A study of the optical absorption of single crystal silicon thin films (0.1-0.4μm thick), grown heteroepitaxially on sapphire substrates and irradiated with different doses of neon and krypton ions at room temperature, has been undertaken as a function of ion dose in the photon energy interval 1.5-3.0 eV. The change in absorptivity is sufficient for generating optical patterns of high contrast by irradiation of selected parts of the water.