Conversion of bright magneto-optical resonances into dark at fixed laser frequency for D2 excitation of atomic rubidium

摘要

Nonlinear magneto-optical resonances on the hyperfine transitions belongingto the D2 line of rubidium were changed from bright to dark resonances bychanging the laser power density of the single exciting laser field or bychanging the vapor temperature in the cell. In one set of experiments atomswere excited by linearly polarized light from an extended cavity diode laserwith polarization vector perpendicular to the light's propagation direction andmagnetic field, and laser induced fluorescence (LIF) was observed along thedirection of the magnetic field, which was scanned. A low-contrast brightresonance was observed at low laser power densities when the laser was tuned tothe Fg=2 --> Fe=3 transition of Rb-87 and near to the Fg=3 --> Fe=4 transitionof Rb-85. The bright resonance became dark as the laser power density wasincreased above 0.6mW/cm2 or 0.8 mW/cm2, respectively. When the Fg=2 --> Fe=3transition of Rb-87 was excited with circularly polarized light in a second setof experiments, a bright resonance was observed, which became dark when thetemperature was increased to around 50C. The experimental observations at roomtemperature could be reproduced with good agreement by calculations based on atheoretical model, although the theoretical model was not able to describemeasurements at elevated temperatures, where reabsorption was thought to play adecisive role. The model was derived from the optical Bloch equations andincluded all nearby hyperfine components, averaging over the Doppler profile,mixing of magnetic sublevels in the external magnetic field, and a treatment ofthe coherence properties of the exciting radiation field.