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.structural color | amorphous photonic structure P hotonic structures of diverse forms have evolved and have been exploited in the biological world to achieve structural coloration (1-5) including ordered structures such as thin films, multilayers, diffraction gratings, and photonic crystals. Ordered photonic structures can produce iridescent structural colors whose coloration mechanisms have been intensively studied and are well understood. For instance, iridescent coloration by photonic crystals is due to their direction-dependent partial photonic bandgaps (6-8). In addition to the ordered categories, there exists another important class of photonic structures that possess only short-range order, namely, amorphous photonic structures (9) that can produce noniridescent coloration. The best known example is the spongy keratin structure in parrot blue feather barbs whose color origin has fascinated scientists (10-16).Incoherent scattering, such as Rayleigh (10) or Mie scattering (1,11,14), was proposed first. Raman opposed the hypothesis of Mie scattering based on his optical observations (12). Dyck challenged the Rayleigh model (13) by the fact that measured reflection spectra disobeyed the prediction of the Rayleigh law and suggested a hypothesis of coherent scattering. Prum and coworkers confirmed convincingly the hypothesis of coherent scattering by performing a Fourier analysis (15) and small-angle X-ray scattering (16). Their results indicated that the sponge possesses short-range order that leads to coherent scattering and the noniridescent coloration.Although noniridescent coloration by the sponge can be conceptually understood by coherent scattering, some fundamental questions remain still to be answered. Here we study the spongy structure in the blue feather barbs of the scarlet macaw (Ara macao) through structural characterization, spectral measurement, and numerical simulation. We aim to uncover the ultimate physical origin of the noniridescent coloration and to give a quantitative description of the structure and its optical response. Our results may help us obtain an in-depth understanding of the ingenious st...

Scales on the elytra of longhorn beetle Anoplophora graafi display diverse non-iridescent colors ranging from blue, green, yellow, and red to purple. By structural characterizations, optical measurements, and theoretical calculations, we found that the scale colors stem from an amorphous photonic structure possessing only short-range order: random close-packing of chitin nanoparticles. Our results showed that direction-independent photonic pseudogaps found in the photon density of states of the random close-packing photonic structure are the ultimate physical origin for non-iridescent coloration of scales. The color steering strategy of scales is ingenious, simply by varying the size of chitin nanoparticles. Revealed natural random close-packing photonic structures and the color steering strategy of scales could render valuable inspiration for the artificial fabrication and design of photonic structures and devices as well.

We conducted structural characterizations, reflection measurements, and theoretical simulations on the iridescent green and purple neck feathers of domestic pigeons (Columba livia domestica). We found that both green and purple barbules are composed of an outer keratin cortex layer surrounding a medullary layer. The thickness of the keratin cortex layer shows a distinct difference between green and purple barbules. Green barbules vary colors from green to purple with the observing angle changed from normal to oblique, while purple barbules from purple to green in an opposite way. Both the experimental and theoretical results suggest that structural colors in green and purple neck feathers should originate from the interference in the top keratin cortex layer, while the structure beyond acts as a poor mirror.

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We report detailed optical measurements and numerical simulations of brown barbules in male peacock tail feathers. Our results indicate that brown coloration is predominantly produced structurally by the two-dimensional (2D) photonic-crystal structure in the cortex layer of a barbule. The constructing strategies of brown coloration revealed by numerical simulations are indeed subtle, which are of great significance in the artificial constructions of mixed structural coloration. It is found that the structural configurations of the 2D photonic-crystal structure such as the lattice constant, the number of periods, and even the interdistance and missing holes between the two melanin layers nearest to the cortex surface, are important in the production of structural brown colors.

The elytra of male beetles Chlorophila obscuripennis (Coleoptera) display an inconspicuous iridescent bluish green color. By structural characterizations we find that the outermost elytral surface comprises a sculpted multilayer, which is the origin of structural coloration. In elytra both structural green and cyan colors are observed which arise from the modulations imposed on the multilayer, leading to a bluish green color by color mixing. The adoption of the sculpted multilayer can render structural coloration inconspicuous, which could be advantageous for camouflage. In addition, it can cause light emergence at nonspecular angles.

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Metalenses have emerged as a new optical element or system in recent years, showing superior performance and abundant applications. However, the phase distribution of a metalens has not been measured directly up to now, hindering further quantitative evaluation of its performance. We have developed an interferometric imaging phase measurement system to measure the phase distribution of a metalens by taking only one photo of the interference pattern. Based on the measured phase distribution, we analyse the negative chromatic aberration effect of monochromatic metalenses and propose a feature size of metalenses. Different sensitivities of the phase response to wavelength between the Pancharatnam-Berry phase-based metalens and propagation phase-reliant metalens are directly observed in the experiment. Furthermore, through phase distribution analysis, it is found that the distance between the measured metalens and the brightest spot of focusing will deviate from the focal length when the metalens has a low nominal numerical aperture, even though the metalens is ideal without any fabrication error. We also use the measured phase distribution to quantitatively characterise the imaging performance of the metalens. Our phase measurement system will help not only designers optimise the designs of metalenses but also fabricants distinguish defects to improve the fabrication process, which will pave the way for metalenses in industrial applications.