Gain, amplification, and lasing in a driven atom-cavity system

Cold atoms coupled to an optical cavity provide an ideal platform for creating lasers without conventional population inversion. Such lasers can generate very high spectral purity, and the output can be precisely controlled by varying the atom-drive or atom-cavity parameters. In this letter, we report experimental observation of gain, amplification, and self-lasing within a continuously operated⁸⁵Rb magneto-optic trap (MOT), in the collective strong coupling regime of the atom-cavity system. The presence of MOT cooling lasers significantly alters the transmission properties of the cavity, and gain is observed in one of the vacuum Rabi peaks. The system makes a transition from an amplifying medium to a self-sustained laser as the MOT parameters are tuned. The underlying lasing mechanism is shown to be Mollow gain in the driven atomic ensemble, and is confirmed through the free-space absorption spectroscopy of the MOT. The free space gain required for the lasing action is significantly smaller in our system as compared to the cases where there is no collective strong coupling.