Phase variation and shape distortion of light pulses in electromagnetically induced transparency media

We have experimentally studied the propagation behaviors of light pulses in an electromagnetically induced transparency (EIT) medium. Both phase variation in different parts of a pulse and distortion of the pulse shape are observed. The consistency between the experimental data and theoretical predictions is satisfactory. Our study explains the physical origins of the phase variation and the shape distortion, shows that the phase variation should be taken into consideration in applications utilizing the EIT scheme in which two-photon detuning must exist, and provides better understanding of the propagation behavior of the slow light.