Millions of years before we began to manipulate the flow of light using synthetic structures, biological systems were using nanometre-scale architectures to produce striking optical effects. An astonishing variety of natural photonic structures exists: a species of Brittlestar uses photonic elements composed of calcite to collect light, Morpho butterflies use multiple layers of cuticle and air to produce their striking blue colour and some insects use arrays of elements, known as nipple arrays, to reduce reflectivity in their compound eyes. Natural photonic structures are providing inspiration for technological applications.

Brilliant iridescent colouring in male butter£ies enables long-range conspeci¢c communication and it has long been accepted that microstructures, rather than pigments, are responsible for this coloration. Few studies, however, explicitly relate the intra-scale microstructures to overall butter£y visibility, both in terms of re£ected and transmitted intensities and viewing angles.Using a focused-laser technique, we investigated the absolute re£ectivity and transmissivity associated with the single-scale microstructures of two species of Morpho butter£y and the mechanisms behind their remarkable wide-angle visibility. Measurements indicate that certain Morpho microstructures re£ect up to 75% of the incident blue light over an angle range of greater than 1008 in one plane and 158 in the other.We show that incorporation of a second layer of more transparent scales, above a layer of highly iridescent scales, leads to very strong di¡raction, and we suggest this e¡ect acts to increase further the angle range over which incident light is re£ected.Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single-scale microstructure to be n (1.56 AE 0.01) + (0.06 AE 0.01)i. This ¢gure is required for theoretical modelling of such microstructure systems.

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The brightest and most vivid colours in nature arise from the interaction of light with surfaces that exhibit periodic structure on the micro-and nanoscale. In the wings of butterflies, for example, a combination of multilayer interference, optical gratings, photonic crystals and other optical structures gives rise to complex colour mixing. Although the physics of structural colours is well understood, it remains a challenge to create artificial replicas of natural photonic structures [1][2][3] . Here we use a combination of layer deposition techniques, including colloidal self-assembly, sputtering and atomic layer deposition, to fabricate photonic structures that mimic the colour mixing effect found on the wings of the Indonesian butterfly Papilio blumei. We also show that a conceptual variation to the natural structure leads to enhanced optical properties. Our approach offers improved efficiency, versatility and scalability compared with previous approaches [4][5][6] .The intricate structures found on the wing scales of butterflies are difficult to copy, and it is particularly challenging to mimic the colour mixing effects displayed by P. blumei and P. palinurus 7,8 . The wing scales of these butterflies consist of regularly deformed multilayer stacks that are made from alternating layers of cuticle and air, and they create intense structural colours (Fig. 1). Although the P. blumei wing scales have previously been used as a template for atomic layer deposition (ALD) 9 , such an approach is not suitable for accurate replication of the internal multilayer structure on large surface areas.The bright green coloured areas on P. blumei and P. palinurus wings result from a juxtaposition of blue and yellow-green light reflected from different microscopic regions on the wing scales. Light microscopy reveals that these regions are the centres (yellow) and edges (blue) of concavities that are 5-10 mm wide, clad with a perforated cuticle multilayer 7 (Fig. 1d,e). For normal light incidence, the cuticle-air multilayer shows a reflectance peak at a wavelength of l max ¼ 525 nm, which shifts to l max ¼ 477 nm for light incident at an angle of 458. Light from the centre of the cavity is directly reflected, whereas retro-reflection of light incident onto the concavity edges occurs by double reflection off the cavity multilayer (Fig. 1g). This double reflection induces a geometrical polarization rotation 10 . If light that is polarized at an angle c to the initial plane of incidence is retro-reflected by the double bounce, it will pick up a polarization rotation of 2c and the intensity distribution through collinear polarisers is therefore given by cos 2 (2c). This leads to an interesting phenomenon: when placing the sample between crossed polarizers, light reflected off the centres of the cavities is suppressed, whereas retro-reflected light from four segments of the cavity edges is detected 10,11 (Fig. 1d, right). In microstructures without this double reflection, both the colour mixing and polarization conversion are absent...

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