Phase conjugation by degenerate four-wave mixing in barium vapor.

We report what is to our knowledge the first observation of phase-conjugate waves by degenerate four-wave mixing in Ba vapor near its 6s(2)(1)S(0) ? 6s6p(1)P(1) resonance transition. A maximum reflectivity of 36% at 10 mbars of Ar buffer gas pressure is observed together with a temporal pulse shortening of 50%. Conical emission in the two pump waves after their passage through the nonlinear medium is also observed. The dependence of the phase-conjugate signal on the pressure of the Ar buffer gas shows a small state-changing collisional decay rate.