Marie Rassart

Diffractive hygrochromic effect in the cuticle of the hercules beetle Dynastes hercules

The elytra from dry specimens of the hercules beetle, Dynastes hercules appear khaki-green in a dry atmosphere and turn black passively under high humidity levels. New scanning electron images, spectrophotometric measurements and physical modelling are used to unveil the mechanism of this colouration switch. The visible dry-state greenish colouration originates from a widely open porous layer located 3 μm below the cuticle surface. The structure of this layer is three-dimensional, with a network of filamentary strings, arranged in layers parallel to the cuticle surface and stiffening an array of strong cylindrical pillars oriented normal to the surface. Unexpectedly, diffraction plays a significant role in the broadband colouration of the cuticle in the dry state. The backscattering caused by this layer disappears when water infiltrates the structure and weakens the refractive index differences.

Optical structure and function of the white filamentary hair covering the edelweiss bracts.

The optical properties of the inflorescence of the high-altitude Leontopodium nivale subsp. alpinum (edelweiss) is investigated, in relation with its submicrometer structure, as determined by scanning electron microscopy. The filaments forming the hair layer have been found to exhibit an internal structure which may be one of the few examples of a photonic structure found in a plant. Measurements of light transmission through a self-supported layer of hair pads taken from the bracts supports the idea that the wooly layer covering the plant absorbs near-ultraviolet radiation before it reaches the cellular tissue. Calculations based on a photonic-crystal model provide insight on the way radiation can be absorbed by the filamentary threads.

Structurally tuned iridescent surfaces inspired by nature

Iridescent surfaces exhibit vivid colours which change with the angle of incidence or viewing due to optical wave interference in the multilayer structure present at the wavelength scale underneath the surface. In nature, one can find examples of iridescent Coleoptera for which the hue changes either greatly or slightly with the angle. Because these species typically make these structures from a single biological material (usually chitin) and air or water as the low refractive index component, they have evolved by adjusting the layer thicknesses in order to display quite different iridescent aspects. Taking inspiration from this proven strategy, we have designed and fabricated periodic TiO2/SiO2 multilayer films in order to demonstrate the concept of structurally tuned iridescent surfaces. Titanium or silicon oxide layers were deposited on a glass substrate using dc reactive or RF magnetron sputtering techniques, respectively. Two structures were designed for which the period and the TiO2/SiO2 layer thickness ratio were varied in such a way that the films displayed radically different iridescent aspects: a reddish-to-greenish changing hue and a stable bluish hue. The fabricated samples were characterized through specular reflectance/transmittance measurements. Modelling of transmittance spectra using standard multilayer film theory confirmed the high quality of the twelve-period Bragg reflectors. The chromaticity coordinates, which were calculated from measured reflectance spectra taken at different angles, were in accordance with theoretical predictions.

Nanomorphology of the blue iridescent wings of a giant tropical wasp, Megascolia procer javanensis (Hymenoptera)

The wings of the giant wasp Megascolia procer javanensis are opaque and iridescent. The origin of the blue-green iridescence is studied in detail, using reflection spectroscopy, scanning electron microscopy, and physical modeling. It is shown that the structure responsible for the iridescence is a single homogeneous transparent chitin layer covering the whole surface of each wing. The opacity is essentially due to the presence of melanin in the stratified medium which forms the mechanical core of the wing.

Possible uses of the layered structure found in the scales of Hoplia coerulea (Coleoptera)

The male of the beetle Hoplia coerulea is known for its spectacular blue-violet iridescence. The blue coloration is caused by the presence of an interesting photonic structure inside the scales which cover the dorsal parts of the insects body. This structure can be described as the stacking of chitin plates supporting arrays of parallel rods. The change of colour of this structure with humidity is investigated, as well as its response to some other external conditions, such as mechanical strain.

Complex optical structure in the ribbon-like feathers of the African open-bill stork

The structural origin of the weak iridescence from the very peculiar ribbon-shaped feathers of the African open-bill stork, Anastomus lamelligerus (Ciconiidae) is investigated, using a combination of spectrophotometry, electron microscopy, and theoretical modelling. The cortex of these feathers can be described as a slab of keratin, transformed into a multilayer by the insertion of thin parallel planes containing harder nodules, disposed sideby- side and oriented along the feather axis. These nodules each show a sperically capped cylindrical shape. An empty cylindrical channel - the vacuole - occupies the long axis of the nodule. These nodules act in a collective and individual way to produce the frequency selection giving rise to the observed dark-green coloration of these special feathers.