Ultrafast Dephasing of Coherent Intersubband Polarizations in a Quasi-Two-Dimensional Electron Plasma

We present the first study of coherent intersubband polarizations in a pure electron plasma by femtosecond four-wave mixing in the midinfrared. Resonantly excited polarizations between consecutive conduction subbands of narrow $\mathrm{GaInAs}/\mathrm{AlInAs}$ quantum wells decay within about 500 fs, much faster than intersubband relaxation of electrons. The relation between the decay rate of the signals and the dephasing rate is analyzed by solving the time-dependent Hartree-Fock equations for the single-particle density matrix. Electron-electron scattering is identified as the main dephasing mechanism.