Credit: P. VUKUSIC, UNIV. EXETER

In man-made optical emitters such as LEDs, paradoxically most of the light remains trapped inside. So physicists have recently begun to exploit structures known as photonic crystals to try to extract more light. But while they have been trying to forge the most efficient devices, it now turns out that the swallowtail butterfly has already mastered the art of emitting light.

The eye-catching Papilio nireus butterfly from eastern and central Asia has wings emblazoned with iridescent blue-green patches (pictured). Pete Vukusic and Ian Hooper have used electron microscopy to learn more about the optical properties of these spots (Science 310, 1151; 2005). Their images reveal that the scales on the butterfly's wings contain an intricate nanostructure of all-natural photonic crystals.

The crystals are just one part of a sophisticated photonic system that provides intense, directed light and endows the butterfly with its striking colours. A two-dimensional photonic crystal ‘slab’, about two micrometres thick, is set within the solid outer wing layer, and consists of hollow cylinders arranged in triangular domains. Highly fluorescent pigment, whose peak excitation wavelength matches that of the blue of the sky, is infused throughout the slab. And layered structures, known as Bragg reflectors, sit about 1.5 micrometres below the crystals.

Photonic crystals can stop light of specific frequencies from propagating within them. In this way, they can be used to limit or inhibit the emission of an optical emitter that is embedded deep within the crystal.

The optical properties of the crystals in this butterfly's scales are such that emission from the fluorescent dye is inhibited only within the crystal plane. Added to the effects of the underlying Bragg reflectors, this means the fluorescent light has nowhere to go but up and out. The effect is clearly seen in the optical-microscope image of a single scale (inset). Notably, similar techniques are used in the latest ultra-efficient LEDs.

An astonishing variety of natural photonic structures are being uncovered not just in butterflies, but also in other insects, birds and fish. Although nature and technology have evolved independently, they have ultimately come up with the same design.