Phase and amplitude variations of optically induced spin transients

We report detailed studies of freely evolving Zeeman coherence (Zeeman quantum beats) observed after optically pumping a gas of Na atoms and suddenly turning off the light field. This Zeeman coherence corresponds to a macroscopic magnetization that precesses in an external magnetic field; it is detected with a cw optical probe beam by using polarization-selective detection of the transmitted light. The amplitude and the phase of the signal show a pronounced variation with laser intensity, laser detuning, and the strength of the magnetic field. This dependence was measured on the 3s2S1/2 ground state of sodium, using the D1 line for optical excitation and detection. The experimental data are compared with theoretical predictions based on a Bloch-type equation of motion for an optically driven spin system in a J = 1/2 ground state.