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Living photonic crystals: Butterfly scales — Nanostructure and optical properties
Materials Science and Engineering C 27 (2007) 941 – 946 www.elsevier.com/locate/msec
Living photonic crystals: Butterfly scales — Nanostructure and optical properties L.P. Biró a,⁎, K. Kertész a , Z. Vértesy a , G.I. Márk a , Zs. Bálint b , V. Lousse c , J.-P. Vigneron d a
c
Research Institute for Technical Physics and Materials Science, H-1525, Budapest, POB 49, Hungary b Hungarian Natural History Museum, H-1088, Budapest, Baross utca 13, Hungary Department of Electrical Engineering, Stanford University, Stanford, California 94305, California, USA and Facultes Universitaires Notre Dame de la Paix, B-5000, Namur, Rue de Bruxelles 61, Belgium d Facultes Universitaires Notre Dame de la Paix, B-5000, Namur, Rue de Bruxelles 61, Belgium Received 3 May 2006; received in revised form 22 September 2006; accepted 25 September 2006 Available online 15 November 2006
Abstract The photonic crystal type nanostructures in the scales of male individuals of two butterfly species: Cyanophrys remus and Albulina metallica were investigated by electron microscopy and reflectance measurements. While the colors of C. remus arise from structures with rigorous long range (dorsal) or short range (ventral) three dimensional (3D) order, the colors of A. metallica are produced by quasi-ordered, layered structures. Surprisingly, the most efficient photonic band gap reflector is the quasi-ordered structure giving the shiny, yellowish green color of the ventral hind wings of A. metallica. All four investigated structures are based on a moderate refractive index contrast between chitin (n = 1.58) and air, the various structures achieve a wide range of biological functions. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Butterfly wing scales; Photonic crystals; Electron microscopy; Reflectance
1. Introduction Photonic crystals [1, 2]] may be regarded as a special case of composite, built from two materials with refractive indices n1 and n2, characterized by a refractive index invariant under the spatial translations of a crystalline lattice, Fig. 1. The periodicity of the structure has to be comparable with the wavelength range in which it will exhibit the photonic crystal type behavior. In welldefined frequency ranges, called “stop bands”, these materials, also called photonic band gap materials (PBG), can prohibit electromagnetic wave propagation along specific directions, even with the very moderate refractive index contrasts found with biological materials [3]. It should be emphasized, that for a complete photonic gap, usually more complex structures are needed than the structure shown in Fig. 1, which is only illustrating the principle of a three dimensional PBG. PBG materials may have numerous applications ranging from optical computing [4] through tunable photonic circuits ⁎ Corresponding author. Tel.: +36 1 3922681; fax: +36 1 3922226. E-mail address: [email protected] (L.P. Biró). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.09.043
[5], more efficient lasers [6] and fibers for optical communications [7], less harmful pigments [8] to photonic paper [9] to mention only a few. While the manufacturing of large area photonic crystals operating in the visible spectrum is still a somewhat challenging and expensive task for laboratory techniques, as the result of extremely refined biological evolutionary paths, periodic dielectric structures which operate in this wavelength range, have evolved in living beings like certain butterflies, for example the exclusively Neotropical butterfly genus Morpho [10–12], birds [13], beetles [14], marine organisms [15] and even in plants [16]. A more comprehensive overview was published recently by Welch [17], the palaeontological aspects were reviewed recently by Parker [18]. In the present paper the characteristic nanostructures in the wing scales, generating the blue and green colors of the lycaenid butterflies: Cyanophrys remus (Hewitson, 1868) and Albulina metallica (Felder and Felder, 1865) (family Lycaenidae), will be examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the reflectance of the investigated structures and their biological role will be
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Fig. 1. Schematic model of a 3D photonic crystal. As a rule of thumb the lattice parameter a has to be of the same order of magnitude as the wavelength for which the PBG will form. Note that for a full PBG in practical cases, usually more complicated structures are needed than the simple cubic case used here for illustration.
discussed. Both butterfly species are colored by PBG nanostructures on both the dorsal and ventral sides and the nanostructures in the scales exhibit different degrees of order.
ing especially in the early morning hours, before the butterfly starts flying. It is interesting to examine more closely in which way evolution, using the same two materials: air and chitin (n = 1.58) produced several different structures, which all exhibit PBG behavior and have very different reflectance and different roles in the survival of the individual and of the species. The nano- and micro-structure of the butterfly scales are revealed by SEM and TEM as shown in Figs. 3 and 4. From the examination of the electron micrographs one may conclude that the four structures represent four different cases, in the sequence of the decreasing order in the structure: i) the dorsal wing surface scales of C. remus are characterized by well developed ridges, which are filled by the same nanostructure as the body of the scale, as shown by the TEM image, the structure has long range order in the full three dimensions (3D), this observation is quantitatively supported by detailed Fourier analysis of the images, published elsewhere [20]; ii) the ventral wing surface scales of the C. remus exhibit locally in the grains a highly ordered structure, a so called face centered cubic (fcc) inverse opal, but there is no long range order as the orientation of the grains is random; iii) the dorsal wing surface scales of A.
