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 μm 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.