Ultrafast Optical Measurements of Defect Creation in Laser Irradiated SiO ₂

(Received 21 December 1996, revised 12 March 1996, accepted 19 March 1996)

Abstract
Optical methods using sub-picosecond laser pulses allow to study the kinetic of defect creation in SiO ₂, caused by an intense electronic excitation. A first intense "pump" pulse is used to create a high density (up to 10 ¹⁹ cm ^-3) of e-h pair. A second, weaker pulse is then used 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 using photoelectron spectroscopy shows that the electrons can reach kinetic energies in the conduction band in large excess of the photon energy, through three-body electron-photon-phonon transitions (a sequential absorption process). "Transient Frequential Interferometry" is used to measure the instantaneous refractive index, i.e. the free carrier density (conduction electrons), and to confirm the existence of the absorption 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 oxides (Al ₂O ₃ and MgO), the trapping process is extremely fast (150 fs), and occurs at all temperatures in the triplet state of the Self Trapped Exciton (STE). A permanent absorption is shown to appear at room temperature only, resulting from the thermal conversion of STE into colored centers. Finally, we study from a theoretical point of view the transport of conduction electrons with help of two different methods: Monte-Carlo simulations, which allow to introduce in a convenient way the effect of the laser field, and solving the time-evolution of the density matrix equations, a more exact treatment in principle required in SiO ₂ because of the strong electron-phonon coupling, but which does not yet allow to include the effect of a strong laser field.