The self‐assembly of films that mimic color‐producing nanostructures in bird feathers is described. These structures are isotropic and have a characteristic length‐scale comparable to the wavelength of visible light. Structural colors are produced when wavelength‐independent scattering is suppressed by limiting the optical path length through geometry or absorption.

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We investigate the mechanism of structural coloration by quasi-ordered nanostructures in bird feather barbs. Small-angle X-ray scattering (SAXS) data reveal the structures are isotropic and have short-range order on length scales comparable to optical wavelengths. We perform angle-resolved reflection and scattering spectrometry to fully characterize the colors under directional and omni-directional illumination of white light. Under directional lighting, the colors change with the angle between the directions of illumination and observation. The angular dispersion of the primary peaks in the scattering/reflection spectra can be well explained by constructive interference of light that is scattered only once in the quasi-ordered structures. Using the Fourier power spectra of structure from the SAXS data we calculate optical scattering spectra and explain why the light scattering peak is the highest in the backscattering direction. Under omni-directional lighting, colors from the quasi-ordered structures are invariant with the viewing angle. The non-iridescent coloration results from the isotropic nature of structures instead of strong backscattering.

Non-iridescent structural colours of feathers are a diverse and an important part of the phenotype of many birds. These colours are generally produced by three-dimensional, amorphous (or quasi-ordered) spongy b-keratin and air nanostructures found in the medullary cells of feather barbs. Two main classes of three-dimensional barb nanostructures are known, characterized by a tortuous network of air channels or a close packing of spheroidal air cavities. Using synchrotron small angle X-ray scattering (SAXS) and optical spectrophotometry, we characterized the nanostructure and optical function of 297 distinctly coloured feathers from 230 species belonging to 163 genera in 51 avian families. The SAXS data provided quantitative diagnoses of the channel-and sphere-type nanostructures, and confirmed the presence of a predominant, isotropic length scale of variation in refractive index that produces strong reinforcement of a narrow band of scattered wavelengths. The SAXS structural data identified a new class of rudimentary or weakly nanostructured feathers responsible for slate-grey, and blue-grey structural colours. SAXS structural data provided good predictions of the singlescattering peak of the optical reflectance of the feathers. The SAXS structural measurements of channel-and sphere-type nanostructures are also similar to experimental scattering data from synthetic soft matter systems that self-assemble by phase separation. These results further support the hypothesis that colour-producing protein and air nanostructures in feather barbs are probably self-assembled by arrested phase separation of polymerizing b-keratin from the cytoplasm of medullary cells. Such avian amorphous photonic nanostructures with isotropic optical properties may provide biomimetic inspiration for photonic technology.

We demonstrate order of magnitude coherent control of total transmission of light through random media by shaping the wavefront of the input light. To understand how the finite illumination area on a wide slab affects the maximum values of total transmission, we develop a model based on random matrix theory that reveals the role of long-range correlations. Its predictions are confirmed by numerical simulations and provide physical insight into the experimental results. PACS numbers: 42.25.Bs, 05.60.Cd, 02.10.Yn A lossless strong scattering medium, that has a thickness L much larger than the elastic mean free path ℓ, is normally opaque to incident beams of light, with only a small fraction, ℓ/L, of the incident photon flux diffusively transmitted. However, it has been known for over two decades that, due to the coherence of elastic scattering, this transmitted flux is not totally random in character, but has subtle correlations that were first discovered in the context of mesoscopic electron transport [1][2][3][4]. One striking implication of these correlations is that an optimally prepared coherent input beam could be transmitted through a strong scattering medium hundreds of mean free paths in thickness with order unity efficiency. These highly transmitting input states are eigenvectors of the matrix t † t, where t is the transmission matrix (TM) of the sample. They were predicted using a random matrix theory approach [1-3] and were termed "open channels".Because the input electron states are not controllable in mesoscopic conductors, the open channel concept was not testable there, except indirectly through other properties such as conductance fluctuations or shot noise [5]. Experimental measurements of the TM through disordered waveguides at microwave frequencies are consistent with the theory developed for this geometry [6][7][8] and imply that open channels should exist, but enhanced transmission has not yet been directly demonstrated in these systems due to the difficulty of imposing an appropriate input waveform. The advent of wavefront shaping methods using a Spatial Light Modulator (SLM) at optical frequencies has reopened the search for this dramatic effect in strong scattering media. It has already been shown that wavefront shaping of input states combined with feedback optimization can enable diverse functions for multiple scattering media in optics [9], causing them to act as lenses [10,11], phase plates [12,13] or spectral filters [14,15]. However coherent control of total transmission, which is a non-local property of the TM, is much more difficult. Some progress in this direction has been made by studying the increase of the total transmission when focusing light through scattering media to wavelength scale spots [16] or by measuring the partial TM and injecting light into calculated singular vectors [17]. In addition, a very recent study highlights effects of the mesoscopic correlation on the transmission properties by measuring a large -but still not complete -TM [18]. We report here a furth...

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Bodies are often made of repeated units, or serial homologs, that develop using the same core gene regulatory network. Local inputs and modifications to this network allow serial homologs to evolve different morphologies, but currently we do not understand which modifications allow these repeated traits to evolve different levels of phenotypic plasticity. Here we describe variation in phenotypic plasticity across serial homologous eyespots of the butterfly Bicyclus anynana, hypothesized to be under selection for similar or different functions in the wet and dry seasonal forms. Specifically, we document the presence of eyespot size and scale brightness plasticity in hindwing eyespots hypothesized to vary in function across seasons, and reduced size plasticity and absence of brightness plasticity in forewing eyespots hypothesized to have the same function across seasons. By exploring the molecular and physiological causes of this variation in plasticity across fore and hindwing serial homologs we discover that: 1) temperature experienced during the wandering stages of larval development alters titers of an ecdysteroid hormone, 20-hydroxyecdysone (20E), in the hemolymph of wet and dry seasonal forms at that stage; 2) the 20E receptor (EcR) is differentially expressed in the forewing and hindwing eyespot centers of both seasonal forms during this critical developmental stage; and 3) manipulations of EcR signaling disproportionately affected hindwing eyespots relative to forewing eyespots. We propose that differential EcR expression across forewing and hindwing eyespots at a critical stage of development explains the variation in levels of phenotypic plasticity across these serial homologues. This finding provides a novel signaling pathway, 20E, and a novel molecular candidate, EcR, for the regulation of levels of phenotypic plasticity across body parts or serial homologs.

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