nature has simple ways of making structures and materials
that are still unobtainable with our million-dollar instruments and
engineering strategies.”
The study by Bartl, University of Utah chemistry doctoral
student Jeremy Galusha and colleagues is set to be published later this
week in the journal Physical Review E.
The beetle is an inch-long weevil named Lamprocyphus augustus.
The discovery of its scales’ crystal structure represents the
first time scientists have been able to work with a material with the
ideal or “champion” architecture for a photonic
crystal.
“Nature uses very simple strategies to design
structures to manipulate light – structures that are beyond
the reach of our current abilities,” Galusha says.
Bartl and Galusha now are trying to design a synthetic version
of the beetle’s photonic crystals, using scale material as a
mold to make the crystals from a transparent semiconductor.
The scales can’t be used in technological devices
because they are made of fingernail-like chitin, which is not stable
enough for long-term use, is not semiconducting and doesn’t
bend light adequately.
The University of Utah chemists conducted the study with
coauthors Lauren Richey, a former Springville High School student now
attending Brigham Young University; BYU biology Professor John Gardner;
and Jennifer Cha, of IBM’s Almaden Research Center in San
Jose, Calif.
Researchers are seeking photonic crystals as they
aim to develop optical computers that run on light (photons) instead of
electricity (electrons). Right now, light in near-infrared and visible
wavelengths can carry data and communications through fiberoptic
cables, but the data must be converted from light back to electricity
before being processed in a computer.
The goal – still years away – is an
ultrahigh-speed computer with optical integrated circuits or chips that
run on light instead of electricity.
“You would be able to solve certain problems that we
are not able to solve now,” Bartl says. “For
certain problems, an optical computer could do in seconds what regular
computers need years for.”
Researchers also are seeking ideal photonic crystals to
amplify light and thus make solar cells more efficient, to capture
light that would catalyze chemical reactions, and to generate tiny
laser beams that would serve as light sources on optical chips.
“Photonic crystals are a new type of optical
materials that manipulate light in non-classic ways,” Bartl
says. Some colors of light can pass through a photonic crystal at
various speeds, while other wavelengths are reflected as the crystal
acts like a mirror.
Bartl says there are many proposals for how light could be
manipulated and controlled in new ways by photonic crystals,
“however we still lack the proper materials that would allow
us to create ideal photonic crystals to manipulate visible light. A
material like this doesn’t exist artificially or
synthetically.”
The ideal photonic crystal – dubbed the
“champion” crystal – was described by
scientists elsewhere in 1990. They showed that the optimal photonic
crystal – one that could manipulate light most efficiently
– would have the same crystal structure as the lattice of
carbon atoms in diamond. Diamonds cannot be used as photonic crystals
because their atoms are packed too tightly together to manipulate
visible light.
When made from an appropriate material, a diamond-like
structure would create a large “photonic bandgap,”
meaning the crystalline structure prevents the propagation of light of
a certain range of wavelengths. Materials with such bandgaps are
necessary if researchers are to engineer optical circuits that can
manipulate visible light.
The new study has its roots in Richey’s science fair
project on iridescence in biology when she was a student at
Utah’s Springville High School. Gardner’s group at
BYU was helping her at the same time Galusha was using an electron
microscope there and learned of Richey’s project.
Richey wanted to examine an iridescent beetle, but lacked a
complete specimen. So the researchers ordered Brazil’s
Lamprocyphus augustus from a Belgian insect dealer.
The beetle’s shiny, sparkling green color is
produced by the crystal structure of its scales, not by any pigment,
Bartl says. The scales are made of chitin, which forms the external
skeleton, or exoskeleton, of most insects and is similar to fingernail
material. The scales are affixed to the beetle’s exoskeleton.
Each measures 200 microns (millionths of a meter) long by 100 microns
wide. A human hair is about 100 microns thick.
Green light – which has a wavelength of about 500 to
550 nanometers, or billionths of a meter – cannot penetrate
the scales’ crystal structure, which acts like mirrors to
reflect the green light, making the beetle appear iridescent green.
Bartl says the beetle was interesting because it was
iridescent regardless of the angle from which it was viewed –
unlike most iridescent objects – and because a preliminary
electron microscope examination showed its scales did not have the
structure typical of artificial photonic crystals.
“The color and structure looked
interesting,” Bartl says. “The question was: What
was the exact three-dimensional structure that produces these unique
optical properties"”
The Utah team’s study is the first to show that
“just as atoms are arranged in diamond crystals, so is the
chitin structure of beetle scales,” he says.
Galusha determined the 3-D structure of the scales using a
scanning electron microscope. He cut a cross section of a scale, and
then took an electron microscope image of it. Then he used a focused
ion beam – sort of a tiny sandblaster that shoots a beam of
gallium ions – to shave off the exposed end of the scale, and
then took another image, doing so repeatedly until he had images of 150
cross-sections from the same scale.
Then the researchers “stacked” the images
together in a computer, and determined the crystal structure of the
scale material: a diamond-like or “champion”
architecture, but with building blocks of chitin and air instead of the
carbon atoms in diamond.
Next, Galusha and Bartl used optical studies and theory to
predict optical properties of the scales’ structure. The
prediction matched reality: green iridescence.
Many iridescent objects appear that way only when viewed at
certain angles, but the beetle remains iridescent from any angle. Bartl
says the way the beetle does that is an “ingenious
engineering strategy” that approximates a technology for
controlling the propagation of visible light.
A single beetl
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