Collision-enhanced Hanle resonances and transverse optical pumping in four-wave light mixing in Na vapor

Collision-enhanced Hanle resonances in the 3 ${}_{}{}^2{}_{}{}^{}S_{1/2}^{}{}_{}{}^{}$ electronic ground state of Na atoms have been studied in a phase-conjugate four-wave light-mixing geometry. Narrow resonances in zero magnetic field have been observed at very high inert-buffer-gas pressures, with a width of 21 mG, or less than 10 kHz, full width at half-maximum. The intensity and the width of these resonances has been studied as a function of buffer-gas pressure, of alkali-metal vapor pressure, of detuning from the $D_1$ and $D_2$ Na resonances, and of the intensity of the incident light beams. Saturation phenomena and power broadening are observed. The results are explained theoretically in terms of transversely pumped optical gratings. The equivalence of this description with collision-induced coherence gratings in the limit of low light intensities is demonstrated.