  The analysis of negative birefringence optically induced in major air
components (Loriot et al., [1, 2]) is revisited in light of the recently
reported plasma grating-induced phase-shift effect predicted for strong field
pump-probe experiments (Wahlstrand and Milchberg, [3]). The nonlinear birefrin-
gence induced by a short and intense laser pulse in argon is measured by
femtosecond time-resolved polarimetry. The experiments are performed with
degenerate colors, where the pump and probe beam share the same spectrum, or
with two different colors and non-overlapping spectra. The in- terpretation of
the experimental results is substantiated using a numerical 3D+1 model
accounting for nonlinear propagation effects, cross-beam geometry of the
interacting laser pulses, and detec- tion technique. The model also includes
the ionization rate of argon and high-order Kerr indices introduced by Loriot
et al. enabling to assess the contribution of both terms to the observed
effect. The results show that the ionization-induced phase-shift has a minor
contribution compared to the high-order Kerr effect formerly introduced, the
latter allowing a reasonably good reproduction of the experimental data for the
present conditions.
