Homogeneous broadening and excitation-induced dephasing of intersubband transitions in a quasi-two-dimensional electron gas

The irreversible decay of coherent intersubband polarizations in n-type modulation-doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ 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 $5\times {10}^{10}\hspace{0.5em}{\mathrm{cm}}^{-2}$ the dephasing time $T_2=320\mathrm{}$ fs accounts fully for the 4 meV intersubband linewidth. For $6\times {10}^{11}\hspace{0.5em}{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^2$ and strong excitation, athermal electron distributions show enhanced dephasing rates.