Geometric phase in optics and angular momentum of light
  Physical mechanism for the geometric phase in terms of angular momentum exchange is elucidated. It is argued that the geometric phase arising out of the cyclic changes in the tranverse mode space of the Gaussian light beams is a manifestation of the cycles in the momentum space of the light. Nonconservation of angular momentum in the spontaneous parametric down-conversion for the classical light beams is proposed to be related with the geometric phase. 
S C Tiwari