Time-resolved study of laser-induced breakdown in dielectrics

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