Proc. Natl Acad. Sci. USA 111, 15630–15634 (2014)

When looking at a typical diffraction-grating surface (such as a compact disc), colours with increasing wavelength appear more prominent at increasing observation angle. This is not the case for a spot on the dorsal side of the wings of male butterflies of the family Pierella luna: the coloured spots have periodically arranged curled-up scales, each with a ribbed nanostructure perpendicularly aligned to the wing's surface, that act as a diffraction grating with inverse colour order (the spots change from red to blue as the observation angle increases). Mimicking P. luna's diffraction-grating structure, Joanna Aizenberg and colleagues have now fabricated grating arrays of nanostructured micrometre plates that display an inverse sequence in diffracted colour. The researchers also show that the diffraction pattern of the artificial photonic structure can be controlled by varying both the pitch of the ribbed patterns on the plates and the interplate distances, and that it can be dynamically modulated by changing the tilt angle of the plates (through mechanical shearing). The work should inspire the development of advanced photonic devices.