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...