Abstract: The absorption and dispersion properties of an N-type four-level system with Doppler broadening effect were investigated. In this system we considered two coherent fields and a probe field. When the second coherent field is absent, this system is a single three-level configuration, and the absorption line is just one clear electromagnetically induced transparency (EIT) window. While the second coherent field exists, there are two EIT windows which become wide and deep with the coherent intensity increasing. The position width and absorbance of the transparency windows can be manipulated by adjusting the detuning of the driving field. In addition, we also found that the medium shows a large refractive index at each transparency window, and the group velocity of the probe laser can be reduced.

Key words: electromagnetically induced transparency, coherent field, N-type, group velocity