Bioinspired Micrograting Arrays Mimicking the Reverse Color Diffraction Elements Evolved by the Butterfly Pierella Luna

$Bioinspired Micrograting Arrays Mimicking the Reverse Color Diffraction Elements Evolved by the Butterfly Pierella Luna$ Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation England, G., M. Kolle, P. Kim, M. Khan, P. Munoz, E. Mazur, and J. Aizenberg. 2014. “Bioinspired Micrograting Arrays Mimicking the Reverse Color Diffraction Elements Evolved by the Butterfly Pierella Luna.” Proceedings of the National Academy of Sciences 111 (44) (October 6): 15630–15634. doi:10.1073/pnas.1412240111. Published Version doi:10.1073/pnas.1412240111 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:27417440 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP 1 69 2 Bio-inspired micro-grating arrays: mimicking the 70 3 71 4 reverse color diffraction elements evolved by the 72 5 73 6 74 7 butterfly Pierella luna 75 8 * *†§ ‡ ‡ * * *‡§ 76 9 Grant England , Mathias Kolle , Philseok Kim , Mughees Khan , Philip Muñoz , Eric Mazur & Joanna Aizenberg 77 10 78 *School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138 †Department of Mechanical Engineering, Massachusetts Institute of 11 Technology, Cambridge, MA, 02139 ‡Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02138 §Corresponding authors 79 12 80 13 Submitted to Proceedings of the National Academy of Sciences of the United States of America 81 14 82 15 Recently, diffraction elements that reverse the color sequence nor- tion effect results from the local morphology of individual scales 83 16 mally observed in planar diffraction gratings have been found in within the colored spot on the fore wings (12). The top parts 84 17 the wing scales of the butterfly Pierella luna. Here, we describe the of the scales are curled upwards, orienting lines of periodically 85 18 creation of an artificial photonic material mimicking this reverse arranged cross-ribs perpendicular to the wing surface and the 86 19 color-order diffraction effect. The bio-inspired system consists of wing surface normal. (Fig. 1C,D). Light incident at an angle onto 87 20 ordered arrays of vertically oriented micro-diffraction gratings. the curled parts of the scales is diffracted by the cross-rib structure 88 21 We present a detailed analysis and modeling of the coupling of acting as a diffraction grating, with a periodicity of ∼400nm. The 89 22 diffraction resulting from individual structural components and alignment of the grating perpendicular to the surface results in the 90 23 demonstrate its strong dependence on the orientation of the reverse color sequence that can be observed in angularly resolved 91 24 individual miniature gratings. This photonic material could provide reflection spectra and in the diffraction pattern (Fig. 1E,F). 92 25 a basis for novel developments in bio-sensing, anti-counterfeiting The identification of this unusual diffraction effect on the 93 26 and efficient light managementSubmission in photovoltaic systems and light wings of P. luna PDF(12) provided inspiration for the development 94 27 emitting diodes. of a bio-inspired photonic system that incorporates vertically 95 28 oriented micro-diffraction gratings with sub-micrometer peri- 96 j j 29 Bio-inspired optics Micro-gratings Biophotonics odicity analogous to the key features observed in the natural 97 30 structure. In addition, the artificial system displays a periodic 98 31 Three-dimensional photonic crystals (1-5), materials with arrangement of the individual vertical gratings in large arrays with 99 32 two-dimensional micro- or nano-sized periodic morphologies (6- two-dimensional micro-scale periodicity. This structural feature, 100 33 8) and one-dimensional multilayer configurations (9) have been which is not found in the natural organism, enriches the optical 101 34 identified as the primary cause of structural coloration in a wide signature of the artificial system via coupling of the diffractive 102 35 variety of non-related biological organisms. In contrast, surface- modes of the two present hierarchical morphologies. In the fol- 103 36 confined diffraction elements for the separation of incident light lowing, we discuss the optical properties of the artificial system 104 37 into specific colors are less abundant in nature and have only been and demonstrate that the modification of either one of the grating 105 38 discovered in a handful of organisms (10), including a fossil poly- morphologies changes the diffraction signature in predictable 106 39 chaete (7), the sea mouse Aphrodita sp. (6) and some flowering ways. 107 40 plants (11). Recently, diffraction elements that reverse the color 108 41 sequence normally observed in planar diffraction gratings have 109 42 been found in the scales of the butterfly Pierella luna (12). 