  An unambiguous proof of lasing in an active nanocavity with ultrahigh
spontaneous emission coupling factor (beta = 0.65) is presented. To distinguish
the subtle lasing threshold features from possible material-related phenomena,
such as saturable absorption in the gain medium, a series of active
nanocavities with different values of beta have been designed to systematically
approach the high-beta device. The demonstration of the lasing threshold is
obtained through the observation of the transition from thermal to coherent
light photon statistics that is well understood and identified in the beta << 1
lasing regime. The systematic investigation allows a more definitive validation
of the onset of lasing in these active nanocavities.
