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.

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figure 1. Photonic structures in nature
a) The elytra and the ventral side of the Japanese jewel beetle display bright and iridescent colours that show a strong angular dependence caused by a multilayer interference stack shown in the electron micrograph.
b) The wings of the Madagascan sunset moth are covered in colourful scales reflecting green, yellow, red and violet light in different regions of the wings. The scales are highly curved so that the observer only perceives light reflected from part of the scales, which creates the impression of texture due to the juxtaposition of bright and dark regions. A cross-section through one scale reveals the origin of the colour which arises from a regular multilayer arrangement of cuticle layers held apart at a well-defined distance by regularly spaced cuticle pillars.
c) The luscious blue of the peacock’s feathers results from a two-dimensional photonic crystal-like structure in the feather barbules.2, 3, 4

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.

figure 2.- Floral diffraction structures
a) The base of Hibiscus trionum petals shows iridescence overlying red pigment, resulting from regular micro-striations on the petal surface.
b) Scanning electron microscopy reveals the striations, found on the petal surface of Tulipa kolpakowskiana. The inset visualises the diffraction caused by the petal surface.
c) A cross-section of the petal surface shows the periodically protruding striations forming a diffraction grating which is modulated by
d) a sinusoidal wave imposed by the flower cells.5

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.

Multilayered structures
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.

figure 3.- A sculpted multilayer
a) The Indonesian butterfly Papilio blumei.
b) The bright green areas on the wings result from colour mixing on the scales which reflect yellow and blue from different microscopic regions.
c) High resolution optical micrograph of a scale in unpolarised light (left) and between crossed polarisers (right).
d) Scanning electron micrograph of the scale surface. The observed concavities are clad by an air-cuticle multilayer

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.

figure 4.- Copying the butterfly’s photonic structure
a) A structurally identical artificial mimic, observed at normal light incidence (left) and at grazing illumination (right). The artificial structure was manufactured using a combination of colloidal self-assembly, electrodeposition and atomic layer deposition.
b) Micrograph of the artificial copy in unpolarised light (left) and between crossed polarisers (right).
c) Cross-sectional view of the oxide multilayer cladding on the concavity wall.
d) A structurally simplified mimic with superior optical performance observed in specular reflection direction (left) and in retro-reflection (right).
e) Micrographs taken in unpolarised light (left) and between crossed polarisers (right).
f) Cross-sectional view of the simplified structure.
g) h) i) A motif encoded in the artificial butterfly structure, achieved by implementing an additional photolithographic step in the manufacturing procedure. The photonic structure reflects very distinct colours depending on the angle of illumination and the observation position.

Drawbacks
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.

References
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.