2. Experimental results and discussion The butterfly samples used in the present work were obtained from the collections of the Hungarian Natural History Museum, Budapest, Hungary. All the specimens investigated were males. The species A. metallica belongs to the subfamily Polyommatinae and inhabits high altitude open habitats in the Himalayas. The species C. remus belongs to the subfamily Theclinae and occurs in the tropical and subtropical forest zone of southeast Brazil, where it lives in forest edges at low or moderate elevations. For SEM examination pieces of wings were attached to the sample holders and covered by 20 nm of sputtered gold to avoid charging. A LEO 1540XB FESEM/FIB was used for SEM investigations. The TEM samples were prepared by incorporation of wing pieces in plastic blocks, followed by ultramicrotome sectioning which resulted in 70 nm thick slices. The samples were examined in TECNAI 10 transmission electron microscope. Reflectivity measurements were carried out with an Avaspec 2048/2 fiber-optic spectrometer, both in specular arrangement under normal incidence and using a 3 cm integrating sphere in order to collect all the light reflected under any angle of emergence. An Avaspec diffuse, white standard was used as comparison sample for reflectance measurements. As seen in Fig. 2a and b, both butterfly species have blue dorsal color (with role in sexual signaling) [19], while their ventral coloration, used as cryptic color, to help avoiding the attacks of predators, is greenish. The ventral wing surfaces of the C. remus have a matt pea green coloration, while only the ventral hind wing surface of A. metallica is shiny greenishyellow. It is worth pointing out that C. remus lives in forests and the matt pea green color is helpful in hiding the butterfly against the diffuse green background of the vegetation. On the other hand A. metallica lives in a dew-rich, grassy environment at high altitude, the shiny greenish color is useful for camouflag-
Fig. 2. Composed photographs of male butterfly individuals representing C. remus (a) and Albulina metallica (both Lepidoptera: Lycaenidae) (b). Left side shows the dorsal surface of the wings, while the right side the ventral side, respectively.
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Fig. 3. Electron micrographs of the scales on the wings of the C. remus: (a) dorsal scale SEM micrograph, note the well developed ridges; (b) dorsal scale TEM micrograph, note the filled ridges and long range order in the structure; (c) ventral scale SEM micrographs, note the granular filling below the plane of the thin ridges; (d) ventral scale TEM micrographs, note the local order within each grain and the random orientation of neighboring grains. All scale bars correspond to 5 μm.
metallica show a characteristic, quasi-ordered structure, which is often called by the entomologists “pepper pot” structure [21]; iv) the ventral wing surface scales of A. metallica also have a quasi-ordered pepper pot type structure, but characterized by different dimensions as compared with the dorsal side. As seen from the TEM images, in fact both the dorsal and ventral scales of A. metallica exhibit a layered structure in which the thickness of the chitin filled layers is different: 80 nm for the dorsal scales and 60 nm for the ventral scales. Additionally, within the air/ chitin filled layers the effective refractive index depends on the average size of the holes and the walls separating them, which is again different for the dorsal and ventral scales. This quantity, which we call filling factor, i.e., the ratio of hole diameter to the width of the chitinous walls separating the holes is: 1.8 for the dorsal scales and 2.2 for the ventral scales. The normal incidence specular reflectance measurements (Fig. 5) revealed that for each of the above structures a characteristic reflectance peak can be found, Table 1. To allow the comparison of the spectra measured, each spectrum was normalized to its maximum in the range of 250–850 nm. In fact, the spectral distribution of intensities in the light reflected from the scales of butterflies built of chitin – a transparent bio-polymer – is determined by the content of pigment: in our case, brown melanin [3] (also a bio-polymer and a very efficient absorber in the UV and blue range of the visible [22, 23]). A second factor is the structure itself when the scale contains PBG material. Melanin alone would yield a continuously increasing reflectance from 500 nm – the value of absorption edge determined from Fig. 1 of Ref. [22] – towards 800 nm, this would result in a brownish color. The PBG material
