Experimental techniques for the study of natural photonic structures

This thesis presents a study into structural colours that exist in natural samples, the principle aim of which is to produce experimental methods by which these colours may be examined and evaluated. In order to achieve this, previously observed structures are described, electromagnetic theory is summarised and a series of samples are examined constituting examples of the structures present in nature. The first sample discussed is the multilayer in the epicuticle of the buprestid beetle, C. raja. In order to evaluate the refractive indices of the layers contained within this structure, existing optical techniques are used to establish absolute reflection spectra for a number of angles of incidence in both linear polarisations. The approximate design for the structure is obtained by electron microscopy and modelled using Fresnel's equations. This model is then refined by a recursive least squares fitting routine to obtain the refractive indices. The second sample is the diffuse white scattering structure in the scales of two white beetles, Lepidiota stigma and Cyphochilus spp.. The reflection from these scales is measured and found to be brilliantly white due to the irregular internal structure of the scales. Comparison of the Fast Fourier Transforms of TEM images of the internal structure with the diffraction pattern obtained from monochromatic laser light diffracting through a single scale demonstrate a link between this structure and photonic effects. The third sample type are found in the scales of the large true weevils, Eupholus schoenherri pettiti and E. magnificus. These scales are shown to have a domained structure in which the domains were oriented differently to each other. Single domains are shown to exhibit different colours at different orientation. The final sample is the highly regular 2-dimensional diffraction grating observed in a marine diatom, Coscinodiscus wailesii. Diffraction is demonstrated by measuring the in-plane diffraction from a single frustule for both monochromatic laser light and white light, showing an enhanced transmission for red wavelengths. Subsequent imaging of the transmitted diffraction pattern allows for the calculation of the transmitted power in each diffracted order.Engineering and Physical Sciences Research Council (EPSRC

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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

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