Two-dimensional optical spectroscopy by periodic excitation of sublevel coherence with sub-Doppler resolution

The response of ground-state spin coherence to narrow-band periodic optical excitation is studied theoretically and experimentally. The dependence of the induced oscillating magnetization on the transverse magnetic field and on the frequency of the laser radiation is calculated for a four-level model system from a Bloch-type equation, yielding a two-dimensional resonance line. The application of this two-dimensional high-resolution Zeeman and optical spectroscopy is demonstrated experimentally in the case of the D₁ line of atomic rubidium vapor. Resolution in the kilohertz range is achieved, which allows clear spectral separation of the two isotopes ⁸⁵Rb and ⁸⁷Rb, showing overlapping lines in the conventional optical absorption spectrum.