Ordered Structures from Nanoparticles

In nature many colors arise from ordered structures with periodicities in the submicron range. One prominent example is the gemstone opal. We discuss the origin of the brillant colors of opal and its artificial counterparts: colloidal crystals and three dimensional photonic crystals. In addition we point out possibilities how to prepare colloidal crystals and related materials with inverted structure. Such materials show promising characteristics for innovative future developments in the fields of opitcs. The physical properties of ordered nanoparticular matter and their sometimes spectacular consequences like the iridescence of opals (see Figure for a photograph of a typical stone) are discussed in this short general review. Also, types of colloidal crystals, and fabrication techniques directed at obtaining certain effects are discussed.