Ultrafast dephasing of continuum transitions in bulk semiconductors

We have studied the coherent emission from bulk semiconductors by spectrall y resolved and spectrally integrated four-wave mixing (FWM) with broadband 16-fs pulses in the carrier density range from x 10(16) cm(-3) to 6 x 10(17 ) cm(-3), in GaAs, only continuum transitions are excited, while both excit onic and continuum transitions are excited in Al0.06Ga0.94As. The decay of the FWM signal for positive time delays unambiguously reflects the dephasin g of the continuum, irrespective of the excitation of excitonic transitions . We find identical ultrafast sub-20-fs decay times of the coherent emissio n from continuum transitions in GaAs and Al0.06Ga0.94As, independent of the excess excitation energy. These decay times depend only weakly on the carr ier density for densities below 10(17) cm(-3). The continuum dephasing is a nalyzed considering carrier-carrier and carrier-LO-phonon scattering in a r elaxation rate approach. The excitonic transitions in Al0.06Ga0.94As domina te the FWM signal at negative time delays.