Ultrafast intraband spectroscopy of electron capture and relaxation in InAs/GaAs quantum dots

The electron capture and relaxation dynamics in self-assembled InAs/GaAs quantum dots (QDs) is investigated by means of interband-pump-intraband-probe spectroscopy. By tuning femtosecond infrared pulses into resonance with intraband transitions between confined QD states and the wetting layer continuum, the electron population of the QD ground and first excited states is determined as a function of time delay after the interband pump. Our experiments indicate that the most efficient relaxation pathway into the QD ground state is the stepwise relaxation through the excited states of the dot. The capture time at room temperature decreases from 2.8 to 1.5 ps with increasing excitation density above a certain threshold, and changes only slightly at low excitation densities. At low temperature (T=5 K), we determine a longer capture time of 4.7 ps. (C) 2003 American Institute of Physics.