Space-Time Observation of an Electron Gas in SiO2

Abstract

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...

Excitation and relaxation dynamics in ultrafast laser irradiated optical glasses

“…In fused silica, a material of technological interest and thoroughly investigated, several particular physical manifestations were observed. Saeta et al [5] and Audebert et al [6] indicated more than a decade ago a characteristic fast sub-ps relaxation channel for excited carriers in self-induced lattice distortions with the formation of self-trapped excitonic (STE) states and their conversion to Frenkel pairs (nonbridging oxygen and E centers). This is indicative for a still rigid matrix, whereas as the level of excitation increases, the relaxation times become long, suggesting a decrease in the trapping capacity, consequence of vibrational molecular activity in low viscosity phases [7] .…”

Does complex absorption behavior leading to conditioning and damage in KDP/DKDP reflect the electronic structure of initiators?

“…Although we do not know the parameters directly, there are some hints. For example, elegant experiments 11 have shown the lifetime of conduction band electrons in fused silica to be about 1 ps and we assume that value here. The pump-probe experiments suggest a lifetime τ 1 on the order of 1 ns and that is also assumed.…”

Finite-Difference Time Domain Analysis of Ultrashort Pulse Laser Light Propagation under Nonlinear Coupling

“…hoton ionization rate W MPI ∝ I 6 figure shows Keldysh's theory dotted line), and six photon abection for inverse bremsstrahlung ( 9 ) where the momentum transfer collision time τ c = 2.33×10 -14 s. In the third term on RHS of Eq. (7), τ r denotes electron recombination with a characteristic time of τ r = 150 fs for fused silica (19) . Although τ r is reported to depend on electric field, it is treated as a constant value in this paper.…”