Coherent nonlinear dynamics of intersubband excitations in a two-dimensional electron gas

Femtosecond midinfrared four-wave mixing is used to study the ultrafast dynamics of coherent intersubband polarizations in n-type modulation-doped GaAs/AlGaAs quantum wells. Time-integrated four-wave mixing yields a dephasing time of T2 1⁄4 320 fs; independent of excitation density. Time-resolved four-wave mixing unambiguously shows that the intersubband transition is purely homogeneously broadened by intraband Coulomb scattering within the low-density ðn 1⁄4 5 10 cm Þ electron plasma. The nonlinear propagation of tailored midinfrared pulses is studied by electro-optic sampling, which allows direct time-resolved measurements of the electric field transients. Excitation pulses of only 1 pJ energy are needed to observe partial Rabi flops of up to 601 in the Bloch sphere. r 2002 Elsevier Science B.V. All rights reserved. PACS: 42.65.Sf; 42.55.Px; 05.60.Gg; 78.47.+p