  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.
