Ab initiomolecular dynamics simulation of femtosecond laser-induced structural modification in vitreous silica

Femtosecond laser pulse induced structural changes in silica glass and their role in changing the refractive index of the glass have been investigated using ab initio molecular dynamics simulation methods based on finite-temperature density functional theory. The average nearest-neighbor Si-O, Si-Si, and O-O distances are found to increase during laser irradiation due to the weakening of bonds resulting from the thermalization of electrons. These changes in the nearest-neighbor distances are almost completely recovered during the postirradiation evolution of the glass structure. However, persistent structural changes are found to involve the formation of three-coordinated Si atoms and nonbridging oxygens that correspond to the defect species of Si ${E}^{\ensuremath{'}}$ centers and nonbridging oxygen hole centers, respectively. These defects give rise to optical absorptions that increase the refractive index of silica glass through a Kramers-Kronig mechanism.