High-resolution studies of collision-induced population grating resonances in optical four-wave mixing in sodium vapor.

Optical four-wave mixing resonances in sodium vapor induced by collisions with invert buffer gases (He,Ar,Xe,N2) are characterized at high resolution (5 MHz) using lasers tuned near the D-lines of Na. These resonances are interpreted in terms of coherent scattering from a population grating established by collisional absorption of the incident radiation. We show that this is caused by collision-induced optical pumping which introduces the ground state lifetime into the grating relaxation physics. Finally, we present data illustrating interference between the collision-induced population grating resonances detailed here and collision induced coherent Raman resonances between Zeeman sublevels of the ground state.