Photonic Structures in Nature: Through Order, Quasi-order and Disorder
Nixon, Matthew Robert
Date: 30 April 2014
University of Exeter
PhD in Physics
The majority of colours in the natural world are produced via the wavelength selective absorption of light by pigmentation. Some species of both flora and fauna, however, are particularly eye-catching and visually remarkable as a result of the sub-micron, light-manipulating architecture of their outer-integument material. This thesis ...
The majority of colours in the natural world are produced via the wavelength selective absorption of light by pigmentation. Some species of both flora and fauna, however, are particularly eye-catching and visually remarkable as a result of the sub-micron, light-manipulating architecture of their outer-integument material. This thesis describes detailed investigations of a range of previously unstudied photonic structures that underpin the creation of the interesting visual appearances of several such species of flora and fauna. These structures were examined using a variety of methods, including optical microscopy, scanning and transmission electron microscopy, focused ion-beam milling and atomic force microscopy. This enabled detailed characterisation of the species’ photonic systems. The degree of order discerned in the species’ photonic structures ranged from: ‘ordered’ systems, where multiple layers of two materials produces metallic and often mirror-like reflections; to ‘quasi-ordered’ systems, where an average periodicity of the structure in all directions gives rise to diffuse, coloured scatter; to disordered systems, where no discernible order is observed, which results in a diffuse, broad-band, white appearance. In addition to this, the range of systems also encompassed: periodicities in one-dimension in the form of multilayering; ‘quasi-two-dimensional’ structures in the form of aligned fibres; and three-dimensional structures formed from arrangements of spherical particles. Alongside this experimental characterisation, an in-depth series of supporting theoretical analyses were undertaken. For the one-dimensional systems studied here, the models’ theoretical reflectance was calculated using analytical methods. For other systems, with more complex structural-geometries, theoretical simulations of their electromagnetic response to incident radiation were carried out using finite-difference-time-domain and finite-element-method numerical modelling approaches. Theoretical modelling results were compared to experimental measurements of each sample's optical properties. These were primarily reflectance measurements, which were taken using a range of techniques appropriate for each specific investigation. In addition to this, a synthetic sample, mimicking the white-appearance and remarkable polarisation-dependant reflectance of one insect’s photonic structure, was created using polymer electrospinning. Using these experimental measurements and theoretical simulation predictions, the structural colour production mechanisms adopted by several species of flora and fauna were elucidated.
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