Absorption and dispersion in probe laser in the four-level Y-type atom system

In this paper we have calculated the absorption and dispersion of the probe laser in the four-level Y-type atom system at low light levels and found that if the external light fields are changed,the system would exhibit an electromagnetically induced transparency effect.At the same time,we found that the system could also exhibit enhancement accompanied with a big negative refractive index,and if the light field phase is changed,the absorption and dispersion of the probe laser are also changed.