Structural color[1,2], also known as physical color, is a kind of luster caused by the wavelength of light. When the light is incident on an object with a similar structure period of wavelength, the color is generated by the scattering, interference or diffraction of light. The surface of the organism usually has some non-smooth structures. Light spreads in the micro-structure of the organisms, and it is easy to form structural colors. For example, the colorful scales on the surface of butterfly wings are typical structural colors.

Butterfly[3], belongs to arthropoda, insecta, lepidoptera, hammerhorn suborder animals collectively. There are about 14,000 species in the world, which is mostly distributed in the americas, especially in the amazon basin. There are 1200 species in China. Butterflies are generally brightly colored, with many stripes on the body and rich colors. The wings and body have all kinds of spots. The largest butterfly can reach about 28 ~ 30 centimeters with wings, and the smallest one is only about 0.7 centimeters. The color class of butterfly surface is divided into two categories. One is pigment color. The other is structural color. The colorful patterns on butterfly wings are generally used to hide, camouflage and attract mates.

The cuticle on the scales of these butterflies’ wings is composed of nano- and microscale, transparent, chitin-and-air layered structures. Rather than absorb and reflect certain light wavelengths as pigments and dyes do, these multi scale structures cause light that hits the surface of the wing to diffract and interfere. Cross ribs that protrude from the sides of ridges on the wing scale diffract incoming light waves, causing the waves to spread as they travel through spaces between the structures. The diffracted light waves then interfere with each other so that certain color wavelengths cancel out (destructive interference) while others are intensified and reflected (constructive interference). The varying heights of the wing scale ridges appear to affect the interference such that the reflected colors are uniform when viewed from a wide range of angles. The specific color that’s reflected depends on the shape of the structures and the distance between them. This way of manipulating light results in brilliant iridescent colors, which butterflies rely upon for camouflage, thermoregulation, and signaling.

Artificial structural color materials have also been extensively applied in optoelectronics, as well as in the biochemical and medical areas. As with periodic dielectric units, the structural color materials, termed “photonic crystals” (PhCs), can control the propagation of photons and generate a so-called photonic band gap. By tailoring the PhCs with certain surface microstructures, these structural color materials were imparted with angle-independence, and could thus be used to create decoration, coating, painting, and display units. Through the introduction of defects or cavities, light flow can be guided or trapped, thus creating waveguides or photonic circuits for photonic communication. Such a photonic device can also be integrated with electronic devices, which shed light on optoelectronic applications. Besides guiding the light, structural color materials also sense light. By incorporating responsive polymers into the photonic crystal structural framework, such materials can distinguish and detect a wide range of stimuli through different degrees of color shift, leading to important applications in anti-counterfeiting technology and smart sensors [8]. It is worth noting that, responsive structural color materials have recently attracted considerable attention in biomedical fields, serving as spectroscopic barcodes, label-free sensors, and cell microcarriers for multiplex biomolecule and cell assays [9] and [10].

[1]    https://zh.wikipedia.org/wiki/%E7%BB%93%E6%9E%84%E8%89%B2

[2]    https://baike.baidu.com/item/%E7%BB%93%E6%9E%84%E8%89%B2

[3]    https://baike.baidu.com/item/%E8%9D%B4%E8%9D%B6/5811

[4]    https://asknature.org/strategy/wing-scales-cause-light-to-diffract-and-interfere/

[5]    https://www.materialstoday.com/amorphous/articles/s136970211600095x/

[6]    C. Fenzl, et al. Angew. Chem. Int. Ed., 53 (2014), pp. 3318-3335

[7]    Y.J. Zhao, et al. Acc. Chem. Res., 47 (2014), pp. 3632-3642

[8]    K.R. Phillips, et al. Chem. Soc. Rev., 45 (2016), pp. 281-322

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