Xishuang Banna, a semitropical prefecture located in the south of China near the borders with Laos and Myanmar, is known in China as the “home of the peacock.” Visitors might return home with local fruits, combs made from water buffalo horns, or peacock feathers, such as the one shown in figure 1.
Jian Zi, a physicist at Fudan University in Shanghai, returned from his trip to Banna with an idea for a new research project. “I was really astonished by the stunning beauty of the peacock’s eye pattern. I asked myself, What structures could cause such beautiful colors?” Last October, he, Xiaohan Liu, and other colleagues from Fudan published scanning electron microscope images of some of those structures and described experimentally and theoretically how details in structural form lead to the spectacular patterns that peacocks display to attract peahens. 1
The idea that coloration might arise from structural elements as opposed to pigments is not new. In Opticks, Isaac Newton related iridescence (the change in observed color with viewing angle) to optical interference. In the 1920s, chemical engineer Clyde Mason looked at bird feathers through an optical microscope and reported observations of structural elements responsible for their coloration. Nowadays, scientists find nanoscale architectures in an astonishing variety of plants and animals. 2
Andrew Parker (University of Oxford) and colleagues reported in 2001 that the spines covering the back of the sea mouse Aphrodita consist of a natural photonic structure. 3 The index of refraction varies periodically within such a structure, and as a result, the frequencies of light that can propagate through the structure are confined to so-called photonic bands. (See the article by Sajeev John in Physics Today, Physics Today 0031-9228 44
The eyes have it
About a year before the work of Zi and company, Shinya Yoshioka and Shuichi Kinoshita of Osaka University in Japan reported observations of photonic structures in peacock feather barbules—the straplike “twigs” that come off the branches of a peacock feather. 4 The Osaka researchers looked at the photonic arrays found in yellow and blue barbules and noted that the two arrays differ in both the length and number of their periods. They also backed up their observations with calculations relating structure and color.
The Fudan researchers went further. They considered the structures that reflect the four principal colors—blue, green, yellow, and brown—seen in peacock tails. They also took into account the interference of light that reflects off the front surface of a structure with light that reflects off the back before emerging through the front. That Fabry–Perot interference plays a particularly important role in generating a peacock’s brown color.
The lower panel in figure 1 shows the photonic structure in a green barbule. The dark gray holes are air voids in an array of melanin cylinders. Each cylinder is surrounded by a thin layer of keratin, which gives the lattice its structural integrity and also affects the color reflected by the photonic material. The melanin cylinders form a square array with a spacing of about 150 nm. Some 10 periods fill the region between the barbule’s thin keratin surface and its central core. The lattice in blue barbules is very similar and has a period of 140 nm. Yellow barbules contain a square lattice with a spacing of 165 nm, but with about six periods, which is significantly fewer than in green and blue barbules. The lattice in brown barbules is rectangular rather than square and includes only about four periods.
Because the green and blue barbules have a relatively large number of periods in their photonic structures, they reflect light well and the reflected light is confined to a relatively narrow wavelength band. They also display relatively little Fabry–Perot interference in their reflectance spectra. Figure 2 plots the measured reflectances of the various barbules. Because the broad band associated with yellow barbules ranges from orange to green wavelengths, a peacock’s yellow is not a spectral hue. Part of the peacock’s yellow comes from a small Fabry–Perot contribution at around 450 nm. The keratin scaffolding of the photonic structure plays an important role in setting the location of the interference peak. Absent the keratin, explains Zi, the interference peak would be on the red side of the broad reflection peak.
The reflection spectrum of the brown barbules shows a pronounced interference peak whose contribution is a significant component of the observed color. The structural origin of the peacock’s brown coloration is particularly interesting, notes Pete Vukusic of the University of Exeter in England, because browns in the natural world are generally caused by pigmentation. Zi and colleagues, though, observed iridescence in the brown regions of peacock feathers, a signature of structural color. They also dipped the feathers in glycerin to plug up the air holes in the photonic lattice and change its bandgap structure. After doing so, they confirmed a change in the reflectance spectrum of the brown barbules. Still, melanin qua pigment may be partially responsible for the brown color.
The photonic structures seen in the natural world are, in some ways, not as sophisticated as lab-made photonic crystals. The periodic structure shown in figure 1, for example, is visibly imperfect. On the other hand, emphasizes Joanna Aizenberg of Lucent Technologies’ Bell Labs, natural processes yield nanometer-scale photonic arrays without using harsh chemicals or expensive materials and without expending a great deal of energy. Further, natural self-assembly operates at normal temperature and pressure. Thus, Nature may be able to teach scientists a new approach to the fabrication of technologically useful photonic structures. And in terms of achieving its own goals, notes Aizenberg, Nature does a wonderful job.