We demonstrate the capability of complete thresholdless lasing operation between dressed states of a two-level atom located inside a microscopic cavity engineered in a photonic band-gap material. We distinguish between threshold and thresholdless behaviors by analyzing Mandel's
Q parameter for the cavity field. We find that the threshold behavior depends on whether the spontaneous emission is or is not present on the lasing transition. In the presence of the spontaneous emission, the mean photon number of the cavity field exhibits threshold behavior, indicating that the system may operate as an ordinary laser. When the spontaneous emission is eliminated on the lasing transition, no threshold is observed for all values of the pumping rate, indicating the system becomes a thresholdless laser. Moreover, we find that, under a thresholdless operation, the mean photon number can increase nonlinearly with the pumping rate, and this process is accompanied by a sub-Poissonian statistics of the field. This suggests that the nonclassical statistics can be used to distinguish a nonlinear operation of the dressed-atom laser.
An almost ideal thresholdless laser can be realized in the strong-coupling regime of the light-matter interaction, with Poissonian fluctuations of the field at all pumping powers. Here, we show that this ideal scenario is thwarted by quantum nonlinearities when crossing from the linear to the stimulated emission regime. A universal jump in the normalized intensity correlation function is predicted to occur, the measurement of which could be used to establish a standard fingerprint of the onset of lasing in the strong coupling regime.