We present measurements of probe beam amplification in Na vapor at intermediate density values, in both the counterpropagating and copropagating directions (with respect to the pump beam) in the case of degenerate pump and probe frequencies. Strong gain is observed only in the counterpropagating direction for parameters that preclude the possibility of Raman or Rayleigh gain. We show that the experimental results can be interpreted in terms of the spontaneous formation of a density grating due to the nonresonant interaction between the electromagnetic field and the atoms.

• Received 18 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.2452