Absorption-Free Sub-and Super-Luminal Light Propagation via Quantum Interference

We investigate the dispersive and absorptive properties of an N-type four-level atomic system in three coherent fields,and consider the susceptibility mathematically.It is shown that the group velocity of the probe field exhibits sub-and super-luminal under some conditions,meanwhile the absorption coefficient keeps zero during the occurrence and transition process of sub-and super-luminal pulses.