Abstract

Polarized light propagating through a resonant medium induces not only optical polarizations in the medium, but in general also population differences and coherences between the various substates of the electronic groundstate. These atomic- or sublevel-coherences give rise to many interesting magnetooptical and nonlinear optical effects and play an important role in laser cooling experiments. We present an experimental scheme, based on two-dimensional (2D) Fourier-transform spectroscopy, that allows, for the first time, to observe the creation and evolution of these coherences. The spectra obtained via this method have a very high information content and allow not only the observation of the overall magneto-optical effects, but also the distinction between contributions from all possible transitions between different Zeeman-substates of the hyperfine multiplets. This makes it possible to measure the internal degrees of freedom of the atomic system in terms of the full level systems, eliminating the requirement for simplified model systems that typically neglect the hyperfine interaction. The interpretation of the experimental data requires therefore a comparison with theoretical calculations of the time-evolution based on the complete level structure. These calculations cannot be performed analytically but must rely on numerical simulations. We have performed such simulations that are in good agreement with the experimental data. The interaction between the atomic system and the optical field can be understood in terms of optical pumping and light-shift effects. In addition, the atomic coherences interact with external magnetic fields via electron-Zeeman and nuclear- Zeeman effect. An example of such a simulation is presented in Figure 1. The two-dimensional spectrum shows how coherences between the substates of the F=1 and F=2 hyperfine multiplet of the Na ground state are transferred between different transitions during a pulse of circularly polarized light applied close to the D₁-transition. The four resonance positions are specific for the four possible transitions with |Δm_f| = 1. The ω₁-axis corresponds to the resonance frequency before the pulse, the ω₂-axis to the frequency after the pulse. Resonance lines with ω₁≠ω₂ indicate therefore that the corresponding coherences have been transferred between different transitions by the laser pulse.

© 1992 IQEC