Extraordinary stability of femtosecond direct written structures

We report novel results on the stability of femtosecond direct written structures: a silica sample previously irradiated with ultrashort pulses was annealed at increasing temperatures till 1400° C where it crystallized. Our results show that the birefringent direct written structures are stable till a temperature close to the glass transition of silica. After annealing at a temperature as high as 1100° C the form birefringence of the structures is still present, proving that the nature of the laser induced anisotropy is related to a structural change.