producing the structural, or “physical color” [24] will cause that in a certain wavelength region, the incoming light cannot penetrate into the scale volume. The light will be reflected by the photonic crystal type structure (PBG), therefore it cannot be absorbed by the melanin distributed in the volume of the scale. In a scale containing only melanin or other pigments and no PBG structure, the light may penetrate in the volume of the scale, due to the absorption its intensity will be reduced according to the absorption coefficient of melanin (other pigment). Therefore, when illuminated with white light, it will appear brown in reflected light. In the region of the PBG a reflectance peak will be superimposed on the reflectance curve characteristic for the melanin, i.e., when a PBG is present, in the wavelength region of the photonic band gap, the reflectance will increase (from the usually low value, characteristic for melanin) as the light is reflected before being absorbed by the melanin. The combined reflectance due to the melanin and the PBG will produce together the blue or green coloration seen in Fig. 2 and the spectra in Fig. 5. As a consequence, the color will depend on the melanin content, the efficiency of the PBG material in the scales – the amplitude of the reflectance maximum – and on the bandwidth of the photonic band gap – the full width at half maximum (FWHM) of the peak – these factors together will decide the intensity and the hue of the observed color. To facilitate the comparisons, all spectra in Fig. 5 were normalized to the maximum measured in the range of 250–850 nm, thus relative reflectances are given in the figure. It is worth to point out that three of the four spectra, although not all belong to the same butterfly species, show relative reflectance of the order of 100% at 850 nm, while for the ventral scales of A. metallica the relative reflectance is only 40%
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Fig. 4. Electron micrographs of the scales on the wings of Albulina metallica: (a) dorsal scale SEM micrograph; (b) dorsal scale TEM micrograph; (c) ventral scale SEM micrograph, note the similar but not identical structure with the SEM micrograph in (a); (d) ventral scale TEM micrograph, note similar but not identical structure with the TEM micrograph in (b). The scale bars in (a) and (c) correspond to 2 μm; the scale bars in (b) and (d) correspond to 1 μm.
at this wavelength. This deviation from the general behavior is associated with a strong and broad maximum at 545 nm with the relative amplitude of 100%. This indicates that the ventral scales of A. metallica contain an extremely efficient reflector, which is in agreement with the observation by the naked eye, or the photographic image in Fig. 2b.
Fig. 5. Relative reflectance data under normal incidence specular measurement for the four butterfly wings. To facilitate comparison, each curve was normalized to its maximum in the presentation window.
The two ordered structures, corresponding to the ventral (blue, Fig. 3a and b) scales and to the dorsal (green, Fig. 3c and d) scales of C. remus were modeled structurally and on the basis of the models numerical computations successfully reproduced the experimentally observed reflectance peaks. The model calculation yielded: a strong blue peak at 450 nm for the dorsal scales, while the green peak is composed of weaker blue (469 nm), strong green (498 nm) and strong yellow (550 nm) reflections occurring on the (110), (100) and (111) faces of the fcc inverted opal [20]. The modeling of quasi-regular structures is a more difficult task, the same is valid for the numerical computation of the reflectance. On the other hand the experimental reflectance curves shown in Fig. 5, indicate that the quasi-regular, layered structures – which may be less difficult to manufacture than the structures based on three dimensional long, or short range order – are more efficient when normal incidence specular reflectance is measured. A similar behavior is found when the integrating sphere is used, the PBG material from the ventral scales of A. metallica is the most efficient one, but the differences are reduced as compared with the specular case Table 1. Another interesting feature is that in these layered structures the refractive index contrast that could be achieved by sharply separated chitin/air layers is reduced by the “mixed layers” of air and chitin, still the reflectance achieved in the case of ventral scales of A. metallica is rather strong and has a large bandwidth. To check for possible directional effects reflectance measurements were carried out using an integrating sphere, the peak positions and FWHM values are given in Table 1. The comparison on specular and integrated values in Table 1 shows,
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Table 1 Reflectance peak positions and intensities as measured in normal incidence specular geometry and with the integrating sphere for the two butterflies under investigation, λnormal is the peak position for normal incidence, λintegrated is the peak position for integrated measurements, NRnormal and NRintegrated are the normalized reflectance intensities (see text for details) for specular and integrated measurements respectively, FWHMn and FWHMi are the full width at half maximum values for the peaks for normal incidence and integrated measurements respectively Butterfly
Side
λnormal (nm)
λintegrated (nm)
NRnormal (%)
NRintegrated (%)
FWHMn (nm)
FWHMi (nm)
C. remus
Dorsal Ventral Dorsal
480 545 450 415
93 58 – – 92 100
106 91
560
55 64 81 77 – 100
110 87 152 a 152 a
Ventral
460 541 – – 416 545
A. metallica
a
185
133 227
Due to the close overlap of the two peaks the FWHM value is given for the two peaks together.
