Electromagnetically induced transparency and fourwave mixing in a cold atomic ensemble with large optical depth

We report on the delay of optical pulses using electromagnetically induced transparency (EIT) in an ensemble of cold atoms with an optical depth exceeding 500. To identify the regimes in which four-wave mixing (4WM) impacts on EIT behaviour, we conduct the experiment in both ⁸⁵Rb and ⁸⁷Rb. Comparison with theory shows excellent agreement in both isotopes. In ⁸⁷Rb negligible 4WM was observed and we obtained one pulse-width of delay with 50% efficiency. In ⁸⁵Rb 4WM contributes to the output. In this regime we achieve a delay-bandwidth product of 3.7 at 50% ef ficiency, allowing temporally multimode delay, which we demonstrate by compressing two pulses into the memory medium.