1 S October 1994 OPTICS COMMUNICATIONS zyxw ELSEVIER Optics Communications 111 ( 1994 ) 613-622 Full length article Degenerate parametric emission in dense barium vapour Witold Chalupczak, Wojciech Gawlik, Jerzy Zachorowski zyxwvutsrqponmlkjihgfedcbaZ Instytut Fizyki, Uniwersytet Jagielloriski, ul. Reymonta 4, 30-059 Krakdw, Poland Received 18 April 1994 Abstract Studies of degenerate parametric interaction in laser beam propagation through dense barium vapour are reported. Various transverse instabilities have been observed when two strong, copropagating beams intersect in the medium. Most of them are attributed to parametric interaction of the intersecting beams and their main features are consistent with the existing four-wave mixing formalism. The conditions of their generation and their mutual competition are discussed. 1. Introduction Studies of nonlinear interaction of laser light with gas media are one of the main topics of contempo- rary optics. These studies are concerned with such ef- fects as changes of a probe beam intensity after pass- ing through a strongly perturbed sample, generation of new light beams via wave mixing, and changes of a transverse beam pattern, i.e., transverse instability of the transmitted beams. Most important in the first group are the three familiar absorption features that can be seen in pump-probe experiments with two- level atoms [ 11. For the pump at wL> w. (where wL, o. are the laser and resonance frequencies, respec- tively) the probe is amplified when its frequency op z w, + Q’ due to the three photon scattering (TPS ) where the generalised Rabi frequency a’= ( d2+ Q2)‘12, detuning J=wL-wo, and the Rabi flopping frequency O=E-D/A, E is the electric field ampli- tude of the light wave and D is the electric dipole mo- ment of an atom. For wp= oL-Q’ the probe is ab- sorbed and the third, dispersion-like feature appears for op z wi_. When in addition to the pump and probe beams a third light beam is taken into account various schemes of four-wave couplings should be considered. Boyd et al. [2] analyzed four-wave mixing (FWM) in a strongly driven two-level system and showed that strong amplification of the probe may take place for or = wL, wL? Q’. This can be explained as a parame- tric process in which two oL photons are absorbed and emission of two other photons of frequencies wp and w4, such that 2oL= wp + w4, is stimulated by the probe beam. The four-wave mixing amplitude has its maxima for all three frequencies of the Mollow trip- let. The actual gain of the probe beam depends on the pump laser detuning 8, on the intensities of pump and probe beams [ 3 ] and on the relaxation times. Due to this gain any small signal (laser background, sponta- neous emission) at a proper frequency can generate a well measurable signal. The first observations of non-degenerate FWM in a strongly driven system have been performed in the experiments of Plekhanov et al. [ 41 and of Chalupczak et al. [ 5 ] in which it has been shown that there is a threshold for generation of non-degenerate ( wp# wL) FWM below which the effect has to be externally initiated. In the case where collisions dominate the non-degenerate FWM is quenched and the highest gain occurs for wp% wL. One could then expect generation of para- 0030-4018/94/%07.00 0 1994 Elsevier Science B.V. All zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG rights reserved XWIOO30-4018(94)00394-7