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, 100 (22), 12576-8

Coloration Strategies in Peacock Feathers

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Coloration Strategies in Peacock Feathers

Jian Zi et al. Proc Natl Acad Sci U S A.

Abstract

We report the mechanism of color production in peacock feathers. We find that the cortex in differently colored barbules, which contains a 2D photonic-crystal structure, is responsible for coloration. Simulations reveal that the photonic-crystal structure possesses a partial photonic bandgap along the direction normal to the cortex surface, for frequencies within which light is strongly reflected. Coloration strategies in peacock feathers are very ingenious and simple: controlling the lattice constant and the number of periods in the photonic-crystal structure. Varying the lattice constant produces diversified colors. The reduction of the number of periods brings additional colors, causing mixed coloration.

Figures

Fig. 1.
Fig. 1.
Scanning electron microscope images of barbule structures. (A) Transverse cross section of the green barbule. The outer cortex layer contains a periodic structure. The central part is the medullar layer. Transverse cross section of the cortex under higher magnification is shown for the green (B) and brown (C) barbules. The surface of the cortex is a thin keratin layer. Beneath the surface keratin layer, there is a 2D photonic crystal-like structure. This 2D photonic crystal is made up of an array of melanin rods connected by keratin. The remaining hollows are air holes (dark gray). Melanin rods are parallel to the cortex surface. The melanin rods embedded in the surface keratin layer can be clearly seen. (D) Longitudinal cross section of the green barbule with the surface keratin layer removed.
Fig. 2.
Fig. 2.
Measured reflectance at normal incidence. (A) Measured reflectance of differently colored barbules for E polarization. Blue, green, yellow, and brown lines indicate the results of the blue, green, yellow, and brown barbules, respectively. (B) Measured reflectance of the green barbule with (dark green) and without (green) glycerine infiltrated for E polarization. (C) Measured reflectance of the green barbule for E (green) and H (dark green) polarizations.
Fig. 3.
Fig. 3.
Calculated photonic band structure of an infinite 2D photonic crystal for E (solid lines) and H (dashed lines) polarizations. Frequency is in units of c/a, where c is the speed of light in vacuum and a is the lattice constant. (Inset) The irreducible Brillouin zone is shown. Γ, X, and M are the center, edge center, and corner of the first Brillouin zone, respectively. Note that the Γ–X direction is along the direction normal to the cortex surface.
Fig. 4.
Fig. 4.
Calculated reflectance of generic 2D photonic-crystal structures with a finite number of periods for E polarization. Structure parameters are taken from the measurements. Results for the blue, green, yellow, and brown barbules are indicated by the correspondingly colored lines, and their periods are 10, 10, 6, and 4, respectively.

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