Photon recoil momentum in dispersive media
  A systematic shift of the photon recoil due to the index of refraction of a dilute gas of atoms has been observed. The recoil frequency was determined with a two-pulse light grating interferometer using near-resonant laser light. The results show that the recoil momentum of atoms caused by the absorption of a photon is n\hbar k, where n is the index of refraction of the gas and k is the vacuum wavevector of the photon. This systematic effect must be accounted for in high-precision atom interferometry with light gratings. 
Gretchen K. Campbell, Aaron E. Leanhardt, Jongchul Mun, Micah Boyd,   Erik W. Streed, Wolfgang Ketterle and David E. Pritchard