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.
SALT LAKE CITY — 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.
Quest for the Ideal or ”˜Champion’ Photonic Crystal
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.
On the Path of the Beetle: From BYU to Belgium and Brazil
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 beetle scale is not a continuous crystal, but includes some 200
pieces of chitin, each with the diamond-based crystal structure but each
oriented a different direction. So each piece reflects a slightly different
wavelength or shade of green.
“Each piece is too small to be seen individually by your eye, so what you
see is a composite effect,” with the beetle appearing green from any angle,
Bartl explains.
Scientists don’t know how the beetle uses its color, but “because it is an
unnatural green, it’s likely not for camouflage,” Bartl says. “It
could be to attract mates.”
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