Covert linear polarization signatures from brilliant white two-dimensional disordered wing structures of the phoenix damselfly

The damselfly Pseudolestes mirabilis reflects brilliant white on the ventral side of its hindwings and a copper-gold colour on the dorsal side. Unlike many previous investigations of odonate wings, in which colour appearances arise either from multilayer interference or from wing-membrane pigmentation, the whiteness on the wings of P. mirabilis results from light scattered by a specialized arrangement of flattened waxy fibres and the copper-gold colour is produced by pigment-based filtering of this light scatter. The waxy fibres responsible for this optical signature effectively form a structure that is disordered in two dimensions and this also gives rise to distinct optical linear polarization. It is a structure that provides a mechanism enabling P. mirabilis to display its bright wing colours efficiently for territorial signalling, both passively while perched, in which the sunlit copper-gold upperside is presented against a highly contrasting background of foliage, and actively in territorial contests in which the white underside is also presented. It also offers a template for biomimetic high-intensity broadband reflectors that have a pronounced polarization signature.

Wrinkles enhance the diffuse reflection from the dragonfly Rhyothemis resplendens

The dorsal surfaces of the hindwings of the dragonfly Rhyothemis resplendens (Odonata: Libellulidae) reflect a deep blue from the multilayer structure in its wing membrane. The layers within this structure are not flat, but distinctly 'wrinkled', with a thickness of several hundred nanometres and interwrinkle crest distances of 5 µm and greater. A comparison between the backscattered light from R. resplendens and a similar, but un-'wrinkled' multilayer in the damselfly Matronoides cyaneipennis (Odonata: Calopterygidae) shows that the angle over which incident light is backscattered is increased by the wrinkling in the R. resplendens structure. Whereas the reflection from the flat multilayer of M. cyaneipennis is effectively specular, the reflection from the wrinkled R. resplendens multilayer spans 1.47 steradians (equivalent to ±40° for all azimuthal angles). This property enhances the visibility of the static wing over a broader angle range than is normally associated with a smooth multilayer, thereby markedly increasing its conspicuousness.

Subtle design changes control the difference in colour reflection from the dorsal and ventral wing-membrane surfaces of the damselfly Matronoides cyaneipennis

The hind wings of males of the damselfly Matronoides cyaneipennis exhibit iridescence that is blue dorsally and green ventrally. These structures are used semiotically in agonistic and courtship display. Transmission electron microscopy reveals these colours are due to two near-identical 5-layer distributed Bragg reflectors, one placed either side of the wing membrane. Interestingly the thicknesses of corresponding layers in each distributed Bragg reflector are very similar for all but the second layer from each outer surface. This one key difference creates the significant disparity between the reflected spectra from the distributed Bragg reflectors and the observed colours of either side of the wing. Modelling indicates that modifications to the thickness of this layer alone create a greater change in the peak reflected wavelength than is observed for similar modifications to the thickness of any other layer. This results in an optimised and highly effective pair of semiotic reflector systems, based on extremely comparable design parameters, with relatively low material and biomechanical costs.

Electromagnetic characterization of millimetre-scale replicas of the gyroid photonic crystal found in the butterfly Parides sesostris

We have used three-dimensional stereolithography to synthetically replicate the gyroid photonic crystal (PC) structure that occurs naturally in the butterfly Parides sesostris. We have experimentally characterized the transmission response of this structure in the microwave regime at two azimuthal angles (ϕ) over a comprehensive range of polar angles (θ). We have modelled its electromagnetic response using the finite-element method (FEM) and found excellent agreement with experimental data. Both theory and experiment show a single relatively broad transmission minimum at normal incidence (θ = 0°) that comprises several narrow band resonances which separate into clearly identifiable stop-bands at higher polar angles. We have identified the specific effective geometric planes within the crystal, and their associated periodicities that give rise to each of these stop-bands. Through extensive theoretical FEM modelling of the gyroid PC structure, using varying filling fractions of material and air, we have shown that a gyroid PC with material volume fraction of 40 per cent is appropriate for optimizing the reflected bandwidth at normal incidence (for a refractive index contrast of 1.56). This is the same gyroid PC material volume fraction used by the butterfly P. sesostris itself to produce its green structurally coloured appearance. This infers further optimization of this biological PC beyond that of its lattice constant alone.

