Butterfly-Inspired Photonics Reverse Diffraction Color Sequence

$Butterfly-Inspired Photonics Reverse Diffraction Color Sequence$ COMMENTARY Butterfly-inspired photonics reverse diffraction color sequence Michael H. Bartl1 and plants (3, 14). Structural colors are the Department of Chemistry, and Materials Research Science and Engineering Center, University result of diffraction and specular reflection of Utah, Salt Lake City, UT 84112 of light, in contrast to typical pigments that produce color by light absorption and dif- Biological systems have been an infinite 12). In contrast, in bioinspiration and bio- fuse reflection. Fossil finds date the first reservoir of inspiration ever since humans mimicry, a biological function or activity— examples of structural colors to some 500 million years ago, and the earliest examples started to develop tools and machinery. Just rather than the organism itself—is converted of photonic structures in these fossil records as early scientists and engineers attempted to into an artificial, human-made material or were discovered within insect hairs and spines mimic birds and fish in the development of device. In PNAS, England et al. report an in the form of multilayers and gratings flying machines and submarines, today, new optical micrograting array inspired by a pho- (15). Although these early structures most technologies find their inspiration from bi- tonic structure found in iridescently colored likely were the result of random mutations, ology, such as gecko feet (1), antireflective eye butterfly wings (13). The authors demon- the accompanying coloration effects must lenses (2), iridescent insects (3), and water- strate a micrograting array that not only have presented evolutionary advantages in repellant surfaces (4). Incorporating biologi- mimics the unique diffraction properties of camouflaging, signaling, communicating, cal systems and concepts into technological the biological structure with reversed color- and mimicking (16). Today we find an design can happen in several ways: inspira- order sequence, but also can be designed to enormous variety of photonic structures tion, mimicking, and replication (5). In the tune these optical properties. in biology, including deformable graded- latter, entire organisms or body parts are di- The concept of structural colors is an index lenses, photonic crystals, and various rectly used, and their structural features are omnipresent optical phenomenon in biolog- diffraction gratings (2, 3, 17–19). – replicated into another compound (6 8). ical systems and refers to colors produced A particularly interesting diffraction ele- Examples include 3D photonic crystals from due to the interaction of light with nano- ment was discovered several years ago in the iridescent beetle scales (9, 10) or antireflec- to-microscale structures built into various wings of the butterfly Pierella luna when tive microlens arrays from insect eyes (11, body parts of insects, birds, marine animals, 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 spec- ularity 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 pro- gressively 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 Author contributions: M.H.B. wrote the paper. a conventional horizontal diffraction grating operating in reflection mode. (D) Schematic depiction of a curved but- The author declares no conflict of interest. terfly 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, See companion article on page 15630. diffracted light; WL, white light. 1Email: [email protected]. 15602–15603 | PNAS | November 4, 2014 | vol. 111 | no. 44 www.pnas.org/cgi/doi/10.1073/pnas.1418292111 Downloaded by guest on September 25, 2021 reflection mode of a conventional grating go beyond merely reproducing the optical fraction pattern in terms of wavelength and COMMENTARY with periodicity parallel to the surface (Fig. effects of the biological samples. They or- angular position. 1C). Using simple light-ray geometry argu- ganize vertical scalloped microplates into The work by England et al. is an excellent ments, the authors show this vertical trans- a highly periodic array (13). This array con- example of taking inspiration from some of mission grating produces the exact reversed stitutes its own conventional diffraction the amazing structures and functions opti- color-mode sequence in decomposed white mized by natural evolution and translating light observed from the butterfly wing scales. England et al. report an this inspiration into an artificial material England et al. took the butterfly’s special optical micrograting optimized for technological applications geometry and its unique function as a blue- (13). Although biology provides an awe- print for the fabrication of a new type of array inspired by a inspiring source of unique materials, con- optical grating (13). True to bioinspiration photonic structure cepts, and functionalities, as scientists and and mimicry, the authors did not produce engineers, we have to remind ourselves that exact replicas of the curved butterfly scales, found in iridescently the implementation of photonic structures but rather extracted the crucial structural and colored butterfly wings. varies strongly between biological and tech- functional aspects—the vertical geometry and nological applications. Although biological grating. In addition, coupling of the dif- the transmission mode of the grating—to structures have been optimized to create translate these aspects into an artificial pho- fraction modes of these two gratings (scallops coloration for camouflage, signaling, or tonic material (Fig. 1E). Butterfly scale- and microplate array) was observed. This disguise, the goal for technological applica- inspired diffraction elements were fabricated hierarchical coupling could be controlled by tions is to optimize photonic structures for using a master structure/molding procedure. adjusting both the interplate spacing and the dispersing, guiding, storing, and amplifying For this, a silicon master was created by the scallop pitch and thereby strongly enriched light. In addition, evolution has designed Bosch process, which involves multiple etch- the optical diffraction properties of the biological photonic structures as part of an ing and passivation steps. The silicon master material. Furthermore, the authors tilted entire organism, whereas artificial photonics consisted of an array of micrometer-sized the angle of orientation of the microplates underlie a very different design principle, vertical plates, each with periodic undula- from vertical to 20° off vertical to provide namely integration into devices. The work tions on the surface. The periodic length of another tuning knob for the optical prop- of England et al. elegantly embodies these the surface undulations, termed scallops, was erties of this photonic material. Although design principles by developing bio-inspired designed to be 500 nm, very close to the this tilting leaves the microplate array dif- artificial photonic structures that will pave nano-ribs in the vertical grating structure of fraction properties largely unchanged, it the way for novel applications in sensing, the butterfly scales. The array of scalloped predictively shifts the scallop-induced dif- anticounterfeiting, and light-emitting diodes. silicon plates was then subjected to a dou- ble-molding procedure involving polymeric compounds. First, a negative replica of the 1 Arzt E, Gorb S, Spolenak R (2003) From micro to nano contacts in 12 Pulsifer DP, Lakhtakia A, Martín-Palma RJ, Pantano CG (2010) master structure was cast into polydimethyl biological attachment devices. Proc Natl Acad Sci USA 100(19): Mass fabrication technique for polymeric replicas of arrays of insect 10603–10606. corneas. Bioinspir Biomim 5(3):036001. siloxane (PDMS). The PDMS structure then 2 Zuccarello G, Scribner D, Sands R, Buckley LJ (2002) Materials for 13 England G, et al. (2014) Bioinspired micrograting arrays mimicking served as an intermediary template and was bio-inspired optics. Adv Mater 14(18):1261–1264. the reverse color diffraction elements evolved by the butterfly Pierella subsequently replicated into a UV-curable 3 Srinivasarao M (1999) Nano-optics in the biological world: Beetles, luna. Proc Natl Acad Sci

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