Butterfly-inspired photonics reverse diffraction color sequence.

Biological systems have been an infinite reservoir of inspiration ever since humans started to develop tools and machinery. Just as early scientists and engineers attempted to mimic birds and fish in the development of flying machines and submarines, today, new technologies find their inspiration from biology, such as gecko feet (1), antireflective eye lenses (2), iridescent insects (3), and waterrepellant surfaces (4). Incorporating biological systems and concepts into technological design can happen in several ways: inspiration, mimicking, and replication (5). In the latter, entire organisms or body parts are directly used, and their structural features are replicated into another compound (6–8). Examples include 3D photonic crystals from iridescent beetle scales (9, 10) or antireflective microlens arrays from insect eyes (11, 12). In contrast, in bioinspiration and biomimicry, a biological function or activity— rather than the organism itself—is converted into an artificial, human-made material or device. In PNAS, England et al. report an optical micrograting array inspired by a photonic structure found in iridescently colored butterfly wings (13). The authors demonstrate a micrograting array that not only mimics the unique diffraction properties of the biological structure with reversed colororder sequence, but also can be designed to tune these optical properties. The concept of structural colors is an omnipresent optical phenomenon in biological systems and refers to colors produced due to the interaction of light with nanoto-microscale structures built into various body parts of insects, birds, marine animals, and plants (3, 14). Structural colors are the result of diffraction and specular reflection of light, in contrast to typical pigments that produce color by light absorption and diffuse reflection. Fossil finds date the first examples of structural colors to some 500 million years ago, and the earliest examples of photonic structures in these fossil records were discovered within insect hairs and spines in the form of multilayers and gratings (15). Although these early structures most likely were the result of random mutations, the accompanying coloration effects must have presented evolutionary advantages in camouflaging, signaling, communicating, and mimicking (16). Today we find an enormous variety of photonic structures in biology, including deformable gradedindex lenses, photonic crystals, and various diffraction gratings (2, 3, 17–19). A particularly interesting diffraction element was discovered several years ago in the wings of the butterfly Pierella luna when Vigneron et al. reported that light hitting certain parts of the wings of this butterfly is diffracted in reversed diffraction color-mode sequence compared with a conventional diffraction grating (20). In a conventional grating, white light striking the grating is decomposed such that shorter wavelengths are diffracted less than longer ones; i.e., blue wavelengths exit with an angle closer to specularity followed by green, yellow, and red (Fig. 1A). This sequence is reversed when white light is decomposed by interaction with the diffractive elements (specific cuticle scales) located on the butterfly wings. Here, red wavelengths exit at angles closer to the direction perpendicular to the wing plane, whereas yellow, green, and blue exit at progressively increasing angles (Fig. 1B). The reason for this reversed diffraction behavior lies in the curved shape of each diffracting scale, which positions the grating periodicity (nano-ribs) perpendicular to the wing surface (Fig. 1D). Such a “vertical” grating operates in transmission mode, in contrast to the Fig. 1. Schematic illustration of diffraction properties and grating types. (A) Color-mode sequence obtained from a conventional horizontal diffraction element. (B) Reversed color-mode sequence of diffracted light observed from diffraction elements located on the wings of the butterfly Pierella luna. (C ) Schematic depiction of the structure of a conventional horizontal diffraction grating operating in reflection mode. (D) Schematic depiction of a curved butterfly wing scale with a vertically oriented diffraction element operating in transmission mode. (E ) Schematic depiction of the bioinspired artificial vertical diffraction element fabricated by England et al. (13). DE, diffraction element; DL, diffracted light; WL, white light. Author contributions: M.H.B. wrote the paper.