Type I and type II second harmonic generation of conically refracted   beams
  Type I and type II second harmonic generation (SHG) of a beam transformed by the conical refraction phenomenon are presented. We show that, for type I, the second harmonic intensity pattern is a light ring with a point of null intensity while, for type II, the light ring possesses two dark regions. Taking into account the different two-photon processes involved in SHG, we have derived analytical expressions for the resulting transverse intensity patterns that are in good agreement with the experimental data. Finally, we have investigated the spatial evolution of the second harmonic signals, showing that they behave as conically refracted beams. 
Alex Turpin, Yury V. Loiko, Todor K. Kalkandjiev, Jose Trull, Crina   Cojocaru, and Jordi Mompart
physics.optics