It is becoming increasingly evident that Nature has developed a panoply of sophisticated
optical systems which enable living creatures to control the way they reflect light.
In most circumstances, chemical means (pigments) are used, but
sometimes organisms need to use light efficiently, or particular pigments are
difficult to make, or expensive in terms of energy. They then
resort to structural means, creating composite materials in which
interference and diffraction are used to create striking or
camouflaging colour effects.
We discuss here the structures present in a marine creature, the sea mouse (Aphrodita sp., Polychaeta:Aphroditidae), and a butterfly (Teinopalpus imperialis). We show both exploit partial photonic band gaps to achieve colour effects. In the former case, the sea mouse achieves a striking iridescence of its spines and felt, with a strong peak reflectance which tunes with angle. The iridescent effects are shown in Figure 1 and the microstructure responsible for this is shown in Figure 2.
We compare the results of laser microreflectometer measurements of sea mouse hairs with calculations treating the microstructure as a stack of gratings, and with band structure calculations.
In the case of Teinopalpus imperialis, it achieves a structural and non-iridescent green through the use of a mosaic structure, consisting of a distorted silicon structure. It can also achieve a remarkable structural black by adapting this structure. We comment on the determination of the wing scale structure, and aspects of the resulting optical properties.
We comment on what we may learn from these and other natural systems, and possible applications for the designs they have evolved, and the methods they have developed to achieve their optical microstructures.
Figures and their captions
Figure 1 [.ps 17 MB] Reflectance of light by a sea mouse spine, showing strong colouration in the red.
Figure 2 [.ps 5.5 MB] A micrograph of the wall of a spine, showing hexagonally-packed voids in a chitin matrix, with a spacing of 0.51 microns.
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