Intensity-dependent dispersion under conditions of electromagnetically   induced transparency in coherently prepared multi-state atoms
  Interest in lossless nonlinearities has focussed on the the dispersive properties of $\Lambda $ systems under conditions of electromagnetically induced transparency (EIT). We generalize the lambda system by introducing further degenerate states to realize a `Chain $ \Lambda $' atom where multiple coupling of the probe field significantly enhances the intensity dependent dispersion without compromising the EIT condition. 
Andrew D. Greentree, Derek Richards, J. A. Vaccaro, A. V. Durrant, S.   R. de Echaniz, D. M. Segal, J. P. Marangos