F1.1 Femtosecond buildup of screening and collective effects in photoexcited GaAs: How bare charges get dressed

Abstract. On a femtosecond time scale, we observe how Coulomb screening and collective scattering build up in an extreme nonequilibrium electron-hole plasma in GaAs. To this end, we photogenerate a plasma within 10 fs and probe the subsequent polarization response of the system with uncertainty-limited temporal 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.