Butterflies shine brighter by design
Spongy wing scales help to channel light.
African butterflies have light-emitting wings that share a trick with high-tech light-emitting diodes (LEDs), researchers in Britain have found.
The swallowtail butterflies of eastern and central Africa (Princeps nireus) have bands of blue spots composed of fluorescent scales: these do not simply reflect light; they actively shine. And now physicists Pete Vukusic and Ian Hooper at Exeter University have discovered that the brightness of this emission is boosted by the structure of the wing scales, which channels the fluorescent light in a single direction away from the wing.
Vukusic, who specializes in the optical effects of butterfly wings, saw a description of these butterflies in a book and decided they were worth a closer look. So he and Hooper sent off for some samples. "The first thing we did was to stick them under an ultraviolet light," says Vukusic, to induce fluorescence. "They glowed as brightly as anything I've seen."
A photonic crystal is filled with an array of holes, making it impervious to light in a certain band of wavelengths. Such a crystal can be used to control light beams of those wavelengths: a tunnel passing through it, for example, acts like an optical fibre, because the light cannot escape from the channel by passing into the crystal walls. The crystal can similarly be used to shepherd light in a certain direction, rather than letting it spread out. This technology is used to make some state-of-the-art LEDs brighter.
Scientists make photonic crystals by painstakingly creating regular arrays of microscopic objects such as tiny plastic beads. But nature's imitation of the effect suggests that there might be an easier way.
Wing of light
The swallowtail butterfly uses the blue colour in its wings to signal to other members of its species. The wing scales in these patches are impregnated with natural pigments that absorb the deep blue component of sunlight and emit it as fluorescence with a slightly longer wavelength, in the blue to blue-green part of the spectrum. The butterflies' visual system is closely attuned to this colour.
Under the microscope, the scales of P. nireus have a spongy appearance, being perforated by roughly cylindrical holes from top to bottom.
The two researchers calculate that the photonic crystals of the butterflies' wing scales block light of precisely the colour that the pigment molecules emit. This suggests that the perforated structure has evolved to direct the emitted light outwards, preventing it from being absorbed in the wing, they report in Science1.
The opalescence of opal, as well as the bright blue of other butterfly wings, comes from a similar phenomenon. Vukusic has himself seen other examples of photonic crystals in butterfly wings, but had never before seen them used to boost fluorescence, which makes for an extra bright, startling effect.
Vukusic is excited because the discovery shows that a photonic structure works even if the array of holes is not perfectly regular.
"Nature has shown how little periodicity we can get away with," he says. Such 'imperfect' photonic crystals could be considerably easier to manufacture.
- VukusicP. & HooperI. Science, 310. 1151 (2005).
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