The irreversible decay of coherent intersubband polarizations in n-type modulation-doped GaAs/AlₓGa_1-xAs quantum wells is studied by femtosecond time-integrated and time-resolved four-wave mixing. We provide the first direct evidence for predominant homogeneous broadening of intersubband resonances by intraband electron-electron scattering. Even at a low electron concentration of 5×10¹⁰ cm^-2 the dephasing time T₂=320 fs accounts fully for the 4 meV intersubband linewidth. For 6×10¹¹ electrons/cm² and strong excitation, athermal electron distributions show enhanced dephasing rates.
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Kaindl et al. (2001) studied this question.