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  • Beetle Scales Ideal Crystals
May 2008
SALT LAKE CITY, May 20, 2008 -- The ideal photonic crystal, a diamond-like structure that manipulates visible light, has eluded creation in the lab, but now chemists have discovered that nature has already done the work by forming the 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," said 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."
This inch-long Brazilian beetle accomplished a task that so far has stymied human researchers. University of Utah chemists determined the beetle glows iridescent green because it evolved a crystal structure in its scales that is like the crystal structure of diamonds. Such a structure is considered an ideal architecture for photonic crystals that will be needed to manipulate visible light in ultrafast optical computers of the future. (Photo courtesy Jeremy Galusha, University of Utah)
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 that could bring ultrafast optical computers closer to reality.

“Nature uses very simple strategies to design structures to manipulate light, structures that are beyond the reach of our current abilities,” chemistry doctoral student and team member Jeremy Galusha said.

This isn't the first time scientists have looked to nature for its photonic designs. The tiny photonic scales that help color a Morpho peleides butterfly's wings have been studied for use as biotemplates in fabricating photonic structures such as waveguides (See: Butterfly Wing is Template for Photonic Structures and Butterfly Wings Work Like LEDs), and the ultrawhite Cyphochilus beetle is teaching researchers how to produce brilliant white ultrathin materials and more efficient white light sources (See: Unusually White Beetle Gives Scientists Bright Ideas).

The Utah researchers ordered the beetle from a Belgian insect dealer because Brigham Young University (BYU) student Lauren Richey wanted to examine an iridescent beetle for a project but lacked a complete specimen.

The beetle’s sparkling green color is produced by the crystal structure of its scales, not by any pigment, Bartl said. 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-µm (millionths of a meter) long by 100-µm wide. A human hair is about 100-µm thick.

Green light, with a wavelength of about 500 to 550 nanometers (billionths of a meter), cannot penetrate the scales’ crystal structure, which acts like mirrors to reflect it instead, making the beetle appear iridescent green.

Bartl said 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. The question was: What was the exact three-dimensional structure that produces these unique optical properties?” he said.
Microscopic image showing individual scales attached to the exoskeleton of the beetle Lamprocyphus augustus, and how the scales glow iridescent green because their fingernail-like material has a diamond-like crystal structure that reflects green light. University of Utah chemists are among researchers seeking to create a material with the same structure, which is considered ideal for future ultrahigh-speed optical computers powered by light instead of electricity. (Image courtesy Michael Bartl, University of Utah)
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, he said.

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 the chitin is not stable enough for long-term use, is not semiconducting and doesn’t bend light adequately.

Researchers need photonic crystals as part of their goal 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 fiber optic 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 powered by light.

“You would be able to solve certain problems that we are not able to solve now,” Bartl said. “For certain problems, an optical computer could do in seconds what regular computers need years for.”

Ideal photonic crystals could also be used to amplify light and 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 nonclassic ways,” Bartl said. 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.

He said 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 other scientists 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 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,” Bartl said.

Galusha determined the 3-D structure of the scales using a scanning electron microscope. He cut a cross section of a scale and took an electron microscope image of it. He then 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 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.

Scientists don’t know how the beetle uses its color, but “because it is an unnatural green, it’s likely not for camouflage,” Bartl said. “It could be to attract mates.”

The Utah chemists conducted the study with coauthors Richey, BYU biology professor John Gardner and Jennifer Cha of IBM’s Almaden Research Center in San Jose, Calif. Their work will appear this week in the journal Physical Review E and was funded by the National Science Foundation, American Chemical Society, the University of Utah and BYU.

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A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very...
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
Pertaining to optics and the phenomena of light.
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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