Optical and Structural Characterization of Natural Nanostructures

Abstract: The spectacular biodiversity of our planet is the result of millions of years of evolution. Over this time animals and plants have evolved and adapted to different environments, developing specific behavioral and physical adaptations to increase their chances of survival. During the last centuries human's curiosity has pushed us to study and understand the phenomena and mechanisms of the nature that surrounds us. This understanding has even led to the fields of biomimetics where we seek solutions to human challenges by emulating nature.Scarab beetles (from the insect family Scarabaeidae) have fascinated humans for centuries due to the brilliant metallic shine of their chitin-rich exoskeletons and more recently for their ability to polarize reflected light. This doctoral thesis focuses on the optical characterization of the polarized reflected light from beetles in the Chrysina genus, although beetles from other genera also have been investigated. All the Chrysina beetles studied here share one characteristic, they all reflect left-handed near-circular polarized light. In some cases we also detect right-handed polarized light.We have observed two different main behaviors among the studied Chrysina beetles. Those which are green-colored scatter the reflected polarized light, whereas those with metallic appearance are broadband specular reflectors. We present a detailed analysis of the optical properties with Mueller-matrix spectroscopic ellipsometry combined with optical- and electron-microscopy studies of the exoskeletons. This allow us to create a model that reproduces the optical properties of these structures. The model consists of a chiral (helicoidal) multilayer structure with a gradual change of the pitch and a constant rotation of the optic axis of the layers.Beetles are not alone to have polarizing structures in nature and it is known that many birds and insects have the ability to detect linearly polarized light. This raises the question of whether the polarization properties of the beetles are the direct or indirect results of evolution or just pure coincidence. In order to get a better understanding of the possible reasons of this particular ability, we present a simulation study of different possible scenarios in nature where incoming light could be polarized or unpolarized, and where we consider detectors (eyes) sensitive to different states of polarized light. If the beetles are able to use this characteristic for camouflage, to confuse predators or for intraspecific communication is,however, still unknown and requires further investigation.My research results provide deeper understanding of the properties of light reflected on the beetle's exoskeleton and the nanostructures responsible for the polarization of the reflected light. The developed model could be used as bioinspiration for the fabrication of novel nano-optical devices. My results can also complement biological behavioral experiments aiming to understand the purposes of this specific optical characteristics in nature.