We investigate the population differences between ground-state hyperfine levels (
5S1/2,F=2 and
5S1/2, F=1) of an optically pumped
87Rb beam in a static magnetic field, taking into account the various kinds of atomic coherence (Zeeman, hyperfine, and optical). The pumping lasers are monochromatic and in resonance with the ${(5S}_{1/2},F=1\ensuremath{\leftrightarrow}{5P}_{1/2}{,F}^{\ensuremath{'}}=1)$ transition. By varying the polarization configuration of the pumping lasers, we find that the pumping efficiency is substantially enhanced when a $\ensuremath{\sigma}$-polarized laser and a $\ensuremath{\pi}$-polarized laser are counterpropagating. In this case, the pumping efficiency remains high even in a weak magnetic field, which explains qualitatively experimental data reported previously from a cesium beam experiment.
A new and unusual phenomenon which we call light narrowing is reported and discussed in this paper. We discovered this effect in dense, spin-polarized cesium vapor optically pumped with a cw blue dye laser beam tuned to the second resonance
D1 line (4593 \AA{}). We observe a significant narrowing of the radio-frequency power-broadened magnetic resonance lines (linewidths narrow by as much as a factor of 2.5) when the intensity of the circularly polarized incident dye laser beam is increased by either focusing the beam or by the removal of attenuating filters from the focused beam. The magnetic resonance linewidths in spin-polarized cesium vapor were measured over a wide range of cesium number densities (5\ifmmode\times\else\texttimes\fi{}
1012 ${\mathrm{cm}}^{\ensuremath{-}3}$ \ensuremath{\le}[Cs]\ensuremath{\le}1\ifmmode\times\else\texttimes\fi{}
1016 ${\mathrm{cm}}^{\ensuremath{-}3}$). This corresponds to cesium spinexchange rates of 4.5\ifmmode\times\else\texttimes\fi{}
103 to 9\ifmmode\times\else\texttimes\fi{}
106 ${\mathrm{sec}}^{\ensuremath{-}1}$. For low cesium number densities (5\ifmmode\times\else\texttimes\fi{}
1012 [Cs]3\ifmmode\times\else\texttimes\fi{}
1014 ${\mathrm{cm}}^{\ensuremath{-}3}$) the light-narrowing effect is large (a factor of 2.5) and independent of [Cs]. In the region of 3\ifmmode\times\else\texttimes\fi{}
1014 to 1\ifmmode\times\else\texttimes\fi{}
1015 ${\mathrm{cm}}^{\ensuremath{-}3}$ in number densities, the light-narrowing effect decreases with increasing [Cs]. At high cesium number densities ([Cs]> 1\ifmmode\times\else\texttimes\fi{}
1015 ${\mathrm{cm}}^{\ensuremath{-}3}$) this light-narrowing effect almost completely disappears. In the limit of low-radio-frequency power the magnetic resonance linewidths for focused and unfocused dye laser beam are nearly the same. Experimental observations on this new effect are presented in detail. In the latter part of this paper a self-contained theoretical treatment of the light-narrowing effect is developed. Using Bloch equations in the presence of optical pumping, spin relaxation (diffusion, electron randomization), rapid spin exchange, and radio-frequency magnetic field, expressions for magnetic resonance line shapes are derived. In general, we find good agreement between our experimental results and the theory.
We studied the spectral behavior of a probe transmission in the presence of a counterpropagating pump beam for various laser polarizations and magnetic fields with and without the assistance of an additional repopulation laser beam. These parametric studies enabled us to distinctively extricate the influence of saturation, optical pumping, and resonant light pressure. The enhanced probe absorption resulting from the pump-beam-induced Zeeman substate dressing was maximized when the change in the resonance frequency was suitably compensated by the change in the Doppler shift arising from resonant light pressure. The possibility of tuning the laser frequency locked to saturation absorption spectroscopy was investigated and implemented to a laser cooling and trapping setup.