that although the relative amplitude of the blue maximum corresponding for the dorsal scales of C. remus increased significantly when measured with the integrating sphere – indicating strong non-specular reflectance –, still its amplitude did not increase over the amplitude of the green maximum of the A. metallica. The blue maximum of C. remus has a narrow bandwidth, while the green maximum of the A. metallica, has a very large bandwidth which makes these structures suitable for different applications. As regards the biologic function of these colors, the blue color of the dorsal scales has a role in sexual signaling [19], both in attracting the females and repelling other males. It is perhaps more interesting here to concentrate on the very different, cryptic green of the ventral scales. Two very different tasks were achieved by appropriately modified PBG nanostructures built from the very same materials chitin and air. The species C. remus, living in forests or at forest edges evolved a matt green, diffuse reflector built of randomly oriented photonic crystal grains with fcc inverse opal structure, thanks to which it may blend very efficiently in the diffuse green background of the forest. The A. metallica, living in a high altitude, grassy habitat with rich dew in the morning hours, developed a shiny cryptic color, which may help it to blend efficiently in the glittering dew covered grass. The production of these two greens with very different aspect (matt versus shiny) clearly shows the great flexibility of the natural “designs”, which can be used as templates or blueprints for artificial structures [14]. 3. Conclusions The photonic crystal type nanostructures built of chitin, occurring in the butterfly wing scales of the male individuals representing the species C. remus and A. metallica were investigated by electron microscopy and reflectance spectroscopy. While C. remus has a dorsal metallic blue color arising from a fully ordered 3D structure and a ventral mat green color produced by randomly oriented fcc inverse opal photonic crystal grains, with perfect short range order, but lacking long range order, the A. metallica has metallic blue color on its dorsal wings and a very shiny goldish green color on the ventral side. Surprisingly, both colors of A. metallica are achieved with similar quasi-ordered layered structures with slightly different structural parameters. The examination of four different PBG materials built with the same two components: air and chitin, allowed the
comparison of the efficiency of different structures. The PGB material found in the ventral scales of A. metallica is the most efficient reflector from the four investigated ones. It is worth pointing out that it is not a rigorously ordered structure and it is based on a moderate refractive index contrast. Such quasiordered nanostructures, which may be more easily manufactured than the perfectly ordered 3D nanostructures could prove useful in various practical applications. The investigation of natural designs may yield very useful templates for artificial structures. Acknowledgements The work was supported by EU6 NEST/PATHFINDER/ BioPhot-01915. The work in Hungary was partly supported by OTKA No. 042972. The work in Belgium was partly carried out with support from EU5 Centre of Excellence ICAI-CT-2000– 70029 and from the Inter-University Attraction Pole (IUAP P5/1) on “Quantum-size effects in nanostructured materials” of the Belgian Office for Scientific, Technical, and Cultural Affairs. Correspondence and requests for materials should be addressed to László P. Biró, [email protected]. References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] E. Yablonovitch, T.J. Gmitter, Phys. Rev. Lett. 63 (1989) 1950. [3] L.P. Biró, Zs. Bálint, K. Kertész, Z. Vértesy, G.I. Márk, Z.E. Horváth, J. Balázs, D. Méhn, I. Kiricsi, V. Lousse, J.-P. Vigneron, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 67 (2003) 021907. [4] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Photonic Crystals, Molding the Flow of Light, Princeton University Press, Princeton, 1995. [5] S.F. Mingaleev, M. Schillinger, D. Hermann, K. Busch, Optics Lett. 29 (2004) 2858. [6] K. Srinivasan, P.E. Barclay, O. Painter, J. Chen, A.Y. Cho, C. Gmachl, Appl. Phys. Lett. 83 (2003) 1915. [7] P. Gould, Materials Today (September 2002) 32. [8] R.C. Schroden, M. Al-Daous, Ch.F. Blanford, A. Stein, Chem. Mater. 14 (2002) 3305. [9] H. Fudouzi, Y. Xia, Langmuir 19 (2003) 9653. [10] P. Vukusic, J.R. Sambles, Nature 424 (2003) 852. [11] S. Kinoshita, S. Yoshioka, Y. Fujii, N. Okamoto, Forma 17 (2002) 103. [12] S. Berthier, Iridescences, les couleurs physiques des insectes, SpringerVerlag, Paris, 2003. [13] J.P. Vigneron, J.-F. Colomer, M. Rassart, A.L. Ingram, V. Lousse, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 72 (2005) 061904. [14] J.P. Vigneron, M. Rassart, C. Vandenbem, V. Lousse, O. Deparis, L.P. Biró, D. Dedouaire, A. Cornet, P. Defrance, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 73 (2006) 041905.
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[15] V.L. Welch, J.P. Vigneron, A.R. Parker, Curr. Biol. 15 (2005) R985. [16] J.P. Vigneron, M. Rassart, Z. Vértesy, K. Kertész, M. Sarrazin, L.P. Biró, D. Ertz, V. Lousse, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 71 (2005) 011906. [17] W.L. Welch, in: S. Kinoshita, S. Yoshioka (Eds.), Structural Colors in Biological Systems — Principles and Applications, Osaka University Press, Osaka, 2005, pp. 53–75. [18] A. Parker, J.R. Soc. Interface 2 (2005) 1. [19] L. Lundgren, J. Res. Lepid. 16 (1977) 249.
