Spin-to-orbital angular momentum conversion in focusing, scattering, and   imaging systems
  We present a general theory of spin-to-orbital angular momentum (AM) conversion of light in focusing, scattering, and imaging optical systems. Our theory employs universal geometric transformations of nonparaxial optical fields in such systems and allows for direct calculation and comparison of the AM conversion efficiency in different physical settings. Observations of the AM conversions using local intensity distributions and far-field polarimetric measurements are discussed. 
K. Y. Bliokh, E. A. Ostrovskaya, M. A. Alonso, O. G.   Rodriguez-Herrera, D. Lara, and C. Dainty
physics.optics