The blue coloration of Morpho butterflies has anomalously low angular dependence despite the production of color with a selected wavelength based on an interference effect. A key to the mechanism of the specific Morpho-color was suggested to be the randomness of its scale. Using finite-difference time-domain (FDTD) analysis, the role of different kinds of randomness in the structure of the Morpho butterfly's scale was investigated, which was impossible by conventional analytical calculations. The results revealed that incoherence in the incident light plays an essential role, which cannot be realized only by structural randomness. On the other hand, the lateral and vertical randomness, and the number of random components were found each to have an independent role to realize the specific Morpho-color preventing the sharp reflective angular dependence. The direction obtained by the numerical simulations to analyze optically complex random structures will serve not only to understand the scientific principles, but also to design the optical properties of artificial materials.

Strucural color of some blue Morpho butterflies has a physically mysterious feature, because it has both high reflectivity (>60%) and a single color in too wide angular range (> ± 40° from the normal), which are contradicting with each other from viewpoint of the interference phenomena. We have recently proven the principle of the mystery by extracting the physical essence, and emulating the nano-structures using nano-fabrication techniques. The key was exquisite combination of regular and random structures at nanometer scale. Such artificial structural color was found to concern wide applications, because the Morpho-color can produce a single color without pigment in wide angular range with high reflectivity. Also it makes colors impossible by pigment, and is resistant to fading due to chemical change over longtime. However, we must overcome several "death valleys" for wide industrial applications. A serious problem was low throughput in the fabrication process of nano-patterning by the conventional lithography, which was solved by use of nano-imprinting technique. Next research step is attempts at a precise control of the optical properties, in both terms of real space and wavelength, i.e. angular distribution and spectrum. We could successfully optimize the optical properties by controlling the parameters of nano-structures in the artificial film. In this process, optical simulations and micro-structural observations were taken in account. The optimization was achieved both theoretically and empirically by comparing a series of films fabricated with different nano-patterns. Also the relationship between the structural parameters and the optical properties was analyzed. The reflective characteristics of the optimized film were found to reproduce the optical properties closer to the natural Morpho-blue than the prototypes.

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218 1 大阪大学大学院工学研究科（〒565 0871 大阪府吹田市山田丘 2 1）  2 理化学研究所・播磨研究所/SPring 8（〒679 5148 兵庫県佐用郡佐用町光都 1 1 1）  3 JST さきがけ（〒102 0075 東京都千代田区三番町 5） Conspicuous metallic blue of Morpho butter‰ies is well known and attract interest because it is a brilliant luster of natural beings. The blue is produced by their proteins, which are almost transparent without pigment. The origin of the coloration with high re‰ectivi-ty (＞～60) is then attributed to an interference eŠect based on a periodic structure. However, the interference contradicts the blue that is maintained in too wide angular range (＞±40°from the normal). This mystery has recently been explained with a speciˆc multilayer, which is aˆne combination of regular and random structures at nanometer scale. We proved this hypothesis successfully by emulating the 3D structures by deposition of multilayerˆlm on a nano-patterned substrate. Such artiˆcial structural color can be applied to various industries, because it makes colors qualitatively impossible by pigment, and resistant to fading due to chemical change over longtime. Also we developed a high throughput nano-patterning process by use of nano-imprinting method, and succeeded in controlling the optical properties both in angular and wavelength distribution.