Tunable white light cavity induced by interacting dark resonances

We propose a scheme for a broadband optical cavity induced by interacting dark resonances in a ΛΛ-type four-level atomic system. The interacting dark resonances leads to constructive interference for the nonlinear dispersion and destructive interference for the absorption between two electromagnetically induced transparency windows. The enhanced nonlinear anomalous dispersion is used to broaden the cavity linewidth. The simulation results show that the cavity linewidth could be widened more than one order of magnitude broader than an empty-cavity linewidth. Moreover, the cavity linewidth depends on the intensities and the detunings of coupling fields.