Structural and Photonic Properties of Butterfly Wings Replicated

Lauren I. Rugani

The wings of the Morpho Peleides butterfly may not affect weather patterns across the world, but researchers from Georgia Institute of Technology in Atlanta and Zhejiang University in Hangzhou, China, have discovered that they may influence the fabrication of low-cost, reproducible photonic integrated circuits based on their structural similarities to both a waveguide and a beamsplitter.

An alumina-coated butterfly wing shows different colors caused by changes in the thickness of the coating layer. After removal of the wing template, the optical properties of the wing are preserved. This presents a novel approach for fabricating complex nanostructures using a biological system as a template for achieving unique physical properties. Images courtesy of Xudong Wang and Zhong Lin Wang, Georgia Institute of Technology.

The wings have periodic structures on several levels, beginning with millions of individual scales ∼150 mm long and ∼60 mm wide. Each scale comprises 35 to 40 rows of lamellae evenly spaced about 1.6 mm apart. In turn, each lamella is structured with 1.6-mm-long main ribs spaced 60 nm apart that are bound by sub-ribs 50 nm apart. The periodicity of the resulting unit cells can be considered a two-dimensional photonic crystal, with a photonic bandgap that is responsible for the natural blue/violet color of the wings.

The researchers replicated the wing structure through atomic layer deposition with Al₂O₃. With a growth rate of 1 Å per cycle, varying the number of deposition cycles enabled the team to control the layer thickness. As the thickness increased from 10 to 40 nm in 10-nm intervals, the reflected color of the alumina-coated wing changed from its original blue to green, yellow, orange and pink, respectively. The wing was then removed from the coating, leaving an inverse replica of the periodic nanoscale structure, as exhibited by both scanning electron microscopy and transmission electron microscopy images.

Along with complete replication of the morphologies, the technique also preserved the optical properties of the wing scales. The original wing displayed a reflection at 390 nm. With the 40-nm-thick alumina coating, the peak reflection occurred at ~600 nm, making the wing appear pink. However, when the wing was removed, the alumina replica exhibited optical properties that were much closer to the original, with a reflection peak at 420 nm. This is evidence of a photonic bandgap within the structure, which may again be characterized as a two-dimensional photonic crystal.

The researchers discovered that the scale replica could function as both a waveguide and a beamsplitter, two key components in photonic integrated circuits. The unit cells formed by the main and sub-ribs in the original wing became air nanocylinders connected by a channel at the bottom in the replica. Light of the proper wavelength can propagate within this channel while the nanocylinders act as line defects, essentially creating a waveguide along the lamellae. As the scale widens, the lamellae bifurcate to form natural beamsplitting devices.

Arranging these two components thus would allow the realization of an efficient photonic integrated circuit. Unlike current complex and expensive techniques, the wings provide millions of scales that can be used to fabricate integrated circuits with lower cost and higher reproducibility.

Nano Letters, published online Sept. 28, 2006.