ULTRAFAST OPTICAL MEASUREMENTS OF DEFECT CREATION IN LASER-IRRADIATEDSIO2

Optical methods using sub-picosecond laser pulses allow to study the k inetic of defect creation in SiO2, caused by an intense electronic exc itation. A first intense ''pump'' pulse is used to create a high densi ty (up to 10(19) cm(-3)) of e-h pair. A second, weaker pulse is then u sed to probe the state of the material after an adjustable delay, with a time resolution of the order of 10(-13) s. A first investigation us ing photoelectron spectroscopy shows that the electrons can reach kine tic energies in the conduction band in large excess of the photon ener gy, through three-body electron-photon-phonon transitions (a sequentia l absorption process). ''Transient Frequential Interferometry'' is use d to measure the instantaneous refractive index, i.e. the free carrier density (conduction electrons), and to confirm the existence of the a bsorption by conduction electrons. Transient absorption can be used to monitor the appearance of point defects following the trapping of the free carriers. We show that, contrary to what is observed in other ox ides (Al2O3 and MgO), the trapping process is extremely fast (150 fs), and occurs at all temperatures in the triplet state of the Self Trapp ed Exciton (STE). A permanent absorption is shown to appear at room te mperature only, resulting from the thermal conversion of STE into colo red centres. Finally, are study from a theoretical point of view the t ransport of conduction electrons with help of two different methods: M onte-Carlo simulations, which allow to introduce in a convenient way t he effect of the laser field, and solving the time-evolution of the de nsity matrix equations, a more exact treatment in principle required i n SiO2 because of the strong electron-phonon coupling, but which does not yet allow to include the effect of a strong laser field.