ULTRAFAST PROCESSES IN LASER-IRRADIATED WIDE BANDGAP INSULATORS

Optical methods will be presented which allow a study of the charge tr apping processes occurring in wide bandgap insulators submitted to a h igh density of electronic excitation. Interferometric measurement of t he density of conduction electrons, immediately following their inject ion in the conduction band by an intense subpicosecond laser pulse is used to measure the time evolution of the free carrier density with a resolution of the order of 100 fs. Another method consists in time res olved measurements of specific absorption lines, which are a signature of the appearance of point defects. In the case of SiO2 the initial s tep is always the trapping (in 150 fs) of an electron-hole pair on one Si-O bound, forming a self-trapped exciton in its triplet state, whic h can subsequently decay either radiatively or into a permanent E' cen ter. Strong differences exist however between apparently similar mater ials. For instance, the extremely fast trapping processes discussed fo r SiO,, are not observed in two other important oxides: Al2O3 and MgO. In these cases, the electrons either do not trap, or trap into states which are very close to the conduction band, yielding a quite differe nt signature in the interferometry experiment. Comparison of the self- trapping kinematics in SiO2 and NaCl, combined with Monte-Carlo simula tions shows that the electron-phonon coupling is a decisive parameter in determining the exact nature of the trapping process.