Photonic crystals (PCs) are ubiquitous natural objects found in beetle shells, butterfly wings, and nacre, yet at the same time promising building blocks for all-optical integrated circuits and switches moulding the propagation of light. The underlying principle however is simple: Following nature's example, beautiful colors can be imparted to inherently colorless materials by Bragg reflection taken to a larger length scale: By analogy to electronic semiconductors (the "little brothers"), photonic crystals possess photonic bandstructures including "bandgaps", the latter giving rise to light reflection and thus, color! From a materials perspective, photonic crystals feature a periodic refractive index contrast which can extend into 1, 2, or 3 dimensions. In principle, the library of materials that may constitute PCs is unlimited, rendering PCs an intriguing platform for the creation of highly colourful structures based on inherently colorless materials.
Whereas both top-down and bottom-up approaches to PC reflectors based on standard materials such as SiO2/TiO2 are well established, the fabrication of tunable structural color PCs remains a challenge. Our group pursues the fabrication of stimuli-responsive PCs based on a wide spectrum of inherently functional organic, inorganic and hybrid materials in order to create a platform translating material response into color read-out. Ultimately, this strategy will allow us to use structural color changes as an in situ probe of material transformations and reactions and therefore, as signal transduction pathway in chemooptical sensors.