Scalar propagator in the pp-wave geometry obtained from AdS_5 X S^5
  We compute the propagator for massless and massive scalar fields in the metric of the pp-wave. The retarded propagator for the massless field is found to stay confined to the surface formed by null geodesics. The algebraic form of the massive propagator is found to be related in a simple way to the form of the propagator in flat spacetime. 
Samir D. Mathur, Ashish Saxena and Yogesh K. Srivastava