Synthesis of amorphous silicon/magnesia based direct opals with tunable optical properties

The effective refractive index of silica based artificial opals can be strongly modulated through magnesiothermic and wet etching processes. The magnesiothermic reduction of silica spheres assembled in a fcc lattice produces amorphous silicon/magnesia matrix, which can be easily converted in oxidized porous silicon, preserving the ordered structure. These results are verified by electron microscopies and IR/Raman spectroscopies. The optical properties are analyzed in terms of the experimental reflectance spectra. By comparing the measured data to rigorous calculations, the good quality of the opaline replicas is demonstrated.