We report the first space-time observation of the ultrafast excitation
and relaxation of an electron gas pumped into the conduction band of
a wide band-gap insulator by an intense subpicosecond laser pulse. The
experiment is based on a new phase-sensitive technique which allows a
resolution of 120 fs in time and 5 mum in space. In crystalline and a
morphous SiO2, no evidence of gas expansion has been observed and a me
an trapping time of 150 fs for the photogenerated carriers is measured
.