We report the optical tunability through defect states created in silicon by 1 MeV cobalt ion implantation at room temperature in the fluence range of 5 × 10 13 to 5 × 10 15 ions cm -2 . Atomic force microscopy studies reveal the surface nanostructures with maximum roughness of 0.9 nm at a critical fluence of 5 × 10 15 ions cm-2 which is reduced to 0.148 nm with further increase of fluence. The enhanced native oxide layers after Co ion implantation observed from X-Ray photoelectron spectroscopy studies confirm the presence of surface defects. The combined effect of nanostructures formation and amorphization leads to band gap tailoring. For low fluence, the nanostructures produced on the surface result in an enhanced absorption in the entire UV-Visible region with a simultaneous reduction in band gap of 0.2 eV in comparison to pristine Si whereas high fluence implantation results in interference fringes which signifies the enhancement in refractive index of the top implanted layer ensuing increase in band gap of 0.3 eV. Combined amorphous and crystalline phases of nanostructured surface with tunable optical absorption may have potential applications in solar cell, photovoltaics and optical sensors.