Photonic Bandgap Engineering in Mixed Colloidal Photonic Crystals

We demonstrated that photonic bandgap can be engineered in a manner analogous to the electronic bandgap tuning of the GaAlAs mixed semiconductor system. Two kinds of sub-micron spheres of the same size but of different materials, silica and polystyrene, were randomly mixed and ordered into a densely packed colloidal photonic crystal (CPC). Reflectance spectrum measurements revealed that as the polystyrene sphere composition increases the L-point pseudo-bandgap linearly red-shifts over the range from 1010 nm (for pure silica CPC) to 1090 nm (for pure polystyrene CPC) while the reflectance bandwidth remains constant. This spectral shift is in excellent agreement with the prediction based on the effective-medium theory.