Over the past decade, we have been working to develop intelligent photonic-crystal materials with unique properties, which will be useful in a number of technological areas. These photonic-crystal materials utilize mesoscopically periodic arrays of spherical particles as their active optical elements and are easily fabricated chemically by the use of crystalline-colloidal-array (CCA) self-assembly techniques.
Crystalline colloidal arrays are mesoscopically periodic fluid materials, which efficiently diffract light meeting the Bragg condition. These photonic-crystal materials consist of arrays of colloidal particles that self-assemble in solution into either face-centered-cubic (fcc) or body-centered-cubic (bcc) crystalline arrays (Figure 1), with lattice constants in the mesoscale size range (50-500 nm). Just as atomic crystals diffract x-rays that meet the Bragg condition, CCAs diffract ultraviolet, visible, and near-infrared light, depending on the lattice spacing; the diffraction phenomena resemble those of opals, which are close-packed arrays of monodisperse silica spheres.
The CCA however can be prepared as macroscopically ordered arrays of non-close-packed spheres. This self-assembly is the result of electrostatic repulsions between colloidal particles, each of which has numerous charged surface functional groups. We have concentrated on the development of CCAs that diffract light in the visible spectral region and generally utilize colloidal particles of ~100-nm diameter. These particles have thousands of surface charges, which result from the ionization of surface sulfonate groups. The nearest-neighbor distances are often >200 nm.