Bichromatic electromagnetically induced transparency in cold rubidium atoms

In a three-level atomic system coupled by two equal-amplitude laser fields with a frequency separation 2$\delta$, a weak probe field exhibits a multiple-peaked absorption spectrum with a constant peak separation $\delta$. The corresponding probe dispersion exhibits steep normal dispersion near the minimum absorption between the multiple absorption peaks, which leads to simultaneous slow group velocities for probe photons at multiple frequencies separated by $\delta$. We report an experimental study in such a bichromatically coupled three-level $\Lambda$ system in cold $^{87}$Rb atoms. The multiple-peaked probe absorption spectra under various experimental conditions have been observed and compared with the theoretical calculations.