110 43 Inspired by this biological light manipulation strategy, we de- Significance 111 44 vised an artificial material morphology mimicking the butterfly’s 112 45 diffraction effect by creating periodic arrays of vertically oriented In the course of evolution, many organisms have developed 113 46 individual micro-diffraction gratings. In addition to the butterfly- unique light manipulation strategies that rely on intriguing 114 47 inspired reverse color order diffraction arising from each in- combinations of a broad range of optical effects generated by 115 dividual micro-grating, the periodicity between the individual materials with sophisticated multi-scale hierarchical structural 48 arrangements. By exploiting the optical principles underly- 116 49 gratings causes diffraction on a different length scale, leading ing natural structural color, we can generate new photonic 117 50 to complex intensity distributions in experimentally measured materials. Researchers have only just begun to match na- 118 51 angularly resolved reflection spectra. An in-depth analysis of ture's morphological and compositional complexity in man- 119 the observed diffraction phenomenon complemented by optical made materials using nanofabrication. We present a bioin- 52 spired photonic material that mimics the reverse color order 120 53 modeling revealed a strong dependence of the optical signature diffraction found in the butterfly Pierella Luna. Exploiting and 121 54 on the orientation of the gratings. Such an effect can only be seen improving the butterfly's strategy, we create new photonic 122 55 because of the hierarchical nature of the superposed, orthogonal materials that increase our basic understanding of the optical 123 grating features. To further elucidate the role of the different interplay of hierarchical structures and provide a platform for 56 the development of novel photonic devices. 124 57 structural components for the emerging reflection spectra, the 125 58 initially vertically oriented individual micro-gratings were sub- Reserved for Publication Footnotes 126 59 jected to a tilt, resulting in a predictable change of the surface's 127 60 optical signature. 128 61 The dorsal side of the fore and hind wings of P. luna males 129 62 are dull brown in diffuse ambient illumination (Fig. 1A, left). 130 63 When exposed to directional illumination at grazing incidence, 131 64 a coin-sized spot on each forewing displays an angle-dependent 132 65 color variation across the whole visible spectrum (Fig. 1A, right). 133 66 The color changes from red to blue with increasing observation 134 67 angle unlike the variation from blue to red normally observed in 135 68 conventional diffraction gratings (13). This reverse color diffrac- 136 www.pnas.org --- --- PNAS Issue Date Volume Issue Number 1--?? 137 vidual plates well preserved. The silicon master is formed using 205 138 the Bosch process (15), in which multiple etching and passivation 206 139 steps give rise to the periodic undulations on the micro-plate 207 140 surface. The pitch and height of these grating structures can be 208 141 controlled by adjusting the etching parameters (16, 17). Here, 209 142 they are chosen to be comparable to the spacings and dimensions 210 143 of the diffraction-inducing micro-ribs on the P. luna scales (Fig. 211 144 2B), and hence are expected to cause a similar diffraction effect. 212 145 It is important to notice that in the biological system, the 213 146 periodicity of the diffraction grating-supporting scales is of the 214 147 order of 80 ± 10 µm along their length and 60 ± 10 µm perpen- 215 148 dicular to the scale axis (Fig. 1B). Due to these large distances 216 149 between diffraction elements and a non-negligible amount of 217 150 irregularity in the location of individual scales, no coherence is ob- 218 151 served for light diffracted from adjacent scales. The overall color 219 152 splitting only results from the diffraction caused by the cross-rib 220 153 gratings on the individual scales, which is confirmed by variable 221 154 angle spectroscopy and by diffraction microscopy measurements 222 155 (Fig. 1D). Unlike in the biological system, the individual micro- 223 156 diffraction gratings in the artificial system are intentionally ar- 224 157 ranged in a highly periodic manner, which is expected to result in 225 158 a richer diffraction signature and provide additional possibilities 226 159 of tailoring the interaction of light beyond the diffraction induced 227 160 by the scallops on the plates. 228 161 Variable observation-angle spectroscopy performed on the 229 162 Submissionartificial system servesPDF to spectrally and angularly resolve parts of 230 163 the complex diffraction pattern (Fig. 2C). For each measurement 231 164 the plane of light incidence is chosen to be perpendicular to the 232 165 surface of the individual micro-plates. The light incidence angle 233 166 θI is fixed and the observation angle θD is varied in the plane of 234 167 light incidence to capture light reflected in an angular range of 235 168 ±75° around the sample surface normal.

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