Structural origin of the brown color of barbules in male peacock tail feathers

We report detailed optical measurements and numerical simulations of brown barbules in male peacock tail feathers. Our results indicate that brown coloration is predominantly produced structurally by the two-dimensional (2D) photonic-crystal structure in the cortex layer of a barbule. The constructing strategies of brown coloration revealed by numerical simulations are indeed subtle, which are of great significance in the artificial constructions of mixed structural coloration. It is found that the structural configurations of the 2D photonic-crystal structure such as the lattice constant, the number of periods, and even the interdistance and missing holes between the two melanin layers nearest to the cortex surface, are important in the production of structural brown colors.