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.