  Conventional lasers make use of optical cavities to provide feedback to gain
media. Conversely, mirrorless lasers can be built by using disordered
structures to induce multiple scattering, which increases the effective path
length in the gain medium and thus provides the necessary feedback. These
so-called random lasers potentially offer a new and simple mean to address
applications such as lighting. To date, they are all based on condensed-matter
media. Interestingly, light or microwave amplification by stimulated emission
occurs also naturally in stellar gases and planetary atmospheres. The
possibility of additional scattering-induced feedback (that is, random lasing)
has been discussed and could explain unusual properties of some space masers.
Here, we report the experimental observation 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 of the cloud 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.

Atomic Physics;Disordered Systems and Neural Networks;Optics