Identification of the stimulated-emission threshold in high-{beta} nanoscale lasers through phase-space reconstruction

Nanoscale lasers sustain a few optical modes so that the fraction of spontaneous emission {beta} funnelled into the useful (lasing) mode is high (of the order of 10{sup -1}) and the threshold, which traditionally corresponds to an abrupt kink in the light-in-light-out curve, becomes ill defined. We propose an alternative definition of the threshold that is based on the dynamical response of the laser and is valid even for {beta}=1 lasers. The laser dynamics is analyzed through a reconstruction of its phase-space trajectory for pulsed excitations. Crossing the threshold, brings about a change in the shape of the trajectory and in the area contained in it. An unambiguous determination of the threshold in terms of this change is shown theoretically and illustrated experimentally in a photonic-crystal laser.