[20] K. Kertész, Zs. Bálint, Z. Vértesy, G.I. Márk, M. Rassart, V. Lousse, J-P. Vigneron, L.P. Biró, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 74 (2006) 021922. [21] J.N. Eliot, Bull. Br. Mus. Nat. Hist., Entomol. 28 (1973) 371. [22] H. Ou-Yang, G. Stamatas, N. Kollias, J. Invest. Dermatol. 122 (2004) 492. [23] J.B. Nofsinger, S.E. Forest, J.D. Simon, J. Phys. Chem., B 103 (1999) 11428. [24] N. Tamaoki, Adv. Mater. 13 (2001) 1135.
Living photonic crystals: Butterfly scales — Nanostructure and optical properties L.P. Biró a,⁎, K. Kertész a , Z. Vértesy a , G.I. Márk a , Zs. Bálint b , V. Lousse c , J.-P. Vigneron d a
c
Research Institute for Technical Physics and Materials Science, H-1525, Budapest, POB 49, Hungary b Hungarian Natural History Museum, H-1088, Budapest, Baross utca 13, Hungary Department of Electrical Engineering, Stanford University, Stanford, California 94305, California, USA and Facultes Universitaires Notre Dame de la Paix, B-5000, Namur, Rue de Bruxelles 61, Belgium d Facultes Universitaires Notre Dame de la Paix, B-5000, Namur, Rue de Bruxelles 61, Belgium Received 3 May 2006; received in revised form 22 September 2006; accepted 25 September 2006 Available online 15 November 2006
Abstract The photonic crystal type nanostructures in the scales of male individuals of two butterfly species: Cyanophrys remus and Albulina metallica were investigated by electron microscopy and reflectance measurements. While the colors of C. remus arise from structures with rigorous long range (dorsal) or short range (ventral) three dimensional (3D) order, the colors of A. metallica are produced by quasi-ordered, layered structures. Surprisingly, the most efficient photonic band gap reflector is the quasi-ordered structure giving the shiny, yellowish green color of the ventral hind wings of A. metallica. All four investigated structures are based on a moderate refractive index contrast between chitin (n = 1.58) and air, the various structures achieve a wide range of biological functions. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Butterfly wing scales; Photonic crystals; Electron microscopy; Reflectance
1. Introduction Photonic crystals [1, 2]] may be regarded as a special case of composite, built from two materials with refractive indices n1 and n2, characterized by a refractive index invariant under the spatial translations of a crystalline lattice, Fig. 1. The periodicity of the structure has to be comparable with the wavelength range in which it will exhibit the photonic crystal type behavior. In welldefined frequency ranges, called “stop bands”, these materials, also called photonic band gap materials (PBG), can prohibit electromagnetic wave propagation along specific directions, even with the very moderate refractive index contrasts found with biological materials [3]. It should be emphasized, that for a complete photonic gap, usually more complex structures are needed than the structure shown in Fig. 1, which is only illustrating the principle of a three dimensional PBG. PBG materials may have numerous applications ranging from optical computing [4] through tunable photonic circuits ⁎ Corresponding author. Tel.: +36 1 3922681; fax: +36 1 3922226. E-mail address: [email protected] (L.P. Biró). 0928-4931/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2006.09.043
[5], more efficient lasers [6] and fibers for optical communications [7], less harmful pigments [8] to photonic paper [9] to mention only a few. While the manufacturing of large area photonic crystals operating in the visible spectrum is still a somewhat challenging and expensive task for laboratory techniques, as the result of extremely refined biological evolutionary paths, periodic dielectric structures which operate in this wavelength range, have evolved in living beings like certain butterflies, for example the exclusively Neotropical butterfly genus Morpho [10–12], birds [13], beetles [14], marine organisms [15] and even in plants [16]. A more comprehensive overview was published recently by Welch [17], the palaeontological aspects were reviewed recently by Parker [18]. In the present paper the characteristic nanostructures in the wing scales, generating the blue and green colors of the lycaenid butterflies: Cyanophrys remus (Hewitson, 1868) and Albulina metallica (Felder and Felder, 1865) (family Lycaenidae), will be examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), the reflectance of the investigated structures and their biological role will be
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Fig. 1. Schematic model of a 3D photonic crystal. As a rule of thumb the lattice parameter a has to be of the same order of magnitude as the wavelength for which the PBG will form. Note that for a full PBG in practical cases, usually more complicated structures are needed than the simple cubic case used here for illustration.
discussed. Both butterfly species are colored by PBG nanostructures on both the dorsal and ventral sides and the nanostructures in the scales exhibit different degrees of order.
