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~i,m in space. In crystalline and amorphous Si02, no evidence of gas expansion has been observed and a mean trapping time of 150 fs for the photogenerated carriers is measured. PACS numbers: 78.47.+p, 72.20.Ht When an ultrashort and intense laser field propagates inside a dielectric medium, it induces a strong polarization field and, if the intensity is high enough, it produces a high density of electrons and holes. This space-time dependent problem is extremely intricate because it involves a number of nonlinear effects such as multiphoton excitation, free carrier absorption, photoemission, electronphonon interaction, exciton generation, and carrier-carrier interaction, all these basic processes occurring in the presence of a high intensity laser field. Moreover, these quasifree charges and the likely presence of ultrafast created defects, as well as self-induced effects such as self-phase modulation or self-focusing, can dramatically affect optical wave propagation [1,2]. Understanding the fundamental aspects of laser-dielectric interaction requires detailed knowledge of the energy balance between the laser field, the electrons, and the lattice. Indeed, this is the cornerstone of crucial problems such as the creation of intrinsic defects such as self-trapped excitations [3,4], E' centers in Si02 [5,6], or laser heating of free electrons which can lead to optical breakdown [7 -9]. Recently, pump-probe experiments with subpicosecond resolution have been carried out to observe the onset of absorption bands attributed to laser-induced intrinsic defects [6,10,11]. This type of experiment can give valuable information about the dynamics of the relaxation process. It is, however, focused on the analysis of a particular defect and the resulting information is restricted to the imaginary part of the index which is proportional to the absorption cross section at a given frequency, while the real part of the index is hidden. Moreover, to our knowledge, the very first parts of the relaxation process, concerning the photogenerated carriers in the conduction band, remain unexplored in wide band-gap insulators.In this Letter, we apply a new and very sensitive technique [12,13] allowing us to measure the induced polarization and the excitation density during and after a pump pulse with a temporal and spatial resolution of 120 fs and 5~~m, respectively. The principle of this experiment is based on interferences in the frequency domain. The target is probed with two collinear femtosecond pulses separated by a fixed time delay b, t (b, t = 1 ps in this experiment).These two pulses are then analyzed in a spectrometer.The power spectrum of this sequence of twin pulses is a fringe system whose period is inversely proport...
