Natural photonic engineers

The structural colors of animals have provided an attractive means of advertisement for those reflectors designed and manufactured by humans. Over the past 30 years, optics well-known to the physicist have been emerging, example after example, in nature. But suddenly animals are becoming the first stage in the optical design process. Biologists and physicists have begun serious optics-based projects where results will be supplied by evolution. But it is the recent finding of photonic crystals in animals that has really triggered a surge in interest. Animals, it would seem, have plenty to teach us, not only in terms of the design of their optical structures, but also in their engineering.At a recent car show in Sydney, the demonstration of a new four-wheel-drive revealed a novel type of paint as well. The vehicle appeared blue from the front…but changed entirely to green when viewed from behind! The manufacturers had replaced pigments with ‘structural colors’. The secret behind this coating lies in its self-positioning reflector. Small fragments of thin-films, which cause colored reflections, are mixed within a matrix of transparent paint. When the paint is applied, the thin-films float to the surface so that all are orientated similarly, parallel with the car body, and reflect the same wavelength of light in each direction. As the direction of observation changes, different wavelengths are observed. These thin-film particles are, essentially, butterfly scale analogues.