Subpicosecond plasmon response: Buildup of screening

The screened Coulomb Keldysh propagator is evaluated in the time-domain for ultra-short-pulse laser excitation of a semiconductor using the nonequilibrium Green-function technique. The buildup of screening in the time interval during and shortly after the pulse excitation is treated. A time-dependent plasmon pole approximation is derived and shown to give an excellent description of the dynamics of screening after a resonant femtosecond pulse.