Orbital Angular Momentum in Noncollinear Second Harmonic Generation by   off-axis vortex beams
  We experimentally study the behavior of orbital angular momentum (OAM) of light in a noncollinear second harmonic generation (SHG) process. The experiment is performed by using a type I BBO crystal under phase matching conditions with femtosecond pumping fields at 830 nm. Two specular off-axis vortex beams carrying fractional orbital angular momentum at the fundamental frequency (FF) are used. We analyze the behavior of the OAM of the SH signal when the optical vortex of each input field at the FF is displaced from the beam's axis. We obtain different spatial configurations of the SH field, always carrying the same zero angular momentum. 
Fabio Antonio Bovino, Matteo Braccini, Maurizio Giardina, Concita   Sibilia
physics.optics