COLLECTIVE ATOMIC RECOIL LASER (CARL) OPTICAL GAIN WITHOUT INVERSION BY COLLECTIVE ATOMIC RECOIL AND SELF-BUNCHING OF 2-LEVEL ATOMS

We suggest a novel tunable laser concept, the Collective Atomic Recoil Laser (CARL) which unifies the physics of the FEL and of the atomic l asers. We demonstrate that a cold beam of two-level particles driven c oherently by a counter-propagating resonant wave can amplify exponenti ally a co-propagating optical probe up to a saturation value through a n instability very similar to that of a high gain FEL. In addition, th e two level atoms undergo collective recoil and exponential self-bunch ing in space and form a longitudinal grating on the scale of the wavel ength of the amplified optical signal.