Summary form only given. In previous studies, we have shown that the grating photo-induced by femtosecond laser pulses in a SF59 lead silica glass is not formed instantaneous and continues to evolve at the millisecond time scales after excitation. The grating reflectivity was shown to depend on the pump power density and on the number of exciting pulses [1]. The grating was directly written onto the SF glasses by imaging a master grating, which is illuminated by femtosecond laser pulses (1mJ, ~50fs, 1KHz, ~800nm) [2]. The formation of the grating was shown to result from defects photo-induced by the two photon-absorption of the femtosecond laser pulses [1,2]. Theses defects induce a change of the real (Δnr) and imaginary (Δni) part of refractive index of the glass. However our previous experiments were not able to reveal the time scale for the formation of these defects or to evidence the impact of the two photons absorption on Δnr or Δni. Indeed for a given laser power density, one expects to observe an increase of Δnr and therefore of the grating reflectivity as two-photon absorption coefficient α2 is increased. To evidence the time scale for the formation of these defects, we have measured the evolution of the reflectivity of the photo-induced grating on the picoseconds time scale. As shown in Fig.1a, we found that depending on the pump power density, a rapid growth of the reflectivity is recorded in about 1.5ps to 4ps (Fig.1a). In a good agreement with the formation of the effects in glasses, this indicates that photo-induced defects are produced on few picoseconds [3]. Now Δnr should depends on the number of defects Ndef induced by two-photon absorption: The higher the α2 of the glass, the higher Ndef and the larger (Δnr). In lead silica glasses α2 is linearly proportional to the PbO molar density [4]. This phenomenon is related to the shift of the edge absorption band of the glass toward the longer wavelength with the PbO glass content (Fig.1b). As expected, Fig.1c indicates that, at 1 kHz femtosecond pulse repetition rate and for moderate power density, Δnr increases linearly with PbO molar fraction. Moreover, as the power density is further increased, we notice that Δnr saturate. The value of Δnr at saturation depends on and increases with PbO molar density. The ensemble of measurements that contributes to the understanding of the index modification of glasses photo-induced by femtosecond pulses will be discussed in more details during our presentation.< Réduire