Electromagnetically induced gratings in a degenerate open two-level system

We have experimentally investigated laser diffraction on electromagnetically induced gratings in a degenerate open two-level system. The experiment employs the standard backward four-wave mixing configuration and the open ${6S}_{1/2}\mathrm{}(F=4\mathrm{})\mathrm{}\to {6P}_{3/2}{(F}^{\prime }=4)$ transition of cold cesium atoms. Population and coherence gratings are accessed for different relative polarizations of the incident laser beams. Natural and subnatural linewidths are observed depending on which frequency of the incident beams is scanned and reveal the effect of the atomic motion on the diffraction spectra. A simple theoretical model using a three-level $\Lambda$ system is in reasonable agreement with the observed results and allowed us to make a close connection with the phenomenon of electromagnetically induced transparency.