  Nonlinear optics experiments measuring phase shifts induced in a weak probe
pulse by a strong pump pulse must account for coherent effects that only occur
when the pump and probe pulses are temporally overlapped. It is well known that
a weak probe beam experiences a greater phase shift from a strong pump beam
than the pump beam induces on itself. The physical mechanism behind the
enhanced phase shift is diffraction of pump light into the probe direction by a
nonlinear refractive index grating produced by interference between the two
beams. For an instantaneous third-order response, the effect of the grating is
to simply double the probe phase shift, but when delayed nonlinearities are
considered, the effect is more complex. A comprehensive treatment is given for
both degenerate and nondegenerate pump-probe experiments in noble and diatomic
gases. Results of numerical calculations are compared to a recent transient
birefringence measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and a
recent spectral interferometry experiment [Wahlstrand et al., Phys. Rev. A 85,
043820 (2012)]. We also present results from two new experiments using
spectrally-resolved transient birefringence with 800 nm pulses in Ar and air
and degenerate chirped pulse spectral interferometry in Ar. Both experiments
support the interpretation of the negative birefringence at high intensity as
arising from a plasma grating.
