Peter Vukusic

Quantified interference and diffraction in single Morpho butterfly scales

Brilliant iridescent colouring in male butterflies enables long–range conspecific 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 butterfly visibility, both in terms of reflected and transmitted intensities and viewing angles. Using a focused–laser technique, we investigated the absolute reflectivity and transmissivity associated with the single–scale microstructures of two species of Morpho butterfly and the mechanisms behind their remarkable wide–angle visibility. Measurements indicate that certain Morpho microstructures reflect up to 75% of the incident blue light over an angle range of greater than 100° in one plane and 15° 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 diffraction, and we suggest this effect acts to increase further the angle range over which incident light is reflected. Measurements using index-matching techniques yield the complex refractive index of the cuticle material comprising the single–scale microstructure to be n = (1.56+0.01) + (0.06 ±0.01)i. This figure is required for theoretical modelling of such microstructure systems.

Mimicking the colourful wing scale structure of the Papilio blumei butterfly.

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. 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.

Directionally Controlled Fluorescence Emission in Butterflies

Recently developed, high-efficiency, light-emitting diodes use two-dimensional photonic crystals to enhance the extraction of otherwise internally trapped light and multilayer reflectors to control the direction of light emission. This work describes the characterization of a naturally evolved light-extraction system on the wing scales of a small group of Papilio butterflies. The efficient extraction of fluorescence from these scales is facilitated by a two-dimensional photonic crystal slab that uses a multilayer to help control emission direction. Its light-extraction function is analogous to that of the light-emitting diode.

Bio‐Inspired Band‐Gap Tunable Elastic Optical Multilayer Fibers

The concentrically-layered photonic structure found in the tropical fruit Margaritaria nobilis serves as inspiration for photonic fibers with mechanically tunable band-gap. The fibers show the spectral filtering capabilities of a planar Bragg stack while the microscopic curvature decreases the strong directional chromaticity associated with flat multilayers. Elongation of the elastic fibers results in a shift of the reflection of over 200 nm.

Structural colour: Now you see it — now you don't

The dazzling iridescence seen in some hummingbirds and tropical butterflies arises from natural optical phenomena, the brightest of which originate in nanoscale structures that produce ultra-high reflectivity and narrow-band spectral purity. Here we investigate the coloration of male Ancyluris meliboeus Fabricius butterflies, which have patches of unusual microstructure on their ventral wing scales. We find that this highly tilted, multilayered arrangement produces a bright iridescence of broad wavelength range and generates a strong flicker contrast from minimal wing movement.

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 nano–structure of the wing scales of these regions contributes significantly to their black appearance.

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.

A protean palette: colour materials and mixing in birds and butterflies

While typically classified as either ‘structural’ or ‘pigmentary’, bio-optical tissues of terrestrial animals are rarely homogeneous and typically contain both a structural material such as keratin or chitin and one or more pigments. These base materials interact physically and chemically to create colours. Combinations of structured base materials and embedded pigment molecules often interact optically to produce unique colours and optical properties. Therefore, to understand the mechanics and evolution of bio-optical tissues it is critical to understand their material properties, both in isolation and in combination. Here, we review the optics and evolution of coloured tissues with a focus on their base materials, using birds and butterflies as exemplar taxa owing to the strength of our current knowledge of colour production in these animals. We first review what is known of their base materials, and then discuss the consequences of these interactions from an optical perspective. Finally, we suggest directions for future research on colour optics and evolution that will be invaluable as we move towards a fuller understanding of colour in the natural world.

Development of a prototype gas sensor using surface plasmon resonance on gratings

Abstract A good basis has been established for the development of a prototype gas sensor using the phenomenon of surface plasmon resonance. By exciting a surface plasmon on a metallic diffraction grating that is twisted azimuthally so its grooves are not perpendicular to the plane of incidence, and with suitable choice of input and output polarization, a resonance maximum is detected (as opposed to the usual resonance minimum). the operation of the sensor is based on the measurement of this resonance maximum on a background of weak signal and incorporates a sensing head made remote from both the source and detector by means of fibre optics. Its use is demonstrated by sensing remotely the condensation of ≈0.9 nm of isopropyl alcohol onto a silver-coated grating surface.

Pterin pigment granules are responsible for both broadband light scattering and wavelength selective absorption in the wing scales of pierid butterflies.

A small but growing literature indicates that many animal colours are produced by combinations of structural and pigmentary mechanisms. We investigated one such complex colour phenotype: the highly chromatic wing colours of pierid butterflies including oranges, yellows and patterns which appear white to the human eye, but strongly absorb the ultraviolet (UV) wavelengths visible to butterflies. Pierids produce these bright colours using wing scales that contain collections of minute granules. However, to date, no work has directly characterized the molecular composition or optical properties of these granules. We present results that indicate these granules contain pterin pigments. We also find that pterin granules increase light reflection from single wing scales, such that wing scales containing denser granule arrays reflect more light than those with less dense granule collections. As male wing scales contain more pterin granules than those of females, the sexual dichromatism found in many pierid species can be explained by differences in wing scale pterin deposition. Additionally, the colour pattern elements produced by these pterins are known to be important during mating interactions in a number of pierid species. Therefore, we discuss the potential relevance of our results within the framework of sexual selection and colour signal evolution.