Femtosecond buildup of Coulomb screening in photoexcited GaAs probed via ultrabroadband THz spectroscopy

On a femtosecond time scale, we observe how Coulomb screening builds up in a nonequilibrium electron-hole plasma photogenerated in GaAs within 10fs. To this end, the dynamics of the complex dielectric function throughout the mid infrared is directly probed with uncertainty-limited temporal resolution by means of ultrabroadband THz spectroscopy. While the system becomes conducting instantaneously, the onset of collective behavior such as Coulomb screening and plasmon scattering exhibits a delayed rise. The time scale for these phenomena is of the order of the inverse plasma frequency. Our findings support recent quantum kinetic simulations.