Photonic devices are based on regularly structured materials or material combinations with periodicities in the order of the wavelength of visible light. These structures inﬂuence the ﬂow of light, which quantum unit is the photon, similar to the effect that common semiconductors with their periodic crystal lattice have on the propagation of an electric current. These photonic structures have a considerable potential for applications in security printing, as Mathias Kolle and Ullrich Steiner explain in this article.
During the past decade, optically variable devices have improved considerably, which is obvious even to the layman (it suffices to compare the quality of the embossed holograms on credit cards issued three years ago with the ones that are currently available). Although pigment-based inks and dyes can be used for the creation of micro-optic imagery, they cannot create effects such as interference, diffraction or coherent scattering which are the fundamental physical principles found in optically variable devices (OVDs). That is why for instance Kinegrams®, OVDs supplied by OVD Kinegram AG, make use of photonic structures encoding visual information in micro- and macroscopic images.1 The requirement for continuous improvement of optical security labels imposed by the necessity to ensure effective counterfeiting protection can be addressed by taking an alternative approach based on lessons learnt from nature.
Natural optical devices
Many organisms in fauna and ﬂora employ micro- and nanostructures to achieve particularly impressive optical effects such as spectrally selective near-perfect reﬂection, very high transmission and angle-dependent colour variations. Several insects of the order Lepidoptera and Coleoptera (for example the South American butterﬂy Morpho rhetenor, the Japanese jewel beetle Chrysochroa fulgidissima and the Madagascan sunset moth Chrysiridia rhipheus) and birds, such as the peacock, have surface structures that ‘bend’ light according to their speciﬁc requirements (see figure 1). In all cases the speciﬁc optical signature of wing scales, shells or feathers arises from regular micro- to nano-sized periodic surface or volume patterns.
Optically relevant structures
The simplest natural optical devices for colour–selective reﬂection consist of planar multilayers as found in the hardened forewings (‘elytra’) of the Japanese jewel beetle and in the scales of the sunset moth. These multilayers are indeed very similar to the multilayer interference ﬁlters incorporated into the laminates that are used for “wrapping” security documents.
Diffraction gratings are another optically relevant structure found for instance on the ﬂower petal surface of tulip plants5 (see figure 2) and on the dorsal surfaces of spiders6. Sophisticated artiﬁcial analogues, one- and two-dimensional embossed diffraction gratings, are widely used in security printing.
Multilayer laminates and embossed diffraction gratings on their own are certainly easier to replicate and forge than more complex photonic structures consisting of speciﬁc combinations of multilayers interwoven with diffraction gratings or holograms. The ability to manufacture concave or convex multilayered micro-and nanostructures (as opposed to simple planar or square geometries) can be applied to increase the complexity and recognition value and will therefore be able to contribute to improved counterfeit security of optically variable devices.
The Indonesian swallowtail butterﬂies Papilio
palinurus7 and Papilio blumei8 are natural role models for complex multilayered structures. These butterﬂies feature bright green stripes on their wings which change to a blue colour upon grazing light incidence or observation at very shallow angles (see figures 3a and b). The wings are covered in thousands of little scales arranged in a rooftop tile pattern and the surface of each scale has regularly arranged concave indentations. The centres of these concavities reﬂect bright yellow to green while the edges appear blue. In addition, the blue light resulting from reﬂections off the edges undergoes a polarisation rotation that can be observed when placing a scale between crossed polarisers in an optical microscope (see figure 3c). A cross-section of a single scale, imaged by transmission electron microscopy, reveals the intricate concavely shaped multilayer structure responsible for the bright colour (see figure 3d).7 Macroscopically, this juxtaposition of yellow and blue reﬂected from spatially distinct regions on the scale results in a lucid green perceived by the human eye.
