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

Copyright © 2007 Optical Society of America. This paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-7-4351 . Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.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

The nature and structure of the white-reflecting underside ‘scales’ on the hind wing of Pseudolestes mirabilis (Odonata: Pseudolestidae)

This is the final version of the article. Available from the publisher via the DOI in this record.The Hainanese endemic damselfly, Pseudolestes mirabilis, is unique among the Odonata in having brilliant silvery-white reflective areas on the underside of the hind wings in mature males. The light reflected is easily seen to be several times brighter than that from normal white pruinescence. The hind wing upsides have a striking coppery appearance due to the filtering of light reflected from the inside of the reflective area through bright amber tinted wing membranes, colour which results from small amounts of melanin in those parts of the membrane. Visual signals are thus produced from both sides of the wing and may be used to advertise territory occupancy while perched, as well as having an obvious semiotic function in aerial agonistic displays between pairs of males. The structure consists of a deep layer of long, parallel, flat wax fibres, secreted from the faces of cross-veins in individual wing cells over the affected areas. This is a spectacular and novel mode of cuticular wax secretion. The structure adds about 23–27% to the mass of the hind wing, which may explain its unusual shape and shortness. However this character is also present in females, which lack the wax fibres, hence it may be an unusual example of an epigametic male trait being partially expressed in females.P. Vukusic and M.R. Nixon acknowledge the financial support of AFOSR grant FA9550-10-1-0020

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

Copyright © 2009 Optical Society of America. This paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-17-14729 . Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.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

Sculpted-multilayer optical effects in two species of Papilio butterfly

Copyright © 2001 Optical Society of America. This paper was published in Applied Optics and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-40-7-1116. Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law.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

Surface plasmons at the Brillouin zone boundary of an oblique lattice

Copyright © 2015 AIP PublishingIn periodic systems of low-symmetry, the Bragg condition for the complete interference of waves along the contour of the Brillouin zone (BZ) boundary is not generally satisfied. As a result, band-gaps do not necessarily occur at this boundary. This letter demonstrates this experimentally by recording the iso-frequency contours for surface plasmon polaritons (SPPs) supported on a diffraction grating with an underlying 2D oblique Bravias lattice. It is shown that these contours do not intersect the BZ boundary perpendicularly, as the symmetry operations of the lattice place no conditions on the surface wave interference at this boundary.Engineering and Physical Sciences Research Council (EPSRC)HP Labs BristolUSA

Circularly polarised colour of the scarab beetle Chalcothea smaragdina: light scattering by a dual photonic structure

This is the author accepted manuscript. The final version is available from the Royal Society via the DOI in this record.Helicoidal architectures comprising various polysaccharides such as chitin and cellulose have been reported in biological systems. In some cases, these architectures exhibit stunning optical properties analogous to ordered cholesteric liquid crystal phases. In this work, we characterise the circularly polarised reflectance and optical scattering from the cuticle of the beetle Chalcothea smaragdina (Coleoptera: Scarabaeidae: Cetoniinae) using optical experiments, simulations and structural analysis. The selective reflection of left-handed circularly polarised light is attributed to a Bouligand-type helicoidal morphology within the beetle’s exocuticle. Using electron microscopy to inform electromagnetic simulations of this anisotropic stratified medium, the inextricable connection between the colour appearance of C. smaragdina and the periodicity of its helicoidal rotation is shown. A close agreement between the model and the measured reflectance spectra is obtained. In addition, the elytral surface of C. smaragdina possesses a blazed diffraction grating-like surface structure, which affects the diffuse appearance of the beetle’s reflected colour, and therefore potentially enhances crypsis amongst the dense foliage of its rainforest habitat.This work was financially supported by the National Centre of Competence in Research BioInspired Materials and the Ambizione program of the Swiss National Science Foundation SNSF (PZ00P2 168223, to BDW)

Bio-Inspired Band-Gap Tunable Elastic Optical Multilayer Fibers

Engineering and Applied Science

Bright-white beetle scales optimise multiple scattering of light.

Whiteness arises from diffuse and broadband reflection of light typically achieved through optical scattering in randomly structured media. In contrast to structural colour due to coherent scattering, white appearance generally requires a relatively thick system comprising randomly positioned high refractive-index scattering centres. Here, we show that the exceptionally bright white appearance of Cyphochilus and Lepidiota stigma beetles arises from a remarkably optimised anisotropy of intra-scale chitin networks, which act as a dense scattering media. Using time-resolved measurements, we show that light propagating in the scales of the beetles undergoes pronounced multiple scattering that is associated with the lowest transport mean free path reported to date for low-refractive-index systems. Our light transport investigation unveil high level of optimisation that achieves high-brightness white in a thin low-mass-per-unit-area anisotropic disordered nanostructure.The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement n [291349] and USAF grant FA9550-10-1-0020.This is the final published version, also available from Nature Publishing at http://www.nature.com/srep/2014/140815/srep06075/full/srep06075.html

Structural colour from helicoidal cell-wall architecture in fruits of Margaritaria nobilis

The bright and intense blue-green coloration of the fruits of Margaritaria nobilis (Phyllanthaceae) was investigated using polarization-resolved spectroscopy and transmission electron microscopy. Optical measurements of freshly collected fruits revealed a strong circularly polarized reflection of the fruit that originates from a cellulose helicoidal cell wall structure in the pericarp cells. Hyperspectral microscopy was used to capture the iridescent effect at the single-cell level.Leverhulme Trust (F/09-741/G)United States. Air Force Office of Scientific Research (award number FA9550-10-1-0020)Adolphe Merkle FoundationSwiss National Science Foundation (National Centre of Competence in Research Bio-Inspired Materials)Biotechnology and Biological Sciences Research Council (Great Britain) (BBSRC David Phillips fellowship (BB/K014617/1)