Impurity modes in the optical stop bands of doped colloidal crystals.

Colloidal suspensions of charged polystyrene microspheres can self-organize into colloidal crystals, providing a three-dimensional periodic dielectric medium suitable for the study of photonic band structure effects in the infrared, near-infrared, and visible wavelength regimes. Light propagation along any particular direction in such a system displays bands of frequencies for which the propagation of light is strongly inhibited. Substitutional doping of these colloidal crystals with scatterers of slightly different size or different dielectric strength introduces optical disorder into these crystals which is manifested by the appearance of impurity modes within the optical stop bands. These defect modes can be tuned by appropriate selection of the defect volume and the dielectric constants of the dopants. Widening of optical stop bands with increasing impurity peak heights was observed in the doped crystals. @S0163-1829~96!02644-6#