Ultrafast processes in laser irradiated wide bandgap insulators

Optical methods will be presented which allow a study of the charge trapping processes occurring in wide bandgap insulators submitted to a high density of electronic excitation. Interferometric measurement of the density of conduction electrons, immediately following their injection 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 resolved measurements of specific absorption lines, which are a signature of the appearance of point defects. In the case of SiO2 the initial step 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, which can subsequently decay either radiatively or into a permanent E′ center. Strong differences exist however between apparently similar materials. For instance, the extremely fast trapping processes discussed for SiO2, 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 different signature in the interferometry experiment. Comparison of the self-trapping kinematics in SiO2 and NaCl, combined with Monte-Carlo simulations shows that the electron–phonon coupling is a decisive parameter in determining the exact nature of the trapping process.