An experimental study in the variation of the near-circular polarisation properties in Scarabaeoidae beetles

• Main Author: Carter, Ian Edward
• Other Authors: Weir, Kenny ; McCall, Martin W.
• Published: Imperial College London 2017
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.745266

The scientific study of structural colour in nature dates back over 300 years. The structural colours being observed are a result of millions of years of evolution. It occurs in many different forms in nature from diffraction gratings to interference from multi layered structures and from plants to cephalopods. This thesis focuses on the near circularly polarised (CP) reflection from the helicoidally arranged chitin within the shells of selected beetles from the Scarabaeoidae superfamily. An experimental setup was refined in order to measure broadband CP light in reflection from the beetles. A large variation in the left CP reflection was seen both within and between species, genera, subfamilies and families. This included broadband reflection from silver coloured beetles to narrower reflection of dominant green colour (other colours were observed). A wide variety of spectral shapes were observed including single wavelength peaks, multiple oscillations and peaks far apart. In the Chrysina genus several new species were observed to reflect both right and left CP light. Several perturbations of the CP reflection were modelled based upon Berreman calculus. Further to the CP measurements electron, atomic force and scanning near-field optical microscope images were taken of both the helicoidal structure and surface of the beetles. A fuller description of the polarisation of the reflected near CP light from the beetles’ shells was obtained using a broadband Mueller Matrix (MM) ellipsometer, which was built and designed using liquid crystal variable retarders. From the MMs, the ellipticity, the degree of polarisation and azimuth angle were calculated. As with the CP measurements large variations were observed from highly polarised metallic to less polarised matt specimens. This study of the beetle optical properties will help to inspire the design of various optical devices as well as the further biological investigation into beetles’ polarised vision.