We investigate the linear and nonlinear dispersion and absorption properties of a weak probe field in a dielectric slab doped with Λ-type atoms. The effect of a relative phase between applied fields on linear and nonlinear dispersion, absorption and Kerr nonlinearity is discussed. It is demonstrated that the system is phase sensitive when spontaneously generated coherence and an incoherent pump exist simultaneously. The effect of the coupling field intensity on the Kerr nonlinearity is also presented.
