Circularly Polarised Optics in Scarabaeidae
McDonald, Luke Thompson
Thesis or dissertation
University of Exeter
Reason for embargo
Aspects of the research presented in this thesis are still being prepared for/awaiting publication in scientific journals. In addition, Professor Vukusic's research interests include pursuing the research presented in this thesis and expanding the concepts towards the development of synthetic optical devices for technological applications.
In nature, both fundamental and sophisticated ecological functionality alike are driven by the display or, conversely, the concealment of colour and visual contrast. Through scientific investigation, the evolutionary genius manifested in numerous light manipulating biological systems has provided many blueprints for innovative photonic devices and other smart materials. The development of innovative bioinspired materials that harness the optical properties exhibited by chiral structures, such as those found in certain Scarabaeidae, offer widespread potential. In this thesis, the species-specific circularly polarised optical behaviour corresponding to a variety of Scarabaeidae beetles is investigated. The helicoidal morphology giving rise to circularly polarised photonic properties is first described for several beetles displaying a narrowband colour appearance. A structural analysis of the native surface structures encountered in these species is presented alongside a discussion of their role in defining the species' optical characteristics. Experimental measurements of the circularly polarised scattering behaviour are presented and discussed with respect to the electron microscopy and surface morphology measurements comprising the structural analysis. Following this, the origins of the subtle differences in the metallic colour appearance of several broadband coleopteran structures are deciphered. The broadband response of these species is attributed to chirped configurations of the helicoidal lamellar structure, elucidated using electron microscopy. Results of this structural analysis are applied to theoretical simulations of the structures' electromagnetic response and consolidate experimental measurements of the circularly polarised reflectance. Rigorous experimental measurement demonstrated that the circularly polarised spectral response is relatively robust to small structural defects or `biological noise'. Building from this, the intraspecific colour variation displayed in the species Chrysina aurigans is a useful lens through which the nuances of the chirped multilayer structure are further examined. As a result, this work provides detailed insights into the fine-tuning of chirped biological photonic systems. Likewise, the topic of biological wave retarders is afforded significant attention, primarily with reference to the biological half-wave plate encountered in the cuticle structure of Chrysina resplendens. A rigorous analysis of the photonic system in C. resplendens is presented, leading to novel conclusions describing this system's opto-structural relationship. Additionally, this thesis reports, for the first time, the presence of a biological wave retarder in the cuticle structure of Chrysina cupreomarginata.
PhD in Physics