Generalized phase-matching conditions for high harmonics: The role of field-gradient forces

We present an approach to describe the phase matching of high harmonics emitted by laser driven atoms in a nonperturbative regime, for which the atomic response displays an intrinsic intensity-dependent phase. We show that the traditional phase-matching conditions involving conservation of wave vectors should be modified by taking into account the gradient of this atomic phase. We investigate various focusing geometries and interpret the numerical results of Sali`eres et al. [Phys. Rev. Lett. 74, 3776 (1995)]. Within the framework of the two-step model, we demonstrate that the gradient of the intensity-dependent phase can be considered as the canonical momentum gained by the electron in the continuum due to acceleration by field-gradient forces, including in particular the ponderomotive force.