Degenerate phase-conjugate four-wave mixing in a nearly-Doppler-free two-level atomic medium

We present experimental observations of phase-conjugate degenerate four-wave mixing in a two-level atomic system (the 3s S-2(1/2), F = 2, m(F) = 2 --> 3p P-2(3/2), F = 3, m(F) = 3 transition in a diffuse, collision-free, thermal beam of atomic sodium) driven by cw, narrow-band (similar to 250 kHz), stabilized pump and probe fields. The primary measurements that we report are of the peak signal strength of the four-wave-mixing interaction and the bandwidth of the four-wave-mixing; spectrum (as the laser frequency is tuned through the resonant frequency) as a function of the intensity of the pump field. We compare these measurements with the predictions of a theoretical analysis in which extension to include the effect of the small transverse velocity (similar to 3 m/sec) of the atomic beam is critical. These measurements provide a direct verification of the theory of this important nonlinear interaction. [S1050-2947(98)06010-7].