Photonic Crystal Structure of Wing Scales in Sasakia Charonda Butterflies
Total Page：16
File Type：pdf, Size：1020Kb
Materials Transactions, Vol. 51, No. 2 (2010) pp. 202 to 208 Special Issue on Development and Fabrication of Advanced Materials Assisted by Nanotechnology and Microanalysis #2010 The Japan Institute of Metals Photonic Crystal Structure of Wing Scales in Sasakia Charonda Butterflies Jirˇina Mateˇjkova´-Plsˇkova´1, Dalibor Jancˇik1, Miroslav Masˇla´nˇ1, Satoshi Shiojiri2 and Makoto Shiojiri3;* 1Centre for Nanomaterial Research, Faculty of Science, Palacky University in Olomouc, Slechtitelu 11, 783 71 Olomouc, Czech Republic 2Matsubara Junior High-school, Kyoto 604-8812, Japan 3Professor Emeritus of Kyoto Institute of Technology, 1-297 Wakiyama, Kyoto 618-0091, Japan The hindwings of the male Sasakia charonda charonda butterflies comprise iridescent purple-blue areas, iridescent white-pearl areas, yellow spots and red spots as well as brown background. We have examined the microstructure of their scales by scanning electron microscopy, for applying their photonic crystal structures to fine light manipulators such as reflection elements in laser diodes. The scales in the yellow spots, red spots and brown background have almost the same structure, which is an optical diffraction grating made of ridges with two cuticle layers. Their difference comes from the contained pigments. The scales in the iridescent purple-blue and white-pearl are also the same in structure. They have seven tilted cuticle layers lapped on the ridges, which also constitute a grating. The widths of the ridge and groove in the grating are different between scales of the two kinds. It is shown that the vivid iridescence is mainly attributed to multiple interferences caused between rays reflected from the seven cuticle layers with air gaps. [doi:10.2320/matertrans.MC200903] (Received July 6, 2009; Accepted September 28, 2009; Published November 18, 2009) Keywords: butterfly, wing scale, Sasakia charonda, great purple emperor, photonic crystal, field-emission scanning electron microscopy 1. Introduction using three lycaenid subfamilies; the Coppers (Lycaeninae), the Hairstreaks (Theclinae) and the Blues (Polyommatinae), The characteristic patterns and vivid coloration of wing that the hue of the various blue colors is characteristic scales of butterflies have lately attracted considerable for the microstructure and nanostructure of the body of the attention as photonic crystals for an excellent tool to scales.6) Prum et al.,7) who have investigated the physical manipulate light. Vukusic et al.1,2) studied the origin of the mechanisms of structural color production in twelve bright green coloration of the wing scales of an Indonesian Lepidopteran species from four families, reported that all male Papilio palinurus butterfly, and revealed that the dual the species are appropriately nanostructured to produce color arises from modulated multilayers, producing the blue visible colors by coherent scattering, indicating that only component as the result of an orthogonal-surface retro- the blue of P. Zalmoxis is a fluorescent pigmentary color. reflection process. They also studied the coloration of a Costa However, Lepidoptera, which is the generic name for Rica male Ancyluris meliboeus Fabricius butterfly, called a butterflies and moths, has about 180000 species in the world, ‘living jewel’, and found highly tilted, multilayered arrange- and only a few species have been investigated on the ment that produces a bright iridescence of broad wavelength structure of wings. range.3) Biro´ et al.,4) who investigated the role of photonic- Recently, we have investigated the microstructure of crystal-type structures in the thermal regulation of a lycaenid scales in male wings of Sasakia charonda charonda butterfly sister species pair, showed that the blue color can be (S. charonda) butterfly.8) It is a species in Apaturinae attributed unambiguously to the fine, sponge-like medium, subfamily of Nymphalodae and is called ‘the great purple called ‘pepper-pot structure’, which appears between the emperor’ in English and ‘ohmurasaki’ in Japanese. Adults ridges and the cross ribs in the scales of the colored butterfly. have dark brown wings with white patches and a small red- Only traces of this structure were found on the scales of the orange spot on each of the hindwings. Male generates purple- discolored butterfly. They also revealed from the thermal blue iridescence in the forewings and hindwings, while the measurements that the high-altitude butterfly reaches a female lacks. We identified and named six types of scales; temperature 1.3–1.5 times the temperature reached by the B1, W1, and R1 in brown background, yellow spots, and red low-altitude butterfly, and then concluded that this is spots, respectively, B2 in iridescent purple-blue and W2 in attributed to the photonic crystal-like behavior of