Discovery of ordered and quasi-ordered photonic crystal structures in the scales of the beetle Eupholus magnificus

The outer wing casings (elytra) of the weevil Eupholus magnificus are marked by yellow and blue bands. We have investigated the scales covering the elytra by using microspectrophotometry, imaging scatterometry, scanning electron microscopy and Fourier transform analysis. We demonstrate that the scales in the yellow elytral bands comprise highly ordered 3D photonic crystal structures, whereas the scales of the blue bands comprise quasi-ordered 3D photonic structures. Both systems, highly contrasting in their periodic order, create approximately angle-independent colour appearances in the far-field. The co-existence of these two contrasting forms of 3D structural order in the same single species is certainly uncommon in natural biological systems and has not been reported in the photonic literature.

Measuring and modelling optical scattering and the colour quality of white pierid butterfly scales

Colouration in butterfly wings is due to the interaction of light with a covering of scales on both wing surfaces. A combination of nanostructure in the scales, which reflect or scatter light, and absorption from chemical pigments in the scales and wing substrate create the final colour appearance. We compared the wing scale morphology of the pierid butterfly Pieris rapae (Small White) to the reflectance spectra from its wings. Its wing scales contain a dense array of pterin pigment beads. A positive correlation between bead-array density and wing reflectance, at wavelengths where the pigment does not absorb, was identified and characterised. We observed, however, that light scatter from these beads does not account for all of the broadband light scatter observed from the wings. The rest of the scale structure plays an important role in achieving high light scatter. Furthermore, combining the underlying scattering and absorption mechanisms within the butterfly scales enabled us to quantify the optical characteristics of the samples using CIELab colour theory.

Physical methods for investigating structural colours in biological systems

Many biological systems are known to use structural colour effects to generate aspects of their appearance and visibility. The study of these phenomena has informed an eclectic group of fields ranging, for example, from evolutionary processes in behavioural biology to micro-optical devices in technologically engineered systems. However, biological photonic systems are invariably structurally and often compositionally more elaborate than most synthetically fabricated photonic systems. For this reason, an appropriate gamut of physical methods and investigative techniques must be applied correctly so that the systems' photonic behaviour may be appropriately understood. Here, we survey a broad range of the most commonly implemented, successfully used and recently innovated physical methods. We discuss the costs and benefits of various spectrometric methods and instruments, namely scatterometers, microspectrophotometers, fibre-optic-connected photodiode array spectrometers and integrating spheres. We then discuss the role of the materials' refractive index and several of the more commonly used theoretical approaches. Finally, we describe the recent developments in the research field of photonic crystals and the implications for the further study of structural coloration in animals.

A biological sub-micron thickness optical broadband reflector characterized using both light and microwaves

Broadband optical reflectors generally function through coherent scattering from systems comprising one of three designs: overlapped; chirped; or chaotic multilayer reflectors. For each, the requirement to scatter a broad band of wavelengths is met through the presence of a variation in nanostructural periodicity running perpendicular to the systems' outer surfaces. Consequently, the requisite total thickness of the multilayer can often be in excess of 50 mum. Here, we report the discovery and the microwave-assisted characterization of a natural system that achieves excellent optical broadband reflectivity but that is less than 1 mum thick. This system, found on the wing scales of the butterfly Argyrophorus argenteus, comprises a distinctive variation in periodicity that runs parallel to the reflecting surface, rather than perpendicular to it. In this way, the requirement for an extensively thick system is removed.

Experimental method for reliably establishing the refractive index of buprestid beetle exocuticle

In this study we apply an existing optical characterisation technique to establish reliably the complex refractive indices of layers comprising a natural multilayer reflector in the beetle Chrysochroa raja. Its reflector characteristics, ultrastructure and layer thicknesses were established using electron and optical microscopy. We recorded a significant number of wavelength dependent optical data sets from the same regions of sample using both linear polarisations and from a variety of different angles. These optical data sets were modelled simultaneously in order to significantly reduce the degeneracy of the fitting process. For the C. raja sample in question, the fitted complex refractive indices of both layer types were determined to be n=1.68 k=0.03 and n=1.55 k=0.14.

