Slowdown the light speed in an alexandrite crystal

We have observed a hole in the absorption profile of the homogeneously broadened absorption band of an Alexandrite crystal. With the spectral hole-burning technique that resulting from the periodic modulation of the ground state population at the beat frequency between the pump and the probe fields, we observed the slowdown the light propagation with the group velocity as slow as 12.5 m/s at room temperature. And the group velocity in the Alexandrite crystal depends on the amplitude modulation f