Refractive Index Reduction at the Surface of Co/Cu-Doped Silicate Glasses Induced by Femto-Second Laser Irradiation

Abstract

Femto-second laser pulses (800 nm, 11 nJ/pulse, 82 MHz) were irradiated on the surface of Co- or Cu-doped silicate glasses (70SiO₂•20NaO₂•10Al₂O₃ doped with 20 mol% of CoO or CuO, respectively) for 1 s to modify its surface, which was observed by a scanning probe microscope and an ellipsometer. Some samples exhibited refractive index reduction without any structural modification, which was brought about by local increase in fictive temperature through spot heating and quenching. With the aid of a computer simulation using a finite element method, such a modification is found to occur when the maximum surface temperature is less than 1000°C for the Cu-doped glass.