Butterfly wings could lead to new optics

Amanda D. Francoeur, amanda.francoeur@laurin.com

New optical technologies are taking flight, thanks to a technique that can replicate butterfly wings. The manner in which these wings are formed, and their properties of luminosity, could help researchers develop light-emitting devices with enhanced properties or antireflection coverings that increase light absorption in solar cells.

Through a microscope, a butterfly’s wing shows intricate surface features: thousands of colorful scales, both large and small, arranged elaborately in rows. Nanosize photonic structures on the cuticles determine the wing’s physical color and iridescence and provide it with the ability to appear metallic and to change tones at different angles.

“The technique that we have developed can be used to replicate biotemplates with micro- and nanoscale features distributed over planar and curved surfaces that could further the development of highly efficient photonic devices,” said Raúl J. Martìn-Palma, professor of physics at Universidad Autónoma de Madrid and an adjunct professor of materials science and engineering at Pennsylvania State University. “It is a simple, highly reproducible and inexpensive process for fabricating complex nanostructures with biologically inspired functionalities.”

Rotate, rinse and replicate

Martìn-Palma and fellow researchers Akhlesh Lakhtakia, the Charles Godfrey Binder professor of engineering science and mechanics, and Carlo G. Pantano, director of the Materials Research Institute, both at Penn State, applied their method of conformal evaporated film by rotation (CEFR) – a technique based on the combination of thermal evaporation with simultaneous substrate tilting and rotation – while a biological template (butterfly wing) rotated in a low-pressure chamber.

The researchers first coated the wing with a germanium, antimony and selenium (GeSbSe) compound, then thermally evaporated the material at a current of 80 A with a vapor flux aimed at the biotemplate at an angle of 82°. Meanwhile, they continuously rotated the substrate at 60 rpm. As a result, a GeSbSe chalcogenide glass coating formed on the wing at a thickness of about 500 nm.

They then immersed the wing in an 85 percent orthophosphoric acid solution for 72 hours to dissolve its chitinous exterior coating without damaging the outer surface, or the conformal coating. This resulted in a free-standing replica of the wing. Other conventional methods of removing the chitinous coating destroyed nanoscale surface features, while thermal treatment charred the wing.

Two scanning electron microscope images reveal the external structure of a butterfly wing coated with chalcogenide glass. The removal of the wing by submersion in orthophosphoric acid allows the conformal coating and chitinous coating to separate from the wing, resulting in a free-standing replica.

A Philips XL30 scanning electron microscope revealed the coated wing and free-standing replica’s scales as measuring approximately 200 × 50 μm. The lamellae, or ridges of the wing, were raised and approximately 2.5 μm apart and, between the lamellae, were fine tubes composed of netlike reticulum, or latticework. The scales “are intricately shaped with stratification, voids and grooves of complex shapes that result in several optical effects, such as interference, scattering and diffraction,” Martìn-Palma said.

Both the lamellae and reticulum supply the wing with its physical coloring, but because the sample is only several microns in size, color and iridescence could not be seen. So the researchers measured reflectance spectra >200 to 850 nm with a PerkinElmer Lambda 950 UV/VIS/NIR spectrophotometer to observe the optical behaviors in the visible regime of the coated wing and replica. They determined that the colors of each were similar to the other because of a reasonable correspondence between the two spectra in the visible regime. A report on their study was published June 25, 2009, in the journal Bioinspiration & Biomimetics.

Flying into the future

The investigators are trying to improve their process so as to eliminate all chitin from a sample and increase dissolution speed. They also hope to enlarge the replicas within a few months, which could amplify color and iridescence. By replicating the vibrant colors found in a natural wing, they could develop “swatches of fabrics that would sense magnetic fields, temperature changes [and more],” Lakhtakia said. “[The swatches of fabric] could absorb a gas and provide a color signal to indicate that a gas had been sensed.”

The researchers believe that the development of replicating eyes of flies or hornets using CEFR could improve angular vision for cameras, optical sensors, mobile telephones, displays, security surveillance systems and medical devices such as endoscopes.