Time resolved measurements of gratings photo-induced by femtosecond pulses in a lead doped glass

We report on the formation of gratings photo-induced by femtosecond laser pulses in SF59 glass. Depending on the number of pulses used to excite the sample and the pump power density, transient or permanent gratings are induced. We demonstrate that the grating formation is not instantaneous and is produced by laser-induced defects. This results in a change of both the real and imaginary part of the indexof refraction. A simple set-up that records the temporal evolution of both parameters during the laser excitation is also presented. It makes it possible to evaluate the weight of both contributions to the grating diffraction efficiency.