butterfly-inspired photonics reverse diffraction color sequence

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    Butterfly-inspired photonics reversediffraction color sequenceMichael H. Bartl1

    Department of Chemistry, and Materials Research Science and Engineering Center, Universityof Utah, Salt Lake City, UT 84112

    Biological systems have been an infinitereservoir of inspiration ever since humansstarted to develop tools and machinery. Justas early scientists and engineers attempted tomimic birds and fish in the development offlying machines and submarines, today, newtechnologies find their inspiration from bi-ology, such as gecko feet (1), antireflective eyelenses (2), iridescent insects (3), and water-repellant surfaces (4). Incorporating biologi-cal systems and concepts into technologicaldesign can happen in several ways: inspira-tion, mimicking, and replication (5). In thelatter, entire organisms or body parts are di-rectly used, and their structural features arereplicated into another compound (68).Examples include 3D photonic crystals fromiridescent beetle scales (9, 10) or antireflec-tive microlens arrays from insect eyes (11,

    12). In contrast, in bioinspiration and bio-mimicry, a biological function or activityrather than the organism itselfis convertedinto an artificial, human-made material ordevice. In PNAS, England et al. report anoptical micrograting array inspired by a pho-tonic structure found in iridescently coloredbutterfly wings (13). The authors demon-strate a micrograting array that not onlymimics the unique diffraction properties ofthe biological structure with reversed color-order sequence, but also can be designed totune these optical properties.The concept of structural colors is an

    omnipresent optical phenomenon in biolog-ical systems and refers to colors produceddue to the interaction of light with nano-to-microscale structures built into variousbody parts of insects, birds, marine animals,

    and plants (3, 14). Structural colors are theresult of diffraction and specular reflectionof light, in contrast to typical pigments thatproduce color by light absorption and dif-fuse reflection. Fossil finds date the firstexamples of structural colors to some 500million years ago, and the earliest examplesof photonic structures in these fossil recordswere discovered within insect hairs and spinesin the form of multilayers and gratings(15). Although these early structures mostlikely were the result of random mutations,the accompanying coloration effects musthave presented evolutionary advantages incamouflaging, signaling, communicating,and mimicking (16). Today we find anenormous variety of photonic structuresin biology, including deformable graded-index lenses, photonic crystals, and variousdiffraction gratings (2, 3, 1719).A particularly interesting diffraction ele-

    ment was discovered several years ago in thewings of the butterfly Pierella luna whenVigneron et al. reported that light hittingcertain parts of the wings of this butterfly isdiffracted in reversed diffraction color-modesequence compared with a conventionaldiffraction grating (20). In a conventionalgrating, white light striking the grating isdecomposed such that shorter wavelengthsare diffracted less than longer ones; i.e., bluewavelengths exit with an angle closer to spec-ularity followed by green, yellow, and red(Fig. 1A). This sequence is reversed whenwhite light is decomposed by interactionwith the diffractive elements (specific cuticlescales) located on the butterfly wings. Here,red wavelengths exit at angles closer to thedirection perpendicular to the wing plane,whereas yellow, green, and blue exit at pro-gressively increasing angles (Fig. 1B). Thereason for this reversed diffraction behaviorlies in the curved shape of each diffractingscale, which positions the grating periodicity(nano-ribs) perpendicular to the wing surface(Fig. 1D). Such a vertical grating operatesin transmission mode, in contrast to the

    Fig. 1. Schematic illustration of diffraction properties and grating types. (A) Color-mode sequence obtained froma conventional horizontal diffraction element. (B) Reversed color-mode sequence of diffracted light observed fromdiffraction elements located on the wings of the butterfly Pierella luna. (C ) Schematic depiction of the structure ofa conventional horizontal diffraction grating operating in reflection mode. (D) Schematic depiction of a curved but-terfly wing scale with a vertically oriented diffraction element operating in transmission mode. (E ) Schematic depictionof 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.

    The author declares no conflict of interest.

    See companion article on page 15630.

    1Email: michael.bartl@utah.edu.

