Effect of a plasma grating on pump-probe experiments near the ionization   threshold in gases
  Calculations are performed of the phase shift caused by the spatial modulation in the plasma density due to interference between a strong pump pulse and a weak probe pulse. It is suggested that a recent experiment [Loriot et al., Opt. Express v. 17, 13429 (2009)] observed an effective birefringence from this plasma grating rather than from the higher-order Kerr effect. 
J. K. Wahlstrand and H. M. Milchberg
physics.optics physics.atom-ph physics.plasm-ph