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

TL;DR

This study demonstrates how nonlinear magneto-optical resonances in rubidium can be switched from bright to dark by adjusting laser power density or vapor temperature, supported by a detailed theoretical model.

Contribution

It introduces a comprehensive theoretical model based on optical Bloch equations that explains the bright-to-dark resonance conversion in rubidium's D2 line.

Findings

Bright resonances turn dark with increased laser power density.

Temperature increase causes bright-to-dark transition in circular polarization experiments.

The theoretical model agrees well at room temperature but not at higher temperatures.

Abstract

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