The Colouration of Bird Feathers explained by Effective-Medium Multilayer Modelling

Many birds have fascinating colours generated by specialized nanoscopic structures inside their feathers (see also: academy.allaboutbirds.org/how-birds-make-colorful-feathers/). The special optical properties of these structures, such as those found in the feathers of peacocks, ducks, starlings and bird of paradise, are as yet not fully understood. These structures, which are ordered in the range of 100-200 nanometres, are composed of keratin (like that found in hair), melanin and sometimes also air. They are called “photonic crystals” because they can manipulate light in special ways through the precise tuning of these nanometer-sized and often crystal-like structures. My thesis presents an in-depth study of the relationship between the structures and colours of a number of bird species, thus contributing to a better understanding of the relationship between the nanoscopic structures and their spectral properties.
By applying unique optical methods, the scattering of light and colouration properties of microscopic feather samples could be characterised. Furthermore, although the nanostructures are seemingly complex, the optical effects can be understood using relatively simple models. We found that the photonic structures in the feathers can be approximated as a stack of layers, allowing us to interpret our measurements very well by simple optical calculations. Most importantly, I show specifically how the number of layers and their specific structure determine the colour. With the optical model described in this thesis, the colours of many other bird feathers can now be calculated in detail and be quantitatively understood.