It is well known that strongly driven two-level atoms exhibit gain (and loss) at certain frequencies in the vicinity of the excitation frequency. Inside an optical cavity the optical gain can be larger than the losses, and laser oscillation should occur with an ensemble of two-level atoms as the active medium. The authors have experimentally investigated this possibility by studying the fluorescence rate of driven atoms into the modes of a 1-cm-long confocal optical cavity. A beam of Ba atoms, a strong driving laser field tuned near the 553.5-nm Ba transition, and the cavity axis were oriented along the mutually orthogonal axis and made to intersect at the geometric center of the finesse 200 cavity. The intensity of light emitted out one end of the cavity provided a measure of the cavity-mode atomic fluorescence rate and was measured as a function of cavity tuning, atomic beam density, and laser 138 Ba detuning Δ. The results indicate that the maximum atomic beam density used was within a factor of two of the value necessary to produce a gain that exceeds losses. Preliminary work with an apparatus modified to provide higher atomic beam densities strongly suggests that the laser threshold has been reached.
