Ultrafast formation of many-particle interactions in a photoexcited electron-hole plasma

Ultrabroadband THz spectroscopy is employed to observe how many-particle interactions build up in an extreme nonequilibrium electron-hole plasma. The plasma is photogenerated in bulk GaAs via resonant interband absorption of a 10 fs laser pulse. Subsequently, the dynamics of the complex dielectric function throughout the mid-infrared is directly monitored with uncertainty-limited temporal resolution with a single-cycle THz pulse. Field sensitive detection allows us to measure simultaneously real and imaginary part of the complex dielectric function of the plasma in the multi-THz regime. We show that collective phenomena such as Coulomb screening and plasmon scattering exhibit a delayed onset. This observation is explained in terms of the ultrafast formation of dressed quasiparticles. The time scale for this transient behavior 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.