Natural photonic structures found on the cuticle of insects are known to give rise to astonishing structural colours. These ordered porous structures are made of biopolymers, such as chitin, and some of them possess the property to change colour according to the surrounding atmosphere composition. This phenomenon is still not completely understood. We investigated the structure found on the cuticle of the male beetle Hoplia coerulea (Scarabaeidae). The structure, in this case, consists in a 1D periodic porous multilayer inside scales, reflecting incident light in the blue. The colour variations were quantified by reflectance spectral measurements using water, ethanol and acetone vapours. A 1D scattering matrix formalism was used for modelling light reflection on the photonic multilayer. The origin of the reported colour changes has to be tracked in variations of the effective refractive index and of the photonic structure dimensions. This remarkable phenomenon observed for a non-open but still porous multilayer could be very interesting for vapour sensing applications and smart glass windows.
Structurally colored nano-architectures found in living organisms are complex optical materials, giving rise to multiscale visual effects. In arthropods, these structures often consist of porous biopolymers and form natural photonic crystals. A signature of the structural origin of coloration in insects is iridescence, i.e., color changes with the viewing angle. In the scales located on the elytra of the Brazilian weevil <i>Entimus imperialis</i> (Curculionidae), three-dimensional photonic crystals are observed. On one hand, each of them interacts independently with light, producing a single color which is observed by optical microscopy and ranges from blue to orange. On the other hand, the color perceived by the naked eye is due to multi-length-scale light effects involving different orientations of a single photonic crystal. This disorder in crystal orientations alters the light propagation in such a way that the crystal iridescence is removed. <i>Entimus imperialis</i> is therefore a remarkable example of additive photonic colors produced by a complex multi-scale organic architecture. In order to study this specific natural photonic structure, electron microscopy is used. The structure turns out to be formed of a single type of photonic crystal with different orientations within each scale on the elytra. Our modeling approach takes into account the disorder in the photonic crystals and explains why the structure displays bright colors at the level of individual scales and a non-iridescent green color in the far-field.
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