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 first synthesized following the single-step Stober'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 degrees C and 90 degrees 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 reflection 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 reflection spectra were in agreement with the simulation results obtained by using the transfer matrix method. The reflection spectra were blue shifted as the incident angle deviated from the normal direction, which could be justified simply by using a combination of Bragg formula and Snell's Law.