  Nanoscale lasers sustain few optical modes so that the fraction of
spontaneous emission $\beta$ funnelled into the useful (lasing) mode is high
(of the order of few 10$^{-1}$) 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, based on
the dynamical response of the laser, which is valid even for $\beta=1$ lasers.
The laser dynamics is analyzed through a reconstruction of its phase-space
trajectory for pulsed excitation. Crossing the threshold brings about a change
in the shape of the trajectory and in the area contained in it. An unambiguous
definition of the threshold in terms of this change is shown theoretically and
illustrated experimentally in a photonic crystal laser.
