Assessment of environmental spectral ellipsometry for characterising fluid-induced colour changes in natural photonic structures

Porous photonic structures found in several living organisms are known to display colour changes induced upon contact with liquids, vapours and gases. Usually these changes are due to physico-chemical phenomena such as the swelling of the structure enacted by the fluids, vapour physisorption on the pore walls, capillary condensation or a combination of them. Generally, the porous structures are open to outside, leading to fast fluid exchanges with the surrounding environment and consequently fast colour changes. In this article, we first introduce fluid-induced optical changes in living organisms exhibiting porous photonic structures. We explore then the potentiality of environmental ellipsometry for the first time in the context of natural photonic structures through the investigation of the optical response of the male cerulean chafer beetle Hoplia coerulea (Scarabaeidae) upon contact with water, 2-propanol and toluene vapours. In contrast with most of the investigated photonic structures, this beetle's structure is encased by an envelope that mediates liquid exchanges with the environment. Such a study is of great interest in order to understand the underlying biological functions behind these changes as well as in order to develop bioinspired applications such as gas sensors and other environment-responsive coatings.