ing especially in the early morning hours, before the butterfly starts flying. It is interesting to examine more closely in which way evolution, using the same two materials: air and chitin (n = 1.58) produced several different structures, which all exhibit PBG behavior and have very different reflectance and different roles in the survival of the individual and of the species. The nano- and micro-structure of the butterfly scales are revealed by SEM and TEM as shown in Figs. 3 and 4. From the examination of the electron micrographs one may conclude that the four structures represent four different cases, in the sequence of the decreasing order in the structure: i) the dorsal wing surface scales of C. remus are characterized by well developed ridges, which are filled by the same nanostructure as the body of the scale, as shown by the TEM image, the structure has long range order in the full three dimensions (3D), this observation is quantitatively supported by detailed Fourier analysis of the images, published elsewhere [20]; ii) the ventral wing surface scales of the C. remus exhibit locally in the grains a highly ordered structure, a so called face centered cubic (fcc) inverse opal, but there is no long range order as the orientation of the grains is random; iii) the dorsal wing surface scales of A.
2. Experimental results and discussion The butterfly samples used in the present work were obtained from the collections of the Hungarian Natural History Museum, Budapest, Hungary. All the specimens investigated were males. The species A. metallica belongs to the subfamily Polyommatinae and inhabits high altitude open habitats in the Himalayas. The species C. remus belongs to the subfamily Theclinae and occurs in the tropical and subtropical forest zone of southeast Brazil, where it lives in forest edges at low or moderate elevations. For SEM examination pieces of wings were attached to the sample holders and covered by 20 nm of sputtered gold to avoid charging. A LEO 1540XB FESEM/FIB was used for SEM investigations. The TEM samples were prepared by incorporation of wing pieces in plastic blocks, followed by ultramicrotome sectioning which resulted in 70 nm thick slices. The samples were examined in TECNAI 10 transmission electron microscope. Reflectivity measurements were carried out with an Avaspec 2048/2 fiber-optic spectrometer, both in specular arrangement under normal incidence and using a 3 cm integrating sphere in order to collect all the light reflected under any angle of emergence. An Avaspec diffuse, white standard was used as comparison sample for reflectance measurements. As seen in Fig. 2a and b, both butterfly species have blue dorsal color (with role in sexual signaling) [19], while their ventral coloration, used as cryptic color, to help avoiding the attacks of predators, is greenish. The ventral wing surfaces of the C. remus have a matt pea green coloration, while only the ventral hind wing surface of A. metallica is shiny greenishyellow. It is worth pointing out that C. remus lives in forests and the matt pea green color is helpful in hiding the butterfly against the diffuse green background of the vegetation. On the other hand A. metallica lives in a dew-rich, grassy environment at high altitude, the shiny greenish color is useful for camouflag-
Fig. 2. Composed photographs of male butterfly individuals representing C. remus (a) and Albulina metallica (both Lepidoptera: Lycaenidae) (b). Left side shows the dorsal surface of the wings, while the right side the ventral side, respectively.
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Fig. 3. Electron micrographs of the scales on the wings of the C. remus: (a) dorsal scale SEM micrograph, note the well developed ridges; (b) dorsal scale TEM micrograph, note the filled ridges and long range order in the structure; (c) ventral scale SEM micrographs, note the granular filling below the plane of the thin ridges; (d) ventral scale TEM micrographs, note the local order within each grain and the random orientation of neighboring grains. All scale bars correspond to 5 μm.
metallica show a characteristic, quasi-ordered structure, which is often called by the entomologists “pepper pot” structure [21]; iv) the ventral wing surface scales of A. metallica also have a quasi-ordered pepper pot type structure, but characterized by different dimensions as compared with the dorsal side. As seen from the TEM images, in fact both the dorsal and ventral scales of A. metallica exhibit a layered structure in which the thickness of the chitin filled layers is different: 80 nm for the dorsal scales and 60 nm for the ventral scales. Additionally, within the air/ chitin filled layers the effective refractive index depends on the average size of the holes and the walls separating them, which is again different for the dorsal and ventral scales. This quantity, which we call filling factor, i.e., the ratio of hole diameter to the width of the chitinous walls separating the holes is: 1.8 for the dorsal scales and 2.2 for the ventral scales. The normal incidence specular reflectance measurements (Fig. 5) revealed that for each of the above structures a characteristic reflectance peak can be found, Table 1. To allow the comparison of the spectra measured, each spectrum was normalized to its maximum in the range of 250–850 nm. In fact, the spectral distribution of intensities in the light reflected from the scales of butterflies built of chitin – a transparent bio-polymer – is determined by the content of pigment: in our case, brown melanin [3] (also a bio-polymer and a very efficient absorber in the UV and blue range of the visible [22, 23]). A second factor is the structure itself when the scale contains PBG material. Melanin alone would yield a continuously increasing reflectance from 500 nm – the value of absorption edge determined from Fig. 1 of Ref. [22] – towards 800 nm, this would result in a brownish color. The PBG material
