Abstract

We study effects connected with propagation of a strong laser beam in dense medium. Under such excitation a Mollow spectrum perturbed by collisions is observed in a lateral direction. Light emitted in the forward direction couples non-linearly with pump beam and can be amplified [1], Depending on vapour density and incident laser intensity I_I. we observe three regimes: in the first stage (N~10¹³ cm^-3) the forward spectrum is identical with fluorescence spectrum and contains the line at laser frequency ω_L and another one at the AC-Stark-eftect perturbed atomic resonance freguency ω_f [2]. At higher atomic density (N~10¹⁴ cm^-3) four-wave mixing of the laser with the ω_f line occurs producing a forward emission of radiation at ω₄ = 2ω_L - ω_F. Its intensity scales as i_l4 whereas the peak at ω_F scales as I_L² in accordance with the predictions of the dressed-atom model. The phase matching requirement results in emission into a cone around the pump laser direction. For N~5-10¹⁴ cm^-3 a complete parametric coupling develops and both beams depend exponentially on I_L. Such a complete coupling has been already observed in studies of conical emission with laser beam self-focussed to a narrow filament. Our experimental conditions where we avoided filamentation allowed for a better control over laser intensity and for observation of all stages of the coupling.

© 1996 IEEE