Detailed optical study of the transparent wing membranes of the dragonfly Aeshna cyanea

The optical properties of transparent single membranes on the wings of the dragonfly Aeshna cyanea have been investigated. These membranes comprise one central thick cuticular layer covered dorsally and ventrally with typical odonatan wax pruinosity. Optical characterisation of individual membranes reveals they can support optical guided modes comprising differential polarisation reflection. We suggest this may offer an intraspecific signalling channel. The guided modes' characteristics depend on membrane thickness and the nature of the wax pruinosity. We accurately modelled multiple optical data sets simultaneously, thereby inaugurally quantifying the roughness of the pruinosity and the complex refractive indices of the wax and the odonatan cuticle.

Structurally assisted blackness in butterfly scales

Surfaces of low reflectance are ubiquitous in animate systems. They form essential components of the visual appearance of most living species and can explicitly influence other biological functions such as thermoregulation. The blackness associated with all opaque surfaces of low reflectivity has until now been attributed to strongly absorbing pigmentation alone. Our present study challenges this assumption, demonstrating that in addition to the requirement of absorbing pigmentation, complex nano-structures contribute to the low reflectance of certain natural surfaces. We describe preliminary findings of an investigation into the nature of the black regions observed on the dorsal wings of several Lepidoptera. Specifically, we quantify the optical absorption associated with black wing regions on the butterfly Papilio ulysses and find that the nanostructure of the wing scales of these regions contributes significantly to their black appearance.

Remarkable iridescence in the hindwings of the damselfly Neurobasis chinensis chinensis (Linnaeus) (Zygoptera: Calopterygidae)

The bright green dorsal iridescence of the hindwings of Neurobasis chinensis chinensis males, very rare in Odonata, is known to play a significant role in their courtship behaviour. The mechanism responsible for such high contrast and spectrally pure colour has been investigated and found to be optical interference, producing structural colour from distinct laminations in the wing membrane cuticle. The ventral sides of these iridescent wings are dark brown in colour. In a single continuous membrane of wing cuticle, this is an effect that requires a specialized structure. It is accomplished through the presence of high optical absorption (kappa = 0.13) within two thick layers near the ventral surface of the wing, which leads to superior dorsal colour characteristics. By simultaneously fitting five sets of optical reflectivity and transmissivity spectra to theory, we were able to extract very accurate values of the complex refractive index for all three layer types present in the wing. The real parts of these are n = 1.47, 1.68 and 1.74. Although there is often similarly significant dorsal and ventral colour contrast in other structurally coloured natural systems, very few system designs comprise only a single continuous membrane.

Limited-view iridescence in the butterfly Ancyluris meliboeus

Few mechanisms exist in nature that effect colour reflectivity, simultaneously high in spectral purity and in intensity, over a strictly limited portion of solid angle above a surface. Fewer still bring about such colour reflectivity with an angle dependence that is distinct from the colour transition associated with conventional multilayer interference. We have discovered that the ventral wings of the butterfly Ancyluris meliboeus exhibit these optical effects, and that they result from remarkable nano-scale architecture on the wing scales of the butterfly. This nano-structure is in the form of high-tilt multilayering that, as a result of abrupt termination of the multilayers, brings about diffraction concurrently with interference. The product is bright structural colour in a limited angular region over the ventral wing surface that enables remarkably strong flicker and colour contrast through minimal wing movement. The visibility effects associated with its colour, in terms of bright and dark zones of the observation hemisphere over the wing surface, are described. We suggest the purpose of the high-contrast ventral wing visibility associated with A. meliboeus is at-rest signalling; this is distinct from the dorsal wing visibility of other species such as those of the genus Morpho, the function of which is largely for in-flight signalling.

Sculpted-multilayer optical effects in two species of Papilio butterfly

The wing-scale microstructures associated with two species of Papilio butterfly are described and characterized. Despite close similarities in their structures, they do not exhibit analogous optical effects. With Papilio palinurus, deep modulations in its multilayering create bicolor reflectivity with strong polarization effects, and this leads to additive color mixing in certain visual systems. In contrast to this, Papilio ulysses features shallow multilayer modulation that produces monocolor reflectivity without significant polarization effects.