Rapidly Self-Assembling Three-Dimensional Opal Photonic Crystals

We demonstrate a useful technique to rapidly self-assemble silica particles to form opal photonic crystals. Silica suspensions were rst synthesized following the single-step St ober's method, with controllable particles diameters ranging from 200 to 800 nm. The suspensions were then injected into a cubic cell composed of two glass substrates and two controllable spacers. The self-assembly process was accelerated by heating the cell to an arbitrary temperature between 30 C and 90 C. The opal particles were self-assembled into ordered structures within several minutes. The periodic structures were characterized by checking the scanning electron microscopy images and measuring the re ection spectra. The structure was quite uniform for very large areas and the number of layers could be controlled by using the formation temperature. The measured re ection spectra were in agreement with the simulation results obtained by using the transfer matrix method. The re ection spectra were blue shifted as the incident angle deviated from the normal direction, which could be justi ed simply by using a combination of Bragg formula and Snell's Law.