Intra-band dynamics in single InAs/GaAs quantum dots probed with a free-electron laser

Compared to the vast amount of research done in the past on inter-band transitions in single quantum dots (QDs), transitions within the bands have received much less attention. The main reasons for this are most likely the largely non-radiative character of intra-band transitions and the technical difficulties associated with the corresponding mid- and far-infrared radiation. In our contribution, we approach this challenge by combining conventional micro-photoluminescence (µPL) on low-density annealed InAs/GaAs self-assembled QDs with additional excitation at intra-band (i.e. inter-sublevel) transition energies by pulsed radiation from a free-electron laser (FEL). In contrast to previous studies on ensembles of QDs1–3, using single dots eliminates undesirable effects such as inhomogeneous broadening. The FEL pulse leads to an initial decrease in the PL transient (Fig.1), which we attribute to a temporary redistribution of carriers. This is followed by a pronounced recovery, such that the integrated PL is larger than for a reference transient without FEL excitation. By varying the NIR excitation energy, we find that this increase is due to carriers which are initially present close to but not inside the QD (in the wetting layer or in defect states) and which are freed and/or transported to the dot upon incidence of the FEL pulse.