Space-time observation of an electron gas in SiO2.

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 \ensuremath{\mu}m in space. In crystalline and amorphous Si${\mathrm{O}}_{2}$, no evidence of gas expansion has been observed and a mean trapping time of 150 fs for the photogenerated carriers is measured.