How the Eurasian Jay Expands its Color Palette by Optimizing Multiple Scattering

Abstract

Unlike pigmentary colors, structural colors do not require wavelength-selective absorption of light. Instead, they result from light scattering by nanostructures with a spatial correlation on the length scale of visible wavelengths. Hence, the morphologies of nanostructures critically influence structural colors. Color-producing biophotonic nanostructures have diverse morphologies and are classified into three categories: i) ordered, [3] ii) quasi-ordered, [4] and iii) disordered. [5] Ordered nanostructures produce iridescent colors, while quasi-ordered and disordered nanostructures produce noniridescent colors. [1b,4a,6] Quasi-ordered and disordered nanostructures are particularly interesting, as they preserve viewing-angleindependent optical responses.Biophotonic nanostructures, which produce noniridescent structural colors, are intrinsically disordered. [7] Disordered nanostructures inherently display a white color. However, in some cases, nanostructures display bright, saturated colors. In this instance, they are neither highly ordered nor disordered nanostructures; thus, they are called quasi-ordered nanostructures. Some bird feathers produce colors in this manner: in blue and some green feather barbs, quasi-ordered nanostructures of β-keratin and air in the medullary spongy layer produce brilliant noniridescent colors via coherent light scattering. Research published several decades ago suggested that noniridescent blue colors originate from nanostructured materials via incoherent scattering. [8] As early as 1934, however, Raman's experimental results cast doubt on this hypothesis. [9] Decades later, Dyck supplemented Raman's argument and falsified the incoherent scattering hypothesis, [10] proposing a new hypothesis of coherent scattering. Prum supported this coherent scattering hypothesis by performing a Fourier analysis of 2D transmission electron microscopy (TEM) images [11] and small-angle X-ray scattering (SAXS) patterns. [12] Their results convincingly explain that noniridescent colors in bird feathers result from the coherent scattering of light by quasi-ordered nanostructures.Well-designed disordered nanostructures in nature show extraordinary, brilliant whiteness. Optimally disordered nanostructures in white beetles (Cyphochilus and Lepidiota stigma) display brilliant whiteness (greater than 70% for visible wavelengths). [5,13] The intrascale nanostructures are composed of randomly interconnected chitin networks. Chitin networks possess optimal local order or disorder and structural anisotropy,The production of structural color in nature is still incompletely understood. Multiple scattering exerts critical effects on synthetic disordered systems, but its effects on structural colors in natural materials are not yet well known. Here, electron microscopy, optical modeling, and biomimicry are used to show that variation in the thickness of the feather nanostructures creates periodic color variations in Eurasian jay wing covert feathers, with nanostructures within white feather regions being two t...