Iridescence and nano-structure differences in papilio butterflies

The nano-photonic structures on the wings of three Papilionidae butterflies, Papilio blumei, Papilio ulysses and Papilio peranthus, were investigated. It was observed that the photonic structure is multi-layer with alternate air and cuticle layers forming one-dimensional photonic crystal. The multi-layer structures of the three butterflies differ subtly but are sufficient to account for the differences in their iridescence. The subtleness is more obvious in their polarized reflection results. We performed the simulation of polarized reflection using characteristic matrix method with parameters obtained from SEM images of butterfly wing scales' crosssection. The simulated reflection spectra are matched with the experimental spectra to derive the effective refractive index of the air lamina in the butterfly wing scales. It shows that through varying the optical thickness and periodicities in air/cuticle bilayer stacks, the iridescent color of these three Papilionidae butterflies appear different. The result lays the foundation for mimicking the photonic structures of these butterflies.