Enhancement of the nonlinearity via interacting dark resonances

A scheme for large enhancement of the third-order nonlinearity in a four-level system with double dark resonances is proposed. We obtain an analytical expression for the linear susceptibility and nonlinear susceptibility of a weak probe field and show that the properties of double electromagnetically induced transparency and the Kerr nonlinearity can be modified significantly by changing the ratio of the two coupling fields. A physical understanding of our numerical results using the dressed-state approach is presented. The obtained large nonlinearity with vanishing absorption can be used to generate optical solitons, polarization phase gates and optical Kerr shutters.