Fermat's principle with complex refractive indices and local light-ray   rotation
  We describe local light-ray rotation in terms of complex refractive indices. We show that Fermat's principle holds, and we derive an extended Snell's law. The change in the angle of a light ray with respect to the normal to a refractive-index interface is described by the modulus of the refractive-index ratio, the rotation around the interface normal is described by the argument of the refractive-index ratio. 
Bhuvanesh Sundar, Alasdair C. Hamilton, Johannes Courtial