Time-resolved study of laser-induced breakdown in dielectrics

We present the first femtosecond time-resolved study of the evolution of the laser-excited carrier density with the laser intensity, in various dielectrics of practical interest (SiO2, Al2O3, MgO) both above and below the breakdown threshold. These measurements demonstrate that the high electronic excitation responsible for optical breakdown in these solids is produced by multiphoton absorption by valence electrons, for pulses shorter than a few ps. No sign of electronic avalanche has been observed for such short pulses.