  Bright and dark nonlinear magneto-optical resonances associated with the
ground state Hanle effect have been studied experimentally and theoretically
for D1 excitation of atomic cesium. This system offers the advantage that the
separation between the different hyperfine levels exceeds the Doppler width,
and, hence, transitions between individual levels can be studied separately.
Experimental measurements for various laser power densities and transit
relaxation times are compared with a model based on the optical Bloch
equations, which averages over the Doppler contour of the absorption line and
simultaneously takes into account all hyperfine levels, as well as mixing of
magnetic sublevels in an external magnetic field. In contrast to previous
studies, which could not resolve the hyperfine transitions because of Doppler
broadening, in this study there is excellent agreement between experiment and
theory regarding the sign (bright or dark), contrast, and width of the
resonance. The results support the traditional theoretical interpretation,
according to which these effects are related to the relative strengths of
transition probabilities between different magnetic sublevels in a given
hyperfine transition.
