Butterflies guide optical design

17 Jun 2002

The microscopic structure that gives colour to some butterfly wings could inspire novel devices for telecommunications, data storage and solar cells.

In South Africa, Johan Brink and colleagues are studying the cynandra opis and golden plusia butterflies and the mango moth, all indigenous to various parts of Africa. Brink explained, "These insects do not use pigmentation at all, but instead produce their exceptionally striking colours purely by interference effects from some micro-structure on their wing scales."

Different species exploit different phenomena to paint their wings. For example, a mango moth's wings are covered with a stack of layered thin films, similar to the anti-reflect ion coatings on camera lenses. The colour changes from green to yellow to orange and purple by simply varying the spacing of the layers. This 'structural colour' also varies with angle of view.

In the other two species, the coloration is caused by the many fine straight edges on the wings that act like a complex diffraction grating. In the case of the blue cynandra opis butterfly the 'grating' consists of two sets of veins on top of each other. The wing normally appears dull dark brown, but when backlit it acquires a deep blue colour. As the wing flaps in the sunlight it sends out eye-catching blue flashes. Only the male insects possess the 'grating' so this colouring may be to attract females.

The structure that produces the colours is made from the insect protein chitin, which is exceptionally strong. This means that the colours cannot fade and are highly resistant to wear and tear. If this structure could be applied to large surfaces, it would be a great alternative to paint.

The study of insects' optical properties may also help information storage. "The herring-bone type of grating on the wings of the golden plusia, for example, produces very strong polarization effects from a structure which is much smaller than a wavelength of light. It might be possible to simulate this effect by acousto-optic modulators and store zeros and ones by polarization."

By changing the surface design a highly reflective surface becomes non-reflective. This could have applications as light collectors for solar cells. Another possible application is an insect repellent that works by mimicking the appearance of the insect wings.

SH