Intra-band dynamics in single InAs/GaAs quantum dots under the influence of strong far-infrared excitation

Inter-band transitions in single quantum dots (QDs) have received a huge amount of scientific interest in the recent past. However, mostly due to technical challenges in dealing with mid- and far-infrared radiation, intra-band transitions have not been explored quite as thoroughly. In this work, we combine micro-photoluminescence (µPL) on low-density annealed InAs/GaAs QDs with additional excitation at intra-band transition energies by pulsed radiation from a free-electron laser (FEL). This scheme enables the probing of the single-dot response in spite of the large diameter of the FEL focus. The investigation of single QDs eliminates undesirable effects such as inhomogeneous broadening which has been observed in previous studies on QD ensembles1–3. In the time domain, the FEL pulse leads to an initial decrease in the PL transient (Fig.1), which we attribute to a temporary redistribution of carriers. The subsequent recovery is significantly larger than would be expected for simple redistribution. 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. When investigating at the PL spectrum of a single dot, we observe a marked difference caused by the FEL pulse (Fig. 2), which implies a change of the excitonic state of the QD.