Unexplained high sensitivity of the reflectance of porous natural photonic structures to the presence of gases and vapours in the atmosphere

Structurally coloured natural photonic crystals found in several insects are made of ordered porous chitin structures. In such photonic crystals, colour changes can be induced by relative gas/vapour concentration variations in a mixed atmosphere. For instance, when the composition of the atmosphere changes, the colour of Morpho sulkowskyi buttery is modied. Based on this eect, it is possible to identify closely related gases/vapours. In spite of increasing interests for such sensors, the fundamental mechanisms at the origin of the selective optical response are still not well understood. The point is that refractive index variations resulting from the introduction of a specic gas species in the atmosphere are too small to justify the dramatic changes observed in the optical response. Here, we demonstrate through numerical simulations that indeed gas/vapour-induced refractive index changes are too small to produce a signicant modication of the spectral reectance in a representative 3D periodic model of natural porous nanostructures. For this purpose, we used the rigorous coupled wave analysis (RCWA) method for modelling light scattering from inhomogeneous optical media. The origin of the reported colour changes has therefore to be found in modications of the porous material and their impact on the photonic response.