Photonic natural opals

Photonic crystals have only been studied in the laboratory for two decades, but naturally occurring examples exist, with the best known being the gemstone opal. Opals consist of tiny spheres of silica arranged in a face centred cubic structure. These are thought to have formed from colloidal silica solutions, and the colour depends on the size of the spheres. 

Nanoscale structures such as inverse opals have fascinating photonic properties related to Natural nanostructures with potential applications in optical computing. 

Another important method for photonic crystal fabrication employs colloidal particle self-assembly. A colloidal system consists of two separate phases a dispersed phase and a continuous phase (dispersion medium). The dispersed phase particles are small solid nanoparticles with a typical size of 1-1000 nanometers. Colloidal crystals are three-dimensional periodic lattices assembled from monodispersed spherical colloids. The opals are a natural example of colloidal photonic crystals that diffract light in the visible and near-infrared (IR) spectral regions due to periodic modulation of the refractive index between the ordered monodispersed silica spheres and the surrounding matrix. 

The simplest photonic crystal is nature s opal. The artificial opal is composed of monodispersed spheres of a dielectric, usually silica. Considerable work has been done using latex or polystyrene spheres, but we largely will restrict ourselves here to ceramics. In producing high-quality photonic crystals, care must be taken in each of the three main steps particle synthesis, sedimentation, and sintering. 

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