Coulomb quantum kinetics of degenerate resonant femtosecond four-wave mixing

Resonant time-resolved four-wave–mixing spectra are calculated for direct-gap semiconductors. For pulses shorter than the build-up time of the Coulomb screening (given by the period of a typical plasma oscillation) the dephasing and relaxation of the excited carriers are dominated by the bare Coulomb potential scattering. The quantum kinetics of this regime results in an unusual decay law ∝ exp [ − an τ 3 ] for the integrated four-wave–mixing signal with the delay time τ and the carrier density n . Due to the wave properties contained in the early time regime of quantum kinetics a sideband appears in the spectrum of the time-resolved four-wave mixing with a carrier-density–dependent frequency separation from the main band. The conditions under which the predicted new features can be measured are discussed.