Parametric collective phenomena during the propagation of polychromatic laser pulses in an optically dense resonant medium without population inversion

We experimentally and theoretically study the interaction of broadband polychromatic laser pulses with an optically dense resonant extended medium without population inversion. Experimental (probe field-pumping beam) measurements of the transmission and amplification spectra were carried out in the plasma of a positive neon glow-discharge column containing a large number of metastable atoms. The strong coupling in the field-matter system and the collective behavior of the atomic system in a resonant field were attributable to a high (～10¹² cm^?3) density of atoms at the lower (metastable) level of the optical transitions under consideration and to a relatively low intensity of the interacting laser beams. We observed a broadband weakening of the probe field in the absence of pumping and its strengthening in the line wings in the presence of a strong field. We develop a theoretical model for the parametric amplification of collective interactions in dense extended media based on the solution of the semiclassical Maxwell-Bloch equations for conditions under which the pumping field does not destroy the dipole interaction between atoms through probe-field photons.