Nature’s Photonic Crystal

Researchers have discovered a species of Brazilian beetle that has the unusual trait of reflecting iridescent green from almost any angle. By examining the structure of the beetle’s scales, scientists at the University of Utah found an ideal photonic-crystal structure for visible light–a type of material that optical scientists have been seeking for years.

Three-dimensional periodic structures called photonic crystals are potentially valuable materials for controlling photons; scientists could use photonic crystals operating at visible wavelengths to develop more-efficient solar cells, telecommunications, sensors, and even optical computer chips. A diamond-based structure, in particular, is thought to be the most effective three-dimensional photonic crystal for visible light, because it can reflect a wide band of colors and has high reflectivity. Less light escaping means researchers can better control and manipulate the photons.

Photonic crystals that control visible light have been challenging for scientists to fabricate from appropriate materials, because of how small the periodicity in the structure must be to manipulate wavelengths that short. One- and two-dimensional photonic crystals for visible light have been created, as well as a three-dimensional diamond structure for the longer wavelengths of infrared. A diamond structure that can reflect visible light over all angles for all polarizations has not yet been made. But studying this beetle’s scales may provide new insights into how to construct such a three-dimensional photonic crystal for visible light.

Michael Bartl, a professor at the University of Utah, graduate student Jeremy Galusha, and their colleagues used a very thin slicing technique to discover and model the scales of the Lamprocyphus augustus. Inside each scale, which is about 100 micrometers across and 15 to 20 micrometers thick, is a three-dimensional photonic structure. The structure resembles how carbon atoms arrange in a diamond, and it consists of a crystal lattice with a repeating periodic unit structure of about 300 nanometers, says Bartl. Within a scale, the diamond lattice is positioned at different orientations, giving the beetle its green sheen from almost any angle.

The diamond-structured photonic crystals are among the most difficult to fabricate, says Georgia Tech professor Zhong Lin Wang. “Using biology as a template, this paper shows the possibility of fabricating man-made diamond photonic crystals with well-designed optical performance,” he says.

The beetle’s scales themselves can’t be used for any practical application, because the chitin material is too fragile and not conductive. The group is in the process of molding the beetle scales out of a semiconductor. “We’re making good progress,” says Bartl. Besides using the beetle structure as a mold, he and his colleagues are also studying how the beetle fabricates the structure, in hopes of mimicking the process to create artificial diamond photonic structures.

Applications using photonic crystals “have been more or less restricted to the near infrared spectrum,” says Ayman Abouraddy, a research scientist at MIT. “We already know [that the diamond structure] will be useful; we just don’t know how to make it efficiently. The fact that a beetle–with a down-and-dirty chemical synthesis approach–is able to create quite a clean structure like this is surprising.”