A cold-atom random laser

In conventional lasers optical cavities are used to provide feedback to gain media. Mirrorless lasers can be built by using disordered structures to induce multiple scattering, which increases the path length in the medium, providing the necessary feedback(1). Interestingly, light or microwave amplification by stimulated emission also occurs naturally in stellar gases(2-4) and planetary atmospheres(5,6). The possibility of additional scattering-induced feedback(4,7)-random lasing(8-14)-could explain the unusual properties of some space masers(15). Here, we report experimental evidence of random lasing in a controlled, cold atomic vapour, taking advantage of Raman gain. By tuning the gain frequency in the vicinity of a scattering resonance, we observe an enhancement of the light emission due to random lasing. The unique possibility to both control the experimental parameters and to model the microscopic response of our system provides an ideal test bench for better understanding natural lasing sources, in particular the role of resonant scattering feedback in astrophysical lasers.