producing the structural, or “physical color” [24] will cause that in a certain wavelength region, the incoming light cannot penetrate into the scale volume. The light will be reflected by the photonic crystal type structure (PBG), therefore it cannot be absorbed by the melanin distributed in the volume of the scale. In a scale containing only melanin or other pigments and no PBG structure, the light may penetrate in the volume of the scale, due to the absorption its intensity will be reduced according to the absorption coefficient of melanin (other pigment). Therefore, when illuminated with white light, it will appear brown in reflected light. In the region of the PBG a reflectance peak will be superimposed on the reflectance curve characteristic for the melanin, i.e., when a PBG is present, in the wavelength region of the photonic band gap, the reflectance will increase (from the usually low value, characteristic for melanin) as the light is reflected before being absorbed by the melanin. The combined reflectance due to the melanin and the PBG will produce together the blue or green coloration seen in Fig. 2 and the spectra in Fig. 5. As a consequence, the color will depend on the melanin content, the efficiency of the PBG material in the scales – the amplitude of the reflectance maximum – and on the bandwidth of the photonic band gap – the full width at half maximum (FWHM) of the peak – these factors together will decide the intensity and the hue of the observed color. To facilitate the comparisons, all spectra in Fig. 5 were normalized to the maximum measured in the range of 250–850 nm, thus relative reflectances are given in the figure. It is worth to point out that three of the four spectra, although not all belong to the same butterfly species, show relative reflectance of the order of 100% at 850 nm, while for the ventral scales of A. metallica the relative reflectance is only 40%
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Fig. 4. Electron micrographs of the scales on the wings of Albulina metallica: (a) dorsal scale SEM micrograph; (b) dorsal scale TEM micrograph; (c) ventral scale SEM micrograph, note the similar but not identical structure with the SEM micrograph in (a); (d) ventral scale TEM micrograph, note similar but not identical structure with the TEM micrograph in (b). The scale bars in (a) and (c) correspond to 2 μm; the scale bars in (b) and (d) correspond to 1 μm.
at this wavelength. This deviation from the general behavior is associated with a strong and broad maximum at 545 nm with the relative amplitude of 100%. This indicates that the ventral scales of A. metallica contain an extremely efficient reflector, which is in agreement with the observation by the naked eye, or the photographic image in Fig. 2b.
Fig. 5. Relative reflectance data under normal incidence specular measurement for the four butterfly wings. To facilitate comparison, each curve was normalized to its maximum in the presentation window.
The two ordered structures, corresponding to the ventral (blue, Fig. 3a and b) scales and to the dorsal (green, Fig. 3c and d) scales of C. remus were modeled structurally and on the basis of the models numerical computations successfully reproduced the experimentally observed reflectance peaks. The model calculation yielded: a strong blue peak at 450 nm for the dorsal scales, while the green peak is composed of weaker blue (469 nm), strong green (498 nm) and strong yellow (550 nm) reflections occurring on the (110), (100) and (111) faces of the fcc inverted opal [20]. The modeling of quasi-regular structures is a more difficult task, the same is valid for the numerical computation of the reflectance. On the other hand the experimental reflectance curves shown in Fig. 5, indicate that the quasi-regular, layered structures – which may be less difficult to manufacture than the structures based on three dimensional long, or short range order – are more efficient when normal incidence specular reflectance is measured. A similar behavior is found when the integrating sphere is used, the PBG material from the ventral scales of A. metallica is the most efficient one, but the differences are reduced as compared with the specular case Table 1. Another interesting feature is that in these layered structures the refractive index contrast that could be achieved by sharply separated chitin/air layers is reduced by the “mixed layers” of air and chitin, still the reflectance achieved in the case of ventral scales of A. metallica is rather strong and has a large bandwidth. To check for possible directional effects reflectance measurements were carried out using an integrating sphere, the peak positions and FWHM values are given in Table 1. The comparison on specular and integrated values in Table 1 shows,
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Table 1 Reflectance peak positions and intensities as measured in normal incidence specular geometry and with the integrating sphere for the two butterflies under investigation, λnormal is the peak position for normal incidence, λintegrated is the peak position for integrated measurements, NRnormal and NRintegrated are the normalized reflectance intensities (see text for details) for specular and integrated measurements respectively, FWHMn and FWHMi are the full width at half maximum values for the peaks for normal incidence and integrated measurements respectively Butterfly
Side
λnormal (nm)
λintegrated (nm)
NRnormal (%)
NRintegrated (%)
FWHMn (nm)
FWHMi (nm)
C. remus
Dorsal Ventral Dorsal
480 545 450 415
93 58 – – 92 100
106 91
560
55 64 81 77 – 100
110 87 152 a 152 a
Ventral
460 541 – – 416 545
A. metallica
a
185
133 227
Due to the close overlap of the two peaks the FWHM value is given for the two peaks together.
