Colloidal photonic superlattices

We describe the fabrication and characterization of superlattices formed from colloidal photonic crystals. This represents the first instance in which mid-gap states have been incorporated into a colloidal crystal via a specifically engineered structural modification. The samples used in this study are based on the technique of convective self-assembly, in which a crystal of sub-micron silica spheres grows on a vertical substrate from a liquid suspension. The superlattice structure is formed by the sequential deposition of colloidal photonic crystals using two alternating sizes of silica colloids. These provide a unique format for the study of photonic superlattices, because the dielectric contrast is of the order of one. As a result, it is possible to obtain a large optical stop band and pronounced minibands even if /spl Lambda///spl alpha/ is of the order of 10 or less.