Femtosecond experiments using single-cycle pulses at 30 THz: the buildup of screening in a photoexcited electron-hole plasma

We report on a new class of ultrabroadband THz experiments with a time resolution as high as 30 fs. The measurements rely on single-cycle probe transients which are generated via optical rectification of 10 fs near-infrared pulses in a thin GaSe nonlinear crystal. The electric field of these pulses is directly monitored via ultrabroadband electro-optic detection. On a femtosecond time scale, we observe how Coulomb screening and collective scattering build up in an extreme nonequilibrium electron-hole system in GaAs. To this end, we photogenerate a dense carrier plasma within 10 fs and probe the subsequent polarization response of the system with uncertainty-limited resolution by means of ultrabroadband THz spectroscopy. We show, that the intrinsic material becomes conductive instantaneously upon carrier injection, whereas collective effects such as Coulomb screening and plasmon scattering exhibit a delayed onset Thus, the ultrafast formation of dressed quasiparticles is directly monitored for the first time. The time scale for these phenomena is of the order of the inverse plasma frequency. Thereby, our findings support recent quantum kinetic simulations.