Photon orbital angular momentum in a plasma vortex
  We study theoretically the exchange of angular momentum between a photon beam and a plasma vortex, and demonstrate the possible excitation of photon angular momentum states in a plasma. This can be relevant to laboratory and space plasma diagnostics; radio astronomy self-calibration; and generating photon angular momentum beams. A static plasma perturbation with helical structure, and a rotating plasma vortex are studied in detail and a comparison between these two cases, and their relevance to the physical nature of photon OAM, is established. 
J. T. Mendon\c{c}a, B. Thid\'e, J. E. S. Bergman, S. M. Mohammadi, B.   Eliasson, W. Baan, and H. Then