The formation energies and electronic structure of europium doped zinc oxide has been determined using DFT and many-body $GW$ methods. In the absence of intrisic defects we find that the europium-$f$ states are located in the ZnO band gap with europi
um possessing a formal charge of 2+. On the other hand, the presence of intrinsic defects in ZnO allows intraband $f-f$ transitions otherwise forbidden in atomic europium. This result coorroborates with recently observed photoluminescence in the visible red region [1].
We present an experimental and theoretical study on the gain mechanism in a photonic-crystal-cavity nanolaser with embedded quantum dots. From time-resolved measurements at low excitation power we find that four excitons are coupled to the cavity. At
high excitation power we observe a smooth low-threshold transition from spontaneous emission to lasing. Before lasing emission sets in, however, the excitons are observed to saturate, and the gain required for lasing originates rather from multi-electronic transitions, which give rise to a broad emission background. We compare the experiment to a model of quantum-dot microcavity lasers and find that the number of emitters feeding the cavity must greatly exceed four, which confirms that the gain is provided by multi-electronic states. Our results are consistent with theoretical predictions.
