Tailoring calcium carbonate to serve as optical functional material : examples from biology and materials science

In nature and technology there are many examples of micron sized optical systems based on minerals. The thermodynamically most stable polymorph of calcium carbonate (CaCO3) –calcite– exhibits birefringence. However, nature uses biomineralization approaches to produce CaCO3 based optical functional materials, for example calcite mircrolens arrays as part of the photoreceptor system of brittlestars, amorphous CaCO3 (ACC) plant cystoliths as light scatterers and calcite trilobite eyes. In this short review, we illustrate basic strategies to produce optical materials from CaCO3 by manipulating the material structure. These strategies are driven by the aim to eliminate or reduce the birefringent properties of calcite, or even, to turn this property into an advantage. We report on the formation, structure, and functions of micron sized (hemi-)spherical objects made from CaCO3 with biological origin but also in bio-inspired synthetic examples. We highlight aspects, which pave the way to learn from nature and how to characterize natural and artificial systems. In conclusion, we identify three main possibilities to make optical materials based on CaCO3: (i) orienting the optical axis along the desired light propagation direction, (ii) stabilizing metastable phases, and (iii) producing nanocrystalline structures, which reduce birefringent properties