Optically Buggin'

It's an overcast ugly day here, so this morning while waiting for the sea shanty festival to start, I thought I should get something more serious posted to the site. This one made the rounds of the physical and material science news sites a few weeks ago when the original press release went out. After the paper was actually published I was able to find some time to read it —

"Bugs" and computing are intimately linked throughout computing history (and electronic engineering in general). Moths were the nemesis of early vacuum tube computer operators as they would fry themselves on the circuits and tubes, occasionally causing the tube to fail as well (er...that's the moths that would fry themselves, not the operators). Even before then "bugs" and "debugging" were common vernacular for engineers as they are now for all coders. While it's doubtful that "bugs" or "debug" will exit the jargon, recent discoveries in photonic computing materials are leading to a better reputation for bugs in the computer engineering world.

Researchers at Brigham Young University, IBM'S Almaden Research Center, and the University of Utah have discovered that the weevil Lamprocyphus augustus has scales whose internal structure of chitin is arranged in the same configuration as carbon atoms in diamonds — a lattice configuration that has been described as the ideal configuration for the photonic crystals needed to power future optical computers. Natural diamonds are too dense to use to manipulate visible light. Even though there have been advances in synthetic diamond crystals, none of them have the desired visible light spectrum properties.

Photonic crystal structure of L. augustus
courtesy of Michael Bartl

The weevil's scales are 200µm by 100µm and composed of hundreds of chitin crystals, each with a slightly different orientation. All of them reflect light at 500 to 550nm wavelengths (green), but because of the orientation variation, each crystal reflects a slightly different wavelength back to the viewer, giving the beetle an overall iridescent green color. Because of the structure, the different chitin crystal orientations and their extremely small size, the iridescent effect appears from all angles the beetle is viewed. This is unusual for an iridescent material. Like all iridescent materials the color is not from a pigment, but from the base structure of the material; however, with most iridescent materials, the color and effect changes depending on incident light and viewing angles.

So what?

The lure of using this beetle's scales in photonics is in the structure and iridescent effect of the chitin crystals. The structure matches the "ideal photonics crystal" and the iridescent effect demonstrated is the effect photonics engineers are after — creating a tunable bandgap that will selectively block transmission of certain wavelengths of light through a circuit. The authors hope that their dissection, and reconstruction of the crystals 3D structure will aid in the creation of a synthetic crystal with similar properties, which would help advance optical computing and solar power applications.

Of interest to me is how this research actually came about. It began when one of the authors, Lauren Richey, was pursuing a high school science fair project on biological iridescence. She had a small sample of the weevil and recognized its potential as an iridescent insect, but needed a complete sample for her project. BYU biology professor and co-author John Gardner and his lab were assisting her, while University of Utah grad student and co-author Jeremy Galusha was using the BYU electron microscope and heard about the project. Galusha brought the project to the attention of his advisor and co-author Michael Bartl, a physical and materials chemist with strong interest in photonic crystals.

After getting a complete specimen and conducting painstaking SEM ablation techniques, they were able to unwrap the structure of the chitin crystals of the weevil's scale. Using the chitin as a mold, they may be onto the crystal structure of future photonics. Time will tell...but if it works out, we'll have the weevil L. augustus and a high school science fair project to thank.

For more details on the photonics and materials science aspects see the University of Utah press release, or Physical Review E, where the paper was published online 10 days after the press release, and most reviews, went out.

Galusha, J.W., Richey, L.R., Gardner, J.S., Cha, J.N., Bartl, M.H. (2008). Discovery of a diamond-based photonic crystal structure in beetle scales. Physical Review E, 77(5) DOI: 10.1103/PhysRevE.77.050904