High damage threshold birefringent elements produced by ultrafast laser nanostructuring in silica glass

Birefringent patterning by ultrafast laser nanostructuring in silica glass has been used for space-variant birefringent optics with high durability and high optical damage threshold. We demonstrate that the oblate-shaped birefringent modification (type X) with ultrahigh optical transmission has higher optical damage resistance, comparable to pristine silica glass. The lower damage threshold of nanogratings based modification (type 2) following thermal annealing at 900°C for an hour is improved from 0.96 J/cm² to 1.62 J/cm² for 300 fs laser pulses and approaches the optical damage threshold of type X (1.56 J/cm²). This opens the door to utilize these optical elements for high power laser applications where optical transmission and damage threshold are the key parameters. The lower damage threshold of type 2 modification is related to the relatively high concentration of defects, such as E' centers and oxygen-deficiency centers (ODCs).