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Bug cracks photonic-crystal mystery

Posted: 26 May 2008 ?? ?Print Version ?Bookmark and Share

Keywords:photonic crystal? optical computing? optical chip?

University of Utah researchers suggests that IC makers should take a hint from Mother Nature when pursuing photonic crystals for optical computing. The researchers studying the Brazilian beetle claims the bug's eerie iridescence is evidence of its unique photonic lattice structure!called the "champion" architecture in photonic circles.

Diamonds have it, but they cannot act as photonic crystals because their atoms are packed too tightly together. By replacing the diamond's carbon atoms with cylindrically shaped molecules spaced to match a single wavelength of light, the beetle's scales could hold the key to solving long-standing problems with fabricating 3D diffraction gratings within photonic crystals.

"We are currently utilizing the knowledge we discovered by studying the beetle's scales, and are now finding a way to make the same structure in a much more relevant material!an inorganic semiconductor!about which we hope to announce details later this summer," said University of Utah professor, Michael Bartl.

'Champion' architecture
Since the 1990s, when photonic crystals were most recently popularized, the ideal, or "champion," architecture has been sought for the filtering ability of its band-gap, which depends on the spacing between nodes in a lattice. Just as silicon's band-gap enables transistors to perform not just data-communication tasks, but also amplification and computation, likewise photonic crystals with band gaps should be able to amplify and compute with light instead of electrons. Photonic crystals can also make solar cells more efficient, directing specific wavelengths of light to catalyze chemical reactions, and ultimately enable the fabrication of on-chip lasers for all-optical chips.

Unfortunately, the illusive photonic crystal semiconductor has been easier to imagine than to actually construct. One- and two-dimensional photonic crystalline-like architectures have been fabricated with planar chip processing steps, but 3D photonic crystals required tedious construction methods that have so far met with limited success.

The University of Utah researchers claim to have found a better method by following the example of nature!the iridescent green scales of an inch-long weevil named Lamprocyphus augustus. By dissecting its scales into more than 150 cross-sections, each recorded with an electron microscope, the researchers discovered the secret of its construction technique, and claim to be currently realizing that structure in a semiconductor.

3D green
"The iridescent green scales of the beetle manipulate light by creating a band gap!effectively reflecting just green light and being transparent to all other wavelengths," said Bartl. "The architecture is that of a 3D photonic lattice arranged so that all the other wavelengths besides green can pass through it."

The beetle's scales' overall architecture uses the same symmetry as the atoms in a diamond's lattice, but here the atoms are replaced by cylindrically shaped building blocks made from the organic material, chitin, with air gaps in between. The spacing of the chitin building blocks lets all wavelengths through, except 500- to 550nm (green), acting like a perfect mirror reflecting those wavelengths back, which makes the beetle appear iridescent.

The researchers claim to have discovered why green gets reflected no matter from which angle you view the material!namely, because each 200? x 100? scale is actually composed of over 200 layers, each with the diamond-based crystal structure oriented in a slightly different direction. This three-dimensional hierarchical structure, according to Bartl, extends from the individual chitin molecules to super-molecules to the cylindrical building blocks.

After extensive computer analysis of the chitin's organic crystalline architecture, the researchers have a model that predicts the green reflective powers of its photonic crystal, using its node shape, lattice period, spacing and other parameters. Now that their model is predicting green for the beetle's scales, the researchers hope to use the model to design new photonic characteristics for a transparent semiconductor material. The idea is to get the model to tell them how to build it in 3D using planar semiconductor fabrication tools, such as layer-by-layer molecular beam epitaxy.

Also contributing to this research are University of Utah doctoral candidate Jeremy Galusha; member of the technical staff at IBM's Almaden Research Center Jennifer Cha; Brigham Young University professor John Gardner; and BYU student Lauren Richey.

Bartl's group was funded by the National Science Foundation, the American Chemical Society, the University of Utah and Brigham Young University.

- R. Colin Johnson
EE Times





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