the white-pearl, both of which characterize the male, and B3 in pepper-pot structure, which significantly reduces the pene- the wing edges. The B1, W1, and R1 scales are almost the tration of blue light into the body of the scales. Ve´rtesy same in structure, and the B2 and W2 scales are almost the et al.5) examined the dependence between color and scale same. The difference among the B, W, and R scales is in morphology in species belonging to the cosmopolitan tribe species and content of pigment. The B1, W1, and R1 scales Polyommatini representing Lycaenidae, showing that the have only two layers of cuticle lapped on the ridges. In pepper-pot structure operates as a natural photonic band- contrast with them, the B2 and W2 scales have seven gap material causing increased reflectance in the spectral multilayers of cuticle piled on the ridge. The characteristic range from blue to near ultraviolet (UV). They demonstrated, purple-blue of the male wings was ascribed to the com- bination of the structural and chemical coloration in the B2 *Corresponding author, E-mail: [email protected] scales with melanin. Photonic Crystal Structure of Wing Scales in Sasakia Charonda Butterflies 203 Fig. 1 (a) Photograph of a hindwing of a male S. charonda butterfly. (b) SEM image of B2 scales arranged in the iridescent purple-blue on the wing. (c) Optical microscopy (OM) image of B2 and W2 scales around the boundary between the iridescent purple-blue and iridescent white-pearl areas. (d) OM image of B1 and W1 scales around the boundary between the yellow spot and the brown background. (e) OM image of R1 and B1 scales around the red spot and the brown background. (f) SEM image of B2 scales separated from the wing. Removed scales were observed in this experiment. The structures of the photonic crystals of these scales layer about 10 nm thick to avoid charging effects, similar to would be applicable to fine light manipulators such as our previous observation.8) Scales separated from an un- reflection elements in laser diodes, especially in blue, purple, coated wing were also prepared in the present experiment. and ultraviolet laser diodes. In fact, some investigators have The removed wing scales were not coated with any gold layer produced replicas of the photonic structures of butterfly to observe them in authentic intact natural state. Little scales using atomic layer deposition technique (ALD),9,10) charging effect was present. The hue of the scale’s colors was which is a surface-controlled process of depositing materials examined in an optical microscope (OM). with atomic-layer accuracy.11,12) More structural investiga- tions of the butterfly scales are still required for achieving 3. Results and Discussion tunable photonic properties in the artificial scales. The present paper reports on further observations of the wing Figure 1(a) shows a photograph of a hindwing of a male scales in a male S. charonda butterfly following our previous S. charonda butterfly. Iridescent purple-blue area, iridescent one,8) and discusses their microstructures causing the bright white-pearl area, yellow spots and red spot can be seen as iridescence. well as brown background. In a previous paper,8) the scales in these parts were named B2, W2, W1 and R1, respectively, 2. Experimental Procedure and the scales in the brown background B1. These scales are arranged on the wing as shown in Fig. 1(b). Each scale The male S. charonda used in the present experiment was comprises many ridges which run parallel to each other. The sampled in July, 2008 in Nagano Prefecture in the middle B2 and W2 scales taken with OM are shown in Fig. 1(c), of Japan. Scanning electron microscopy (SEM) observations the B1 and W1 scales in Fig. 1(d) and the R1 and B1 scales were performed in secondary electron detection mode in a in Fig. 1(e). The W1 and W2 scales are semi-transparent, range of 2–15 kV of the accelerating voltage, using a Hitachi the B1 and B2 scales are semi-transparent brown and the SU6600 with a Shottky field-emission gun. The microscope R1 scales are semi-transparent red. In the present SEM guarantees a resolution of 1.2 nm at 30 KV and 3.0 nm at experiment we observed scales removed from the wings, 1 kV. We observed the wing coated with a sputtered gold as shown in Fig. 1(f). Figure 2 shows a SEM image of the 204 J. Mateˇjkova´-Plsˇkova´, D. Jancˇik, M. Masˇla´nˇ, S. Shiojiri and M. Shiojiri Fig. 2 SEM image of a W2 scale separated from the wing. The scale is bent and broken during preparation so that allows us to observe the side view and broken cross-sectional view as well as the top view of the cuticles piled on the ridges. Fig. 3 Computer diffractogram and enlarged SEM images of some areas of the W2 scale in Fig. 2. (a) Computer diffractogram from the area enclosed by the square. (b) The side view and top view of the cuticles piled on the ridges. (c) The cross-sectional view of a ridge with cuties.