Brazilian ‘Photonic Beetle’ Points Way to Ultrafast Computing
Scientists have discovered that their research to build photonic crystals was redundant -- Mother Nature had already produced them with the ideal diamond-like structure, to decorate a beetle's carapace.
Alexander E. Braun, Senior Editor -- Semiconductor International, 9/8/2008 9:10:00 AM
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| This gaudy-looking beetle managed to do what researchers could not: evolve a crystal structure in its scales considered as ideal for photonic crystals, which will be crucial for future ultrafast optical computers. (Source: J. Galusha, University of Utah) |
“Photonic crystals are a completely new class of optical materials that enable the manipulation of light in non-classic ways,” explained Michael Bartl, assistant professor of chemistry and adjunct professor of physics at the University of Utah (Salt Lake City). “Some colors (wavelengths) of light can pass through such a crystal at various speeds, while others are reflected as if the crystal were acting as a mirror.”
Bartl and his group, which includes researchers from Brigham Young University (BYU, Provo, Utah), have been trying to make 3-D photonic crystals with a full photonic bandgap, a concept similar to that of an electronic bandgap, to keep light of a specific frequency from penetrating the crystal. “Proven research in physics demonstrates that if you have such a material, it’ll give you the potential for things like increasing the efficiency of photovoltaic cells, to very low-threshold lasing (light amplification), as well as for establishing optical qbits, which are the cornerstone of some optical computing,” Bartl said.
The Utah researchers had been trying to make different structures with optimized, high-quality architectures. “We struggled,” Bartl said. “Lithography is limited when it comes to 3-D, 100 nm feature size structures. If you use self-assembly techniques with self-assembled silica spheres to produce a photonic opal, you end up with a closed structure; you cannot open the full photonic bandgap with a photonic opal, which is what we are interested in.”
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| SEM image of a cross-sectional view of the photonic crystal structure found in the scales of Lamprocyphus augustus, the green iridescent beetle. (Source: J. Galusha, University of Utah) |
So where does the green arthropod come in? Lauren Richey, now a BYU student, had done a high school science fair project on iridescence in biology. A BYU group was helping her then, and they got her the bug. “The beetle was interesting because it was iridescent regardless of the viewing angle,” Bartl recalled. “We wondered what 3-D structure could produce such unique optical properties.”
The Utah team’s preliminary electron-microscope investigation revealed that the insect’s scales did not have the structure typical of artificial photonic crystals. Using a focused ion beam, 150 cross-sections were made of one of the beetle’s scales in a process akin to tomography, and then digitally put it back together using imaging software. They found out that a single scale is not a continuous crystal, but includes some 200 pieces of chitin, each with the diamond crystal structure, but oriented in different directions, each reflecting a slightly different wavelength of green. “The scale material’s structure had a champion architecture, but used chitin and air instead of the carbon atoms in diamond,” Bartl said. Next, the researchers applied optical studies and theories to predict the properties of the scales’ structure, and their prediction matched reality — green iridescence in the 500-550 nm range.
“We’re working to design a synthetic version of the beetle’s photonic crystals, using the scale structures we discovered on it as a mold for the crystal,” Bartl said. “Although the biological structure is very nice, it has the disadvantage of being made out of chitin, which would not work in any technological application. We want the same structure, but made out of a semiconducting material with a high refractive index.”
The group is currently considering titanium dioxide. It is a wide-bandgap semiconductor transparent to visible light, meaning that it does not absorb photons, something that would result in data loss in a hypothetical optical computer. Another advantage is that the material also has a high dielectric constant and has already been used in things like photovoltaic cell electrodes.
Although the application of photonic crystals is just now beginning, there are many possible uses: as sensors in metrology devices, as light amplifiers to make photovoltaic cells more efficient, to capture light that catalyzes chemical reactions, and to generate small lasers to serve as light sources on optical chips.
The materials that would allow the creation of perfect photonic crystals to manipulate visible light do not yet exist, whether natural or synthetic. They will have to be developed. However, the researchers are presently preparing their results for publication, and appear quite pleased with what they will report.
