Amorphous diamond-structured photonic crystal in the feather barbs of the scarlet macaw

Abstract

Noniridescent coloration by the spongy keratin in parrot feather barbs has fascinated scientists. Nonetheless, its ultimate origin remains as yet unanswered, and a quantitative structural and optical description is still lacking. Here we report on structural and optical characterizations and numerical simulations of the blue feather barbs of the scarlet macaw. We found that the sponge in the feather barbs is an amorphous diamond-structured photonic crystal with only short-range order. It possesses an isotropic photonic pseudogap that is ultimately responsible for the brilliant noniridescent coloration. We further unravel an ingenious structural optimization for attaining maximum coloration apparently resulting from natural evolution. Upon increasing the material refractive index above the level provided by nature, there is an interesting transition from a photonic pseudogap to a complete bandgap.

Fig. 1.

(A) Optical micrograph of scarlet macaw blue feather barbs. Barbs rather than barbules display a vivid noniridescent blue color. (B) Optical micrograph of the transverse cross-section of a blue barb under 100× magnification. (C) Corresponding cross-sectional SEM image of the barb. (D) Close-up cross-sectional SEM image of the spongy keratin structure. (E) Cross-sectional image of an artificially generated model structure based on an atomic model of amorphous silicon (17).

Fig. 2.

(A) Calculated PDOS for the model RAD-PC as a function of reduced frequency d/λ, where d is the rod length and λ is the vacuum wavelength. The PDOS for a homogeneous medium with refractive index n = 1.23, a volume-weighted average, is also given for comparison. (B) Normalized calculated and measured reflection spectra under normal incidence. The calculations were based on observed d = 170 nm.

Fig. 3.

(A) Calculated relative PDOS dip for the model RAD-PC for different rod-volume fractions. Insets are cross-sectional images of the model RAD-PC with volume fractions of 20%, 38%, and 65%. (B) Calculated peak reflectance for the RAD-PC slab with various volume fractions. Circles are calculated data, and the lines are a guide to the eye.

Fig. 4.

Calculated optimal rod-volume fraction of RAD-PCs as a function of the refractive index of the rods. Insets show the PDOS for two cases: one with a photonic pseudogap at refractive index n = 2, and the other with a complete photonic bandgap at n = 3. The threshold for a complete photonic bandgap in these amorphous structures is n ∼ 2.3.