Coherent intersubband polarizations in a quasi-two-dimensional electron plasma studied by femtosecond four-wave-mixing in the mid-infrared

We present the first study of the ultrafast dynamics of coherent intersubband polarizations in a two-dimensional electron plasma using femtosecond four-wave-mixing in the mid-infrared. Experiments with modulation-doped GaInAs/AlInAs multiple-quantum-well samples of different doping concentrations show a fast decay of the coherent third-order intersubband polarization on a sub-500 fs time scale, depending strongly on carrier density and lattice temperature. Electron-electron scattering is identified as the main dephasing scattering mechanism.