Optics in the Natural World: Iridescent Colors of Butterflies and the Twisted Beetle!

Abstract

The study of photonics in nature contains beautiful and diverse examples of sub-wavelength structural features that create observed colors through thin layered or multilayered interference, diffraction, zero order diffraction and light scattering. In this talk I will discuss two such examples: Butterflies and Beetles, both of which attracted the attention of great scientists including Newton, Rayleigh, Michelson and Raman, among others. Structural coloration based on diffraction, multilayer reflection, cholesteric analogues or photonic crystal-like structures is pervasive especially in the world of insects. The color of Papilio palinurus results from microbowl lined with a multilayer of air and chitin. The green color is the result of color mixing of the yellow light reflecting from the bottom of the bowl and the blue light reflecting from the sides of the bowl. We have used breath-figure templated assembly as the starting point to mimic the structure of Papilio palinurus. We were able to produce microbowls which were then coated with a multilayer of alternating titanium oxide and aluminum oxide. The resulting structure exhibits the same color mixing as the original butterfly structure does. In the second part of the talk, we take a closer look at the colors produced by iridescent, metallic green beetle, Chrysina Gloriosa that selectively reflects left circularly polarized light when illuminated with unpolarized light which is observed to possess a nearly hexagonal cellular pattern on its exoskeleton. Using crystallographic concepts and Voronoi analysis of the structure present on the exoskeleton, we determine that these cells (~10 microns each) are organized with pentagons and heptagons, interdispersed typically as clusters, between hexagons. In an optical microscope, each cell appears to contain a bright yellow core, placed in greenish cell that has yellowish borders. Using confocal microscope and the auto-fluorescence of the exoskeleton matrix, we visualize that these cells consist of nearly concentric, nested arcs that lie on surface of a shallow cone. The observed textures are reminiscent of the texture of a cholesteric liquid crystalline phase with a free surface, and present an interesting analogy that provides the basis for structure and color present on beetles.