How many-particle interactions develop after ultrafast excitation of an electron-hole plasma

Electrostatic coupling between particles is important in many microscopic p henomena found in nature. The interaction between two isolated point charge s is described by the bare Coulomb potential, but in many-body systems this interaction is modified as a result of the collective response of the scre ening cloud surrounding each charge carrier(1,2). One such system involves ultrafast interactions between quasi-free electrons in semiconductors(3,4)- which are central to high-speed and future quantum electronic devices. The femtosecond kinetics of nonequilibrium Coulomb systems has been calculated using static(5,6) and dynamical(7,8) screening models that assume the insta ntaneous formation of interparticle correlations. However, some quantum kin etic theories(9-14) suggest that a regime of unscreened bare Coulomb collis ions might exist on ultrashort timescales. Here we monitor directly the tem poral evolution of the charge-charge interactions after ultrafast excitatio n of an electron-hole plasma in GaAs. We show that the onset of collective behaviour such as Coulomb screening and plasmon scattering exhibits a disti nct time delay of the order of the inverse plasma frequency, that is, sever al 10(-14) seconds.