Recoil-induced resonances in cesium: An atomic analog to the free-electron laser

We report on the first expeimental observation of recoil-induced resonances. These resonances, which can be considered as the atomic analog of the grain mechanism of the free-electron laser, are observed on the transmission spectrum of a probe beam making a small angle with a one-dimensional lin{⊥}lin cesium optical molasses. They are interpreted either as stimulated Raman transitions between differently populated velocity groups (the internal atomic state being unchanged), or as stimulated Rayleigh resonances originating from a bunching of the atomic density. Their shape provides information about the transverse velocity distribution of the atoms.