Two-color pump-probe spectroscopy of electron dynamics in superlattices

We report on two-color pump-probe measurements to investigate the intraminiband dynamics of doped GaAs/AlGaAs SLs with different miniband widths smaller or larger than the optical phonon energy. We have analyzed the cooling behaviour at low temperature. We employed infrared pulses from a free-electron laser to excite electrons to the upper miniband at the center of the SL mini-Brillouin zone and the interminiband transition was probed at the zone edge with broadband THz pulses. After relaxation to the ground miniband the electronic distribution is heated up which results in more absorption at the zone edge. When the distribution cools down this induced absorption signal decays, leading to decay times of 40-50 ps for a miniband width smaller and 3.5 ps for a width larger than the optical phonon energy. This difference in time constants can be explained by the new relaxation channel through polar optical phonons. Additionally we performed measurements at room temperature where the lower miniband is already occupied at the zone edge. Thus no induced absorption and fast decay times are observed.