Plasma strings from ultraviolet laser filaments drive permanent structural modifications in fused silica

Long self-trapped femtosecond ultraviolet filaments created in the bulk of pure fused silica are used to induce permanent structural changes in the medium. We monitor the laser pulse propagation as a filamentary structure and the plasma string left at its trail. We investigate and demonstrate the link of the filament-induced plasma to the permanent structural changes left in the medium. Specific electron density thresholds are found for the induced modifications.