Induced birefringence in glass: depletion and enhancement by orthogonal-polarized femtosecond pulses

We show spectacular and different effect of the sequential and simultaneous writing of two perpendicularly polarized ultrashort laser pulse trains on profile and magnitude of induced optical retardation inside fused silica glass. Clear birefringence was observed in the region exposed to linearly polarized pulse train radiation. It found out that the induced birefringence is erasable. It means that, when the sample is irradiated again with pulse train having perpendicular polarization, the induced birefringence is vanished and can be totally erased by optimizing the pulse energy. However, in a simultaneous writing approach, a contradictory result was observed. When the glass substrates were simultaneously (i.e. with an accuracy better than the pulse duration) exposed to two beams with perpendicular polarization the induced birefringence not only remained but also enhanced. Discussion and study on the results of interaction of polarized single ultrashort laser pulse and sequential laser beams (which spatially overlapped) having different polarizations and also change of energy ratio of simultaneously writing pulse trains helps us to analyze different results of the simultaneous interaction of two orthogonally polarized ultrashort laser pulse trains with transparent material. Our results provide pieces of evidence for further understanding the physical mechanism of creation of the birefringence using ultrashort laser pulses. Additionally, they provide the ability to manipulate the transient electron dynamics to control the profile and tailor of the induced birefringence.