The feathers of male peacocks are among nature's most striking displays with greens, blues and other vivid colours, but mixed in there is also brown. It is already known that the bright colours arise from natural photonic crystals - structures with a bandgap that allows transmission of light only at certain frequencies. But where does the brown colour come from?

It had been thought that in the biological world brown originates in pigments, but Yizhou Li and colleagues at Fudan University in Shanghai have extended their work on peacock feathers to find that the browns also arise from photonic crystals.

The team studied the brown barbules - structures on the barbs arrayed on either side of the central stem. They found a subtly different structure for the 2D photonic-crystal lattice in the outer layer (cortex) for the brown barbules compared with the blue, green and yellow. Optical measurements and numerical simulations revealed that differences in the two melanin layers in the photonic-crystal structure, as well as parameters such as lattice constant and number of periods, are important in introducing reflected orange and red components.

Studies of the complex natural structures producing brown could point the way to extending artificial photonic crystals from almost single-wavelength structures to multiple wavelengths.

Further reading

Yizhou Li et al. 2005 Structural origin of the brown color of barbules in male peacock tail feathers Phys. Rev. E 72 010902(R).