By fixing charged colloidal crystals in a poly(acrylamide) hydrogel matrix, we fabricated photonic crystals whose diffraction peak wavelengths were tunable by applying mechanical stress. The reflection spectrum for a single crystal grain was measured by applying microspectroscopy under compression. The photonic band gap wavelength shifted linearly and reversibly over almost the entire visible light wavelength region (460-810 nm).

Aqueous dispersions of charged colloidal silica particles showed a novel one-directional crystal growth by diffusion of a weak base, pyridine. The colloidal crystal consisted of pillar-shaped crystal grains whose height and width were in the order of centimeter and subcentimeter, respectively. The growth process was explainable in terms of (i) the diffusion of pyridine with neutralization reactions between weakly acidic silica surfaces, (ii) charging up of the silica particles, and (iii) the charge-induced crystallization of the dispersions.