Reversed diffraction in bio-inspired photonic materials

Certain butterflies, beetles, and other organisms have evolved intriguing strategies to manipulate their outward appearance through structural coloration, in which perceived color arises from periodic features in the underlying morphology. (See, for instance, the Quick Study by Peter Vukusic, Physics Today, October 2006, page 82 .) Four years ago, Jean Pol Vigneron and colleagues at the University of Namur in Belgium reported a surprising property of male butterflies of the species Pierella luna: A coin-sized iridescent region of their wings will, like a diffraction grating, spectrally decompose incident white light, but the color sequence is the reverse of what one would expect—violet light emerges at a shallow angle, closer to the wing, while red light emerges at a steeper angle, closer to the wing’s perpendicular. A close examination of the wing’s scales revealed the cause: The scales’ ridged tips curl up, away from the wing, forming in essence a vertical transmission grating. Inspired by P. luna, Grant England, Joanna Aizenberg, and colleagues at Harvard University have fabricated an ordered array of vertically oriented microdiffraction gratings, shown here. Each grating mimics a wing scale, and the scallops reproduce the color reversals. But the combination of length scales—the 500-nm periodicity of each micrograting and the orthogonal, micron-scale spacing of the grating array—generates further richness and complexity in the structure’s optical signature. For example, the researchers found that tilting the grating pillars could provide a way to dynamically tune the diffraction pattern. They anticipate that hierarchically structured photonic materials will provide a broad platform for further exploring novel behavior and applications. (G. England et al., Proc. Natl. Acad. Sci. USA 111, 15630, 2014, doi:10.1073/pnas.1412240111 .)