  The four-wave mixing produced with two ultrashort phase-locked w-3w laser
pulses propagating coherently in a two-level system in the infrared spectral
region is shown to depend on the pulses relative phase. The Maxwell-Bloch
equations are solved beyond the rotating-wave approximation to account for
field frequencies which are largely detuned from the atomic resonance. The
relative phase dominating the efficiency of the coupling to the 5w anti-Stokes
Raman component is determined by sign of the total ac Stark shift induced in
the system, in such a way that the phase influence disappears precisely where
the ac Stark effect due to both pulses is compensated. This fundamental quantum
interference effect can be the basis for nonlinear ultrafast optical
spectroscopy techniques.