Developments in replicating photonic structures
Photonic structures similar to the ones found on the swallowtail’s scales promise to be difficult to forge since one needs to have detailed knowledge of the composite material and its local ﬁne-structure. Different routes can be taken to create photonic structures that superﬁcially resemble each other in their optical appearance. However, in order to create an identical counterfeit of a speciﬁc structural design, the procedures used to develop the original have to be intimately known and well-mastered. Consequently, complex photonic structures based on interwoven multilayer and diffraction elements promise versatile applicability for optical labels.
We have recently established a procedure allowing us to faithfully replicate the photonic scale structure of Papilio blumei with an area coverage of several cm². The replicas of the butterﬂy structure are realised in ﬁve steps using a combination of colloid template-assisted electrochemistry and atomic layer deposition. With this procedure, structurally identical replicas of the butterﬂy scale structure can in principle be manufactured on large areas (see figure 4). The employed materials, metals and metal oxides, are common in industry for optical applications and the techniques have already been put to use in an industrial context. Being able to build artiﬁcial photonic structures that are closely based on nature’s blueprints also allows the alteration of the original design to create enhanced optical effects.
A drawback of most currently employed material combinations is their inherent lack of ﬂexibility. Although this does not pose a signiﬁcant problem for photonic applications on reasonably stiff substrates, such as credit cards or labels on branded goods, the mechanical properties of these artiﬁcial butterﬂy structures have to be optimised for their use as optically variable labels on ﬂexible supports such as banknotes. Current development efforts are focussed on the substitution of the metal oxide layers by suitable polymer materials that are likely to even allow dead-folding of the photonic structures due to their high ﬂexibility.
In general, the development and implementation of procedures that ensure satisfactory control of the structural integrity on the nano- and microscale and simultaneously allow for production of the photonic structure on large areas, enabling mass-production of identical features for security documents is not a trivial endeavour. Nevertheless, due to rapid progress in optical technology nowadays many novel processes are developed and established to ensure progressive innovation and improvement of optical labels and counterfeiting protections. Learning and applying tricks from nature for the controlled manipulation of light with photonic structures might help us to continue this trend and to increase the protection of security documents against counterfeiting. However, it is clear that optical signatures can only be a part of the whole, complementing the complete security package which consists of many other haptic, image-based or electronically encoded information.
1 P. Vukusic, “Natural photonics”, Phys. World, 2004, 17, pages 35-39.
2 S. Kinoshita and S. Yoshioka, “Structural Colors in Nature: The Role of Regularity and Irregularity in the Structure”, ChemPhysChem, 2005, 6, pages 1442-1459.
3 M. Kolle, “Photonic structures inspired by nature”, Springer Theses, 2010.
4 J. Zi, X. Yu, Y. Li, X. Hu, C. Xu, X. Wang, X. Liu and R. Fu, “Coloration strategies in peacock feathers”, PNAS, 2003, 100, pages 12576-12578.
5 H. Whitney, M. Kolle, P. Andrew, L. Chittka, U. Steiner and
B. J. Glover, “Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators”, Science, 2009, 323, pages 130-133.
6 A.R. Parker and Z. Hegedus, “Diffractive optics in spiders”,
J. Opt. A: Pure Appl. Opt. 5, 2003, pages 111-116.
7 P. Vukusic, J. R. Sambles and C. Lawrence, “Structural colour: Colour mixing in wing scales of a butterﬂy”, Nature, 2000, 404, page 457.
8 M. Kolle, P. M. Salgard-Cunha, M. R. J. Scherer, F. Huang, S. Mahajan, P. Vukusic, J. J. Baumberg and U. Steiner, “Mimicry of Papilio blumei’s colourful wing scale structure”, Nature Nanotechnology, 2010, 5, pages 511-515.
9 M. Kolle, B. Zheng, N. Gibbons, J. Baumberg and U. Steiner, “Stretch-tuneable dielectric mirrors and optical microcavities”, 2010, 18, pages 4356-4364.
Ullrich Steiner is Professor of the Physics of Materials in the Department of Physics at the University of Cambridge, UK. His areas of research include the physics of pattern formation on surfaces and in thin films and biomimetic formation of sub-micrometre morphologies.