Femtosecond buildup of Coulomb screening in a photoexcited electron-hole plasma

In an extreme nonequilibrium electron-hole plasma in GaAs, we observe how Coulomb screening builds up on a femtosecond time scale. To this end, the dynamics of the complex dielectric function throughout the mid-infrared is probed with uncertainty-limited temporal resolution by means of ultrabroadband THz spectroscopy. We show that the system becomes conducting instantaneously, whereas collective behavior such as Coulomb screening and plasmon scattering exhibits 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. Our findings support recent quantum kinetic calculations of the temporal evolution of the Coulomb interaction after ultrafast excitation of a dense electron-hole plasma.