Spin and Orbital angular momentum propagation in anisotropic media:   theory
  This paper is devoted to study the propagation of light beams carrying orbital angular momentum in optically anisotropic media. We first review some properties of homogeneous anisotropic media, and describe how the paraxial formalism is modified in order to proceed with a new approach dealing with a general setting of paraxial propagation along uniaxial inhomogeneous media. This approach is suitable for describing the space-variant-optical-axis phase plates. 
Antonio Pic\'on, Albert Benseny, Jordi Mompart, and Gabriel F. Calvo
physics.optics quant-ph