    1560215603 | PNAS | November 4, 2014 | vol. 111 | no. 44 www.pnas.org/cgi/doi/10.1073/pnas.1418292111


  • reflection mode of a conventional gratingwith periodicity parallel to the surface (Fig.1C). Using simple light-ray geometry argu-ments, the authors show this vertical trans-mission grating produces the exact reversedcolor-mode sequence in decomposed whitelight observed from the butterfly wing scales.England et al. took the butterflys special

    geometry and its unique function as a blue-print for the fabrication of a new type ofoptical grating (13). True to bioinspirationand mimicry, the authors did not produceexact replicas of the curved butterfly scales,but rather extracted the crucial structural andfunctional aspectsthe vertical geometry andthe transmission mode of the gratingtotranslate these aspects into an artificial pho-tonic material (Fig. 1E). Butterfly scale-inspired diffraction elements were fabricatedusing a master structure/molding procedure.For this, a silicon master was created by theBosch process, which involves multiple etch-ing and passivation steps. The silicon masterconsisted of an array of micrometer-sizedvertical plates, each with periodic undula-tions on the surface. The periodic length ofthe surface undulations, termed scallops, wasdesigned to be 500 nm, very close to thenano-ribs in the vertical grating structure ofthe butterfly scales. The array of scallopedsilicon plates was then subjected to a dou-ble-molding procedure involving polymericcompounds. First, a negative replica of themaster structure was cast into polydimethylsiloxane (PDMS). The PDMS structure thenserved as an intermediary template and wassubsequently replicated into a UV-curableepoxy polymer. Remarkably, this double rep-lication (inverse of the inverse) preserved allof the nano- and microscale structural fea-tures of the master, resulting in a periodicarray of scalloped polymer microplates ori-ented normal to the surface.The diffraction properties of these bio-

    inspired polymeric diffraction elementswere investigated by variable observationangle optical spectroscopy. Indeed, theauthors found strong diffraction from thevertically oriented scalloped microplates;most importantly, diffraction colors dis-played the same reversed sequence as in thebiological structures. However, England et al.

    go beyond merely reproducing the opticaleffects of the biological samples. They or-ganize vertical scalloped microplates intoa highly periodic array (13). This array con-stitutes its own conventional diffraction

    England et al. report anoptical microgratingarray inspired by aphotonic structurefound in iridescentlycolored butterfly wings.grating. In addition, coupling of the dif-fraction modes of these two gratings (scallopsand microplate array) was observed. Thishierarchical coupling could be controlled byadjusting both the interplate spacing and thescallop pitch and thereby strongly enrichedthe optical diffraction properties of thematerial. Furthermore, the authors tiltedthe angle of orientation of the microplatesfrom vertical to 20 off vertical to provideanother tuning knob for the optical prop-erties of this photonic material. Althoughthis tilting leaves the microplate array dif-fraction properties largely unchanged, itpredictively shifts the scallop-induced dif-

    fraction pattern in terms of wavelength andangular position.The work by England et al. is an excellent

    example of taking inspiration from some ofthe amazing structures and functions opti-mized by natural evolution and translatingthis inspiration into an artificial materialoptimized for technological applications(13). Although biology provides an awe-inspiring source of unique materials, con-cepts, and functionalities, as scientists andengineers, we have to remind ourselves thatthe implementation of photonic structuresvaries strongly between biological and tech-nological applications. Although biologicalstructures have been optimized to createcoloration for camouflage, signaling, ordisguise, the goal for technological applica-tions is to optimize photonic structures fordispersing, guiding, storing, and amplifyinglight. In addition, evolution has designedbiological photonic structures as part of anentire organism, whereas artificial photonicsunderlie a very different design principle,namely integration into devices. The workof England et al. elegantly embodies thesedesign principles by developing bio-inspiredartificial photonic structures that will pavethe way for novel applications in sensing,anticounterfeiting, and light-emitting diodes.

    1 Arzt E, Gorb S, Spolenak R (2003) From micro to nano contacts inbiological attachment devices. Proc Natl Acad Sci USA 100(19):1060310606.2 Zuccarello G, Scribner D, Sands R, Buckley LJ (2002) Materials forbio-inspired optics. Adv Mater 14(18):12611264.3 Srinivasarao M (1999) Nano-optics in the biological world: Beetles,butterflies, birds, and moths. Chem Rev 99(7):19351962.4 Mish


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