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

► The effective refractive index of artificial opals has been modulated through magnesiothermic and wet etching processes. ► The magnesiothermic reduction of silica spheres assembled in a fcc lattice produces amorphous silicon/magnesia opals. ► Amorphous silicon/magnesia opals can be easily converted in oxidized porous silicon, preserving the ordered structure. ► a-Si/MgO opals which could be exploited for the fabrication of other photonic crystals with different symmetry. ► The etched structures could be suitable for emitting dye impregnations, taking advantage of the porous structure.