Ultrafast Buildup of a Many-Body Resonance after Femtosecond Excitation of an Electron-Hole Plasma in GaAs

Abstract

On a femtosecond timescale, we observe how Coulomb screening and collective scattering build up in an extreme nonequilibrium electron-hole plasma photoinjected in GaAs. To this end, we generate the plasma via interband excitation with a 10 fs laser pulse. The subsequent polarization response of the system is probed with uncertainty-limited temporal resolution using ultrabroadband terahertz 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 timescale for these phenomena is of the order of the inverse plasma frequency. Our findings support recent quantum kinetic simulations.