that although the relative amplitude of the blue maximum corresponding for the dorsal scales of C. remus increased significantly when measured with the integrating sphere – indicating strong non-specular reflectance –, still its amplitude did not increase over the amplitude of the green maximum of the A. metallica. The blue maximum of C. remus has a narrow bandwidth, while the green maximum of the A. metallica, has a very large bandwidth which makes these structures suitable for different applications. As regards the biologic function of these colors, the blue color of the dorsal scales has a role in sexual signaling [19], both in attracting the females and repelling other males. It is perhaps more interesting here to concentrate on the very different, cryptic green of the ventral scales. Two very different tasks were achieved by appropriately modified PBG nanostructures built from the very same materials chitin and air. The species C. remus, living in forests or at forest edges evolved a matt green, diffuse reflector built of randomly oriented photonic crystal grains with fcc inverse opal structure, thanks to which it may blend very efficiently in the diffuse green background of the forest. The A. metallica, living in a high altitude, grassy habitat with rich dew in the morning hours, developed a shiny cryptic color, which may help it to blend efficiently in the glittering dew covered grass. The production of these two greens with very different aspect (matt versus shiny) clearly shows the great flexibility of the natural “designs”, which can be used as templates or blueprints for artificial structures [14]. 3. Conclusions The photonic crystal type nanostructures built of chitin, occurring in the butterfly wing scales of the male individuals representing the species C. remus and A. metallica were investigated by electron microscopy and reflectance spectroscopy. While C. remus has a dorsal metallic blue color arising from a fully ordered 3D structure and a ventral mat green color produced by randomly oriented fcc inverse opal photonic crystal grains, with perfect short range order, but lacking long range order, the A. metallica has metallic blue color on its dorsal wings and a very shiny goldish green color on the ventral side. Surprisingly, both colors of A. metallica are achieved with similar quasi-ordered layered structures with slightly different structural parameters. The examination of four different PBG materials built with the same two components: air and chitin, allowed the
comparison of the efficiency of different structures. The PGB material found in the ventral scales of A. metallica is the most efficient reflector from the four investigated ones. It is worth pointing out that it is not a rigorously ordered structure and it is based on a moderate refractive index contrast. Such quasiordered nanostructures, which may be more easily manufactured than the perfectly ordered 3D nanostructures could prove useful in various practical applications. The investigation of natural designs may yield very useful templates for artificial structures. Acknowledgements The work was supported by EU6 NEST/PATHFINDER/ BioPhot-01915. The work in Hungary was partly supported by OTKA No. 042972. The work in Belgium was partly carried out with support from EU5 Centre of Excellence ICAI-CT-2000– 70029 and from the Inter-University Attraction Pole (IUAP P5/1) on “Quantum-size effects in nanostructured materials” of the Belgian Office for Scientific, Technical, and Cultural Affairs. Correspondence and requests for materials should be addressed to László P. Biró, [email protected]. References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] E. Yablonovitch, T.J. Gmitter, Phys. Rev. Lett. 63 (1989) 1950. [3] L.P. Biró, Zs. Bálint, K. Kertész, Z. Vértesy, G.I. Márk, Z.E. Horváth, J. Balázs, D. Méhn, I. Kiricsi, V. Lousse, J.-P. Vigneron, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 67 (2003) 021907. [4] J.D. Joannopoulos, R.D. Meade, J.N. Winn, Photonic Crystals, Molding the Flow of Light, Princeton University Press, Princeton, 1995. [5] S.F. Mingaleev, M. Schillinger, D. Hermann, K. Busch, Optics Lett. 29 (2004) 2858. [6] K. Srinivasan, P.E. Barclay, O. Painter, J. Chen, A.Y. Cho, C. Gmachl, Appl. Phys. Lett. 83 (2003) 1915. [7] P. Gould, Materials Today (September 2002) 32. [8] R.C. Schroden, M. Al-Daous, Ch.F. Blanford, A. Stein, Chem. Mater. 14 (2002) 3305. [9] H. Fudouzi, Y. Xia, Langmuir 19 (2003) 9653. [10] P. Vukusic, J.R. Sambles, Nature 424 (2003) 852. [11] S. Kinoshita, S. Yoshioka, Y. Fujii, N. Okamoto, Forma 17 (2002) 103. [12] S. Berthier, Iridescences, les couleurs physiques des insectes, SpringerVerlag, Paris, 2003. [13] J.P. Vigneron, J.-F. Colomer, M. Rassart, A.L. Ingram, V. Lousse, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 72 (2005) 061904. [14] J.P. Vigneron, M. Rassart, C. Vandenbem, V. Lousse, O. Deparis, L.P. Biró, D. Dedouaire, A. Cornet, P. Defrance, Phys. Rev., E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 73 (2006) 041905.
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