Intraband relaxation of photoexcited electrons in GaAs/AlGaAs quantum wells and InAs/GaAs self-assembled quantum dots

We present time-resolved measurements of photoinduced intraband absorption in undoped GaAs/AlGaAs quantum well and InAs/GaAs self-assembled quantum dot structures after interband excitation. By using ultrabroadband mid-infrared pulses we determine the temporal evolution of the intersubband absorption spectrum of a double quantum well sample (subband spacing < longitudinal optical phonon energy) after excitation. From these measurements we determine an intersubband relaxation time of 14 ps at T = 5 K. By tuning mid-infrared pulses into resonance with intraband transitions between confined quantum dot states and the wetting layer continuum, the electron population of the quantum dot ground and first excited states is determined as a function of time-delay after the pump. We find that the most efficient relaxation pathway into the quantum dot ground state is the step-wise relaxation through the excited states of the dot. The capture time at T = 5 K is 4.7 ps.