Systematic Control of Structural Changes in GeO ₂ Glass Induced by Femtosecond Laser Direct Writing

Abstract

We report the structural changes inside germania glass induced by femtosecond laser pulses. Inspection by polarization microscopy and secondary electron microscopy indicate that the periodic nanostructures consist of oxygen defects such as ODC s (oxygen deficient centers) and NBOHC s (nonbridging oxygen hole centers) for laser pulse energy less than 0.2 μJ. However, the glass network was dissociated and O 2 molecules were generated for laser pulse energy greater than 0.4 μJ. Two different structural changes, form‐birefringence and dissociation, were induced in GeO 2 glass, depending on the laser pulse energy. The form‐birefringence exhibited by the nanogratings in GeO 2 glass is larger than that in SiO 2 glass for pulse energy less than 0.2 μJ, as the density of nanovoids enclosed by ODC s in GeO 2 glass is higher than that in SiO 2 glass. Arrhenius plots of the phase retardation caused by the nanogratings in GeO 2 and SiO 2 indicate that the oxygen defects are relaxed at a temperature 100°C above the glass‐transition temperature.