Beetle solves photonic-crystal mystery that's bugged researchers

Seeing 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 three-dimensional 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-550 nanometers (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-by-100 micron 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 (San Jose, Calif.) 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.