the incoherent pumping one, and it justifies to optimize the efficiency β when the device is operated below saturation. On increasing pump power, the rate of emitted photons saturates to the value N$_{sat}$ = κR/ ( κ + R ), which in the bad cavity regime reduces to N$_{sat}$ = R . In order to maximize the rate of emitted photons, one has to maximize the parameter R , or equivalently F $^{∗}$, either by increasing Q$_{eff}$ in the resonant case, or by lowering it, by playing on pure dephasing, in the detuned case. Note that the behavior of the parameter N with respect to the atom-cavity detuning, δ , is dramatically different depending on the atom being saturated or not. If the pump power is low, then N evolves as β ( δ ), whereas if the pump power is high, N evolves like the atom-cavity coupling R ( δ ). Such a change of behavior turns out to be a fruitful method, e.g., to measure the Purcell factor of a single quantum dot coupled to a semiconductor microcavity, as it was evidenced in Ref. [29]. In the previous Section we have evidenced that in the bad cavity regime, the rate N of photons emitted by the cavity first evolves linearly with respect to pump power (same as the atomic population), before reaching an upper bound imposed by the spontaneous emission rate R (while the atomic population gets totally inverted). This limit is due to the saturation of the two-level emitter. A way to overcome it is to reach the stimulated emission regime, where the atom-cavity coupling scales like the number of photons in the cavity. In this case, the system is operated as a single emitter laser. This ideal device where the gain medium is quantified at the single emitter level, is of tremendous conceptual interest, and has motivated many fundamental studies since [30]. The primary interest of the lasing regime is that photons are mostly funneled into the cavity mode, allowing to achieve the highly efficient conversion of the incoherent power carried by the pump into single mode light. Second, as the emitter’s cycling rate is enhanced, it can be pumped at a much higher rate before saturation is reached. In this Section we revisit the single atom laser topics, building on the notion of good cavity regime. We examine to which extent pure dephasing can be a resource in the frame of solid-state lasers, and relate this study to recent experimental demonstrations of single quantum dot lasers. V. SINGLE TWO-LEVEL EMITTER LASER 11