TIME-RESOLVED STUDIES OF CARRIERS DYNAMICS IN WIDE-BAND GAP MATERIALS

Lasers delivering high intensity ultrashort pulses are adequate tools to study the dynamics of irradiation induced modifications in wide ban d gap materials. We present the results of two complementary time-reso lved pump-probe optical experiments. We use a technique of interferome try in the frequency domain to probe the instantaneous modification of the refractive index of the material induced by a pump pulse. The pre sence of free carriers in the conduction band induces a negative phase shift of the probe pulse which is proportional to the excitation dens ity, Comparison between different oxides reveals an extremely contrast ed behaviour: the photoexcited carriers are trapped in 150 fs in SiO2, while they remain quasi-free for more than 50 ps in MgO and Al2O3. Th ese lifetimes are closely connected to the formation of radiation indu ced defects. In particular, using a conventional time-resolved absorpt ion technique in the UV (5.2-5.6 eV), we have studied the intrinsic de fects formation kinetics in SiO2. Our results show that the formation of self-trapped excitons (STE) is occurring in the ultrashort time of 150 fs, in agreement with the free carrier lifetime measured in the ab ove experiment. At low temperature, the STE recombine radiatively, whi le at temperature above the luminescence quenching temperature, a smal l proportion of the STE is converted into permanent defects (O vacanci es).