Trirefringence in nonlinear metamaterials
  We study the propagation of electromagnetic waves in the limit of geometrical optics for a class of nearly transparent nonlinear uniaxial metamaterials for which their permittivity tensors present a negative principal component. Their permeability are assumed positive and dependent on the electric field. We show that light waves experience triple refraction -- trirefringence. Additionally to the ordinary wave, two extraordinary waves propagate in such media. 
Vitorio A. De Lorenci and Jonas P. Pereira
physics.optics cond-mat.mtrl-sci