Nonlinear effects in optical pumping of a cold and slow atomic beam

By photoionizing hyperfine (HF) levels of the Cs state $6{\phantom{\rule{0.16em}{0ex}}}^{2}{P}_{3/2}$ in a slow and cold atom beam, we find how their population depends on the excitation laser power. The long time (around $180\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s})$ spent by the slow atoms inside the resonant laser beam is large enough to enable exploration of a unique atom-light interaction regime heavily affected by time-dependent optical pumping. We demonstrate that, under such conditions, the onset of nonlinear effects in the population dynamics and optical pumping occurs at excitation laser intensities much smaller than the conventional respective saturation values. The ev…