Negative index of refraction in a four-level system with magnetoelectric   cross coupling and local field corrections
  This research focuses on a coherently driven four-level atomic medium with the aim of inducing a negative index of refraction while taking into consideration local field corrections as well as magnetoelectric cross coupling, i.e. chirality, within the material's response functions. Two control fields are used to render the medium transparent for a probe field which simultaenously couples to an electric and a magnetic dipole transition, thus allowing one to test the permittivity and permeability of the material at the same time. Numerical simulations show that a negative index of refraction with low absorption can be obtained for a range of probe detunings while depending on number density and the ratio between the intensities of the control fields. 
F. Bello
cond-mat.mtrl-sci quant-ph