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 lasers. We demonstrate that a cold beam of two-level particles driven coherently by a counter-propagating resonant wave can amplify exponentially a co-propagating optical probe up to a saturation value through an instability very similar to that of a high gain FEL. In addition, the two level atoms undergo collective recoil and exponential self-bunching in space and form a longitudinal grating on the scale of the wavelength of the amplified optical signal.