Optical anisotropy induced by torsion stresses in LiNbO3 crystals:   appearance of an optical vortex
  We report the results of studies of torsion effect on the optical birefringence in LiNbO3 crystals. We have found that twisting of those crystals causes a birefringence distribution revealing non-trivial peculiarities. In particular, it has a special point at the center of cross section perpendicular to the torsion axis where zero birefringence value occurs. It has also been ascertained that the surface of the spatial birefringence distribution has a conical shape, with the cone axis coinciding with the torsion axis. We have revealed that an optical vortex, with the topological charge equal to unity, appears under the torsion of LiNbO3 crystals. It has been shown that, contrary to the q-plate, both the efficiency of spin-orbital coupling and the orbital momentum of the emergent light can be operated by the torque moment. 
Ihor Skab, Yurij Vasylkiv, Viktoriya Savaryn and Rostyslav Vlokh
physics.optics