J.R. Sambles

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.

Structural colour: Colour mixing in wing scales of a butterfly

Green coloration in the animal kingdom, as seen in birds feathers and reptile integument, is often an additive mixture of structurally effected blue and pigmentary yellow. Here we investigate the origin of the bright green coloration of the wing scales of the Indonesian male Papilio palinurus butterfly, the microstructure of which generates an extraordinary combination of both yellow and blue iridescence. The dual colour arises from a modulation imposed on the multilayer, producing the blue component as a result of a previously undiscovered retro-reflection process.

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.

The resistivity of thin metal films—Some critical remarks

Abstract This article highlights the problems associated with interpreting data on the resistivity of thin films. It is pointed out that extreme care must be taken in analysing data and a thorough study should be made of the morphology of the films from which data are taken. Without this, values deduced for parameters such as the surface roughness, the specularity and in particular the product of the bulk resistivity and the bulk mean free path are worthless. Even when the scattering contributions from the surface and the grain boundaries are taken properly into account, because of the high defect and dislocation densities in many of the films studied the values deduced for these parameters are still in doubt.

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.

Cobalt phthalocyanine as a basis for the optical sensing of nitrogen dioxide using surface plasmon resonance

Abstract The technique of surface plasmon resonance has been utilized to investigate the viability of using cobalt phthalocyanine as a basis for an optical sensor for nitrogen dioxide. The response of a thin layer of cobalt phthalocyanine in such a system to exposure of varying concentrations of nitrogen dioxide is measured and commented upon. Changes in the optical parameters of such a film that result from exposure to nitrogen dioxide are extracted using appropriate theory and explanation is given as to their cause.

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.

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 (k = 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.

Photoluminescence from dye molecules on silver gratings

Excited dye molecules on metallic gratings can relax by generating surface plasmon polaritons (SPPs). These non-radiative modes can scatter from the grating emitting light in well-defined direction and with a characteristic polarisation. This paper presents an experimental study of the emission properties of a thin layer of laser dye on a silver grating. It is found that the emission is dominated by light re-radiated by SPPs. The direction and the polarisation of the emission are found to depend on the azimuthal angle between the emitted light and the grating grooves. The experimental results are explained in terms of the momentum matching conditions that govern the coupling between photons and SPPs.