Ultrafast coherent dynamics in quantum wells for multisubband excitation in different density regimes

We present an experimental study of the coherent dynamics in quantum wells for the simultaneous excitation of Lorentzian excitons below the band edge and structured exciton Fano continua corresponding to higher-order subband transitions. Spectrally resolved transient four-wave mixing (FWM) experimen ts with 100 fs pulses were performed on a 500 Angstrom broad GaAs/AlxGa1-xA s quantum well over a wide range of carrier densities. At lower carrier den sities, the dephasing times of the different exciton transitions were deter mined from the width of the narrow resonances in the distinctly structured FWM spectra. These data demonstrate that the decay of the FWM signal from e xciton Fano continua in the time-delay domain is not determined by dephasin g at the lowest carrier densities. This is indicative of quantum interferen ce which has its origin in many-body exciton-continuum coupling. At higher carrier densities, the FWM spectra are much less structured due to stronger dephasing, quantum interference effects lose their importance, and the str uctured exciton Fano continua show properties similar to unstructured conti nua without embedded exciton transitions. [S0